self build waterproof basement formwork membrane concrete insulation

Insulating a home with a basement.

I'm not selling anything here. This page is my conclusion and recommendation after years of exhibitions and going back to customers and asking them how their energy efficiency choices had worked out.

I then go on to discuss what else we could do and what might be more worthwhile.

Most customers had a very warm basement, even without heating, but often the rest of the house did not work out as well as expected.
  self build basement

The EU Directive that bought us energy performance certificates, I think we were 8 years late introducing them, also requires new homes from 2020 to be Nearly Zero Energy Buildings with most of their energy from renewable sources. Definition of NZEB near the bottom of the page.

Clearly we are nowhere near meeting this next target.

The target after that is massively reducing leaked refrigerant from aircon, fridges and heat pump systems, such as ground source and air source. Apparently, leaked refrigerant from a heat pump system often completely negates all the greenhouse gases saved using less energy to get heat.

I had not even heard of this till March 2019, so I doubt we will meet it in the next year or two.

Ministers were told in February 2019 that, from 2025, new homes should not be connected to mains gas or use gas for cooking, heating or hot water if Britain is to meet its legally binding emissions target.

An expert at a workshop (Whole Life Carbon at the Futurebuild exhibition on March 7th 2019) a Passivhaus designer, engineer and architect, described that in order to get data his team analysed a new home just completed by a mass national housebuilder. The SAP calculation approved by building control was a U Value of 0.16. But the actual U Value, mainly due to poor air tightness and cold bridging, was worse than 0.30.
Thermal Mass Explained

"From an energy perspective, it would be difficult to have too much, and generally the more thermal mass the better"
This guide was given free at the Futurebuild Exhibition and is available for free from Click on this image to download it in full.

It will help the person doing your SAP calculation include the benefit of thermal mass.
  • "Revisions to the Standard Assessment Procedure (SAP) for Part L1 of the Building Regulations and more challenging requirements for addressing overheating in new homes."
  • "Ongoing improvements in glazing and window technology, which makes passive solar design more effective (lower heat loss and improved solar gain)."
Part of your SAP calculation will include a figure for TMP. 100 is low, 250 is medium and 450 is high.

Some figures for you to come back and refer to:

Entirely timber frame (and presumably SIPs and ICF) about 70.
Traditional (brick cavity, beam and block, some load bearing and some stud partitions, timber upper floor and roof) about 200.
Entirely heavyweight construction 500 to 650 depending on size and other variables.

A slide from a presentation at Futurebuild.

UK energy consumption
It is a fact that our electricity suppliers can pay 3 times as much for wholesale electricity at 5.30pm as they paid 45 minutes earlier before we got home and switched everything on.

It is likely that we will soon have to pay 3 times more for early evening electricity compared to mid-afternoon electricity.

Setting the washing machine to an afternoon wash is fairly simple, but steps to use a lot less electricity at peak time - especially if we no longer heat with gas - will require careful thought.

You need not be heating your home at peak time at all. If it is warmed a little earlier, at a cheaper electricity rate, your energy might cost a lot less. Or, if you rely on thermal mass, your home might be slightly cool late-afternoon, waiting for cooking and bathing to spread heat via heat recovery ventilation.

Emissions will be less if we aren't heating our living space during peak time because, to meet peak demand, the worst polluting power stations are only turned up at peak time.

On April 24th the Futurebuild people made a number of the presentations at their March exhibition available for free online. You have to register here.
The information in this box is from the presentation: "The Value of Energy Positive Buildings".
The slide above is from the presentation: "Lark Rise Energy Project".

We should be asking ourselves what actually works?

First and foremost, good workmanship - which might require continuous, effective inspection to refuse to accept anything sub standard - works a lot better than rushed work and sub standard materials.

If you are a self-builder you have the chance to demonstrate to the country that carbon, energy use and emissions can genuinely be greatly reduced. And that it need not be expensive to do so. But how?

ICF is bad (my many reasons not to use ICF are two pages further on). Timber frames and SIPs are poor because they don't store energy. Heat pumps are too expensive or don't work unless the heat source is a large body of water.

What worked best of all, and for less cost, was thermal mass and heat recovery ventilation with no cold bridging and excellent air tightness.

The best thermal mass is dense concrete. Cement in the UK is responsible for only 1.47% of our carbon emissions. Yet concrete can massively reduce the 28% of our emissions that are due to domestic heating - and likely to rise as we install more air conditioning unless we do something about it.

What we need from the heating and cooling of our homes is the ability to lose body heat at a comfortable rate. We don't want to be too cold and lose heat too fast, neither do we want to be too hot and unable to cool down. And daytime and night-time might require different room temperatures.

Thermal Mass is the ability of a heavyweight material to store a lot of heat and release it slowly. The denser and the least conductive the better.

Cast insitu concrete, brickwork and stonework are all good.

Concrete blocks are less dense and so less good. Beam and block is less effective than concrete cast in place.

concrete   brick   stone
thermal mass concrete   thermal mass brick   thermal mass stone
Solid walls and floors could be plastered or tiled as well.  

The experts at the Futurebuild workshop discussed the high carbon cost of materials and equipment as well as energy. Apparently, specifying up to 8 times the equipment needed is not uncommon. Neither is it uncommon to completely replace a whole unit instead of replacing just the part that failed. This all wastes carbon emissions from the supply chain.

They went on to explain that sourcing all the building materials and all the materials your equipment is made from, as well as all the transportation; and the end of life costs in removing and replacing anything all add up toward your Whole Life Carbon. A concrete office block completely gutted and refitted when a new tenant moves in and then demolished after only 10 years to be replaced by an even bigger concrete tower, is a terrible waste of carbon.

One of the expert's teams had to quantify the carbon in a new public building. They found that a lot of the timber was Canadian but it had been sold and moved to Southern USA before being purchased and transported to Britain. The total carbon was more than twice Scottish timber that only travelled 300 miles or so. In comparison, how far would your concrete travel? The aggregate, 77% by weight, might be quarried where they batch the concrete. If so usually a maximum of only 20 miles. The cement: a 15th of your concrete might have come from Port Talbot and another 15th from Greece. Another 15th is the water that only travelled as far as the truck. Overall, concrete is greener than many suppose.

Some of the best results the experts at the workshop got anywhere were concrete used where its useful life would be expected to exceed 150 years - which could be your house. The carbon investment in a concrete house becomes insignificant annually if it will be in use unchanged for 150 years.

Another cause of carbon waste the experts were keen to make clear was the effect on Whole Life Carbon designing a building and its services to fully meet the demands for energy during the coldest and the hottest times.

Your heat recovery supplier might well try to supply you with an average one air change per hour with ducting to every room. But will this be too much when everyone is out or asleep?   This sketch came up first page in Google: I think the experts would say this is over-engineering and over-selling.

The experts said that in order for every building in the land to get enough mains energy for air conditioning during the hottest few hours a year and full-bore heating during the coldest few hours a year, the extra national grid infrastructure and extra building equipment and services and energy certain to cope are twice what is required on the other 363 days a year.

They impressed upon us that if our buildings can only cope with 95% the peak demand during the coldest and hottest few hours a year, then the total Whole Life Carbon for everything required (mains supply infrastructure, equipment and energy) is halved.

And the way to smooth out the need for peak energy is thermal mass.

One of the experts said that his Victorian, London home, refurbished to maximise thermal mass, air tightness, insulation outside and heat recovery ventilation maxed at 25oC throughout the heat wave last year. A new build could do even better.

ICF basement house   Case Study. Begun 2012, updated with good news 2014, updated again with very bad news 2017.

In 2012 I built a basement and all the house walls with ICF.

These clients initially thought they had achieved zero energy bills for their new home with U Values around 0.20 rather than 0.10.

You can see a video I made in 2014, two years later, explaining this basement and its heating here.

These clients thought that their fuel bills would be zero because
  1. Photo Voltaics on the roof,
  2. Ground Source Heat Pump bringing heat in,
  3. Heat Recovery Ventilation saving some heat before it is lost outside,
  4. Air Source Heat Pump in the basement heating their water cheaply by extracting even more heat from the still fairly warm air on its way out,
  5. 150mm of ICF insulation above ground,
  6. Roof trusses stuffed with insulation and
  7. Excess electricity sold to the grid.
When I made the video in December 2014 the weather had been very cold already. They calculated that in that first month they spent just £26 on buying energy. The rest came from their GSHP. On an annual basis, after selling excess electricity in Summer, they thought their bills would be about zero.

But when I returned again in April 2017 to catch up on old times, their bills were £1,000 a year for energy.

Something had gone wrong. Actually, two things in particular seemed to have gone wrong.
  1. The GSHP had got expensive. It no longer cost less than £26 a month to heat their house. It was costing 10 times that because the ground had become too cold.

  2. With no thermal mass because of the ICF insulation inside, they got too hot in Summer and installed air conditioning.

The guys selling ICF, timber frames and SIPs are all trying to sell you a building method that wastes the opportunity to store energy in thermal mass - in concrete.

True, it takes less energy to warm the air in a cold house of lightweight construction. But why should the house have got cold?

True, it takes less energy to cool the air in a warm house of lightweight construction. But why should the house have got too warm?

True, all these choices are more air tight than brick and block cavity walls. These days, the mass housebuilders cover over all the holes in the masonry and insulation with airtight plasterboard and skirting board. The cold still gets in.

BUT NOTE: the ICF description only says "making it possible". Not that it will.

And NOTE: the timber frame description puts made from renewable woodland first. Not saving energy.

AND: the SIPS description only compares SIPs to "older technologies".

My conclusion is that ICF, timber frames and SIPs all perform poorly compared to a cleverly designed new house built with maximum thermal mass.

My money is on the future being concrete. That might be reinforced and poured in situ, prefabricated or concrete block walls covered in brick or stone.

I Googled "benefit of ICF / timber frame / SIPs". These came up first:

ICF basement

timber frame basement

SIPs basement

Until March 2019 I was excited by solar powered panels on the roof. But it was telling that at Futurebuild many heat pump, solar panel and Tesla Powerwall suppliers didn't show this year.

The problem is that solar power in Britain is at its maximum when demand is at its minimum. Until we can store electricity solar doesn't have much value.

This BBC correspondent explains quite clearly why domestic wind turbines are sadly a waste of money as well.

The question we are working towards is, should you insulate inside or outside a basement? I answer that lower down.

Much of what you have read so far was added in March 2019. Most of the following was first written after visiting the Homebuilding & Renovating Show 2018. Since then, Ground Source and Heat Source have got official thumbs down and the emphasis is less on cheaper energy and much more on less energy - not just the energy the home uses but also the energy used to build, manufacture and transport the equipment, maintain and eventually demolish the home.

During early 2019, Government was advised that gas should no longer be supplied to new homes (because of the huge collective carbon emiision) and heat pumps needed a water source (because water recently cooled by the pump moves away to quickly be replaced by warmer water), not ground or air source which don't work well enough in winter.

Heat Pumps.

The ground source heat pump and the air source heat pump guys have been trying to let you believe that the efficiency in Summer is so good that even in Winter you save money. But put on the spot many can't tell you that you would save anything on the coldest days when you need the most energy.

They are also sneaky because the savings they claim are against electricity whereas most homes are heated by cheaper gas and oil, so the actual savings for most are far less.

But if, at exhibitions, you ask how much does it all cost, it seems to me that those talking in the realms of £7,000 are the ones who struggle the most to convince you you will always save energy; while those talking about £20,000 are very confident you will save energy all the time, but they cannot promise any financial return on your investment.

However, saving money and saving energy aren't quite the same when it comes to renewables.

My customers who were, I think, disappointed with GSHPs seem to have saved money burying overlapping coils. Lots of pipe, not much soil to get heat from.

The expensive suppliers might tell you to have a straight pipe in a very deep borehole, say 150m deep, perhaps two boreholes.

GSHP basement GSHP basement

The expensive suppliers might be tapping into a far greater heat source than the cheaper guys.

I tend to think that the cheap guys use all the available heat in the ground quickly before the end of Winter; while the expensive guys have such a huge reservoir of energy deep enough to be warmed up by the centre of the earth that their installations succeed more often. But, compared to gas or a home designed to require less energy, the expensive guys will never break even on cost.

So why do people buy cheaper heat pumps?

Maybe because cheaper could be free over 7 years, because of RHI.

The Which? magazine has a guide. Note that RHI is aimed at those 'who are off the gas grid'. The reason why might be lower down this page.

Perhaps the Renewable Heat Incentive, RHI, will cover the whole cost of a small installation more easily than it will cover the whole cost of an expensive scheme; except that with a cheap scheme you probably still need some expensive, carbon-emission-rich energy from the grid and another heating system as well for the days a small heat pump cannot cope.

£20,000, I'm not saying that is what yours needs to cost, might only be partially offset by the RHI yet more likely to provide all your energy cheaply and with substantially reduced-carbon-emissions. But only cheaply if you ignore the huge investment and only lower greenhouse gas emissions if no refrigerant gets lost during servicing.

£4,000 and the RHI might cover the heat pump cost completely, but you might not get much free heat that way and you could still be buying expensive energy from the grid as well.

It seems to me, that ASHPs too small for the job and GSHPs without access to enough soil struggle in cold weather - especially if your home got cold when the temperature dropped suddenly outside and you want a lot of heat fast.

Whereas a larger heat pump heating a house that is well insulated and protected from a sudden drop in temperature outside might cope adequately at all times.

Another technology that caught my attention (in 2018) is Seasonal Thermal Energy Storage. STES.

Icax says on its website: "It is a characteristic of earth that heat only moves very slowly through it - as slowly as one metre a month."

Icax tells us what we hear all the time, that it is very expensive to store electricity. But, they say, it can be virtually free to store heat. What they seem to promote are two systems both with coils alongside each other buried in the ground.
  1. By Summer, solar energy is used to warm up the ground.

  2. By Winter, that same ground has the excess heat taken from it much more efficiently by GSHP because the soil is warm to begin with.
When I put this to heat pump suppliers at an exhibition, they doubted you could have enough surplus energy in summer, from solar panels, to put enough heat into the ground to provide for a whole Winter. Are Icax right? I don't know. There is an academic exploring a similar idea. A chartered engineer and chartered geologist, she is a research fellow whose blog describes her excitement at the idea. Two of her blog pages here: testing and monitoring.

Should we be storing heat in the soil beneath our basements?

We would need the heat to be very deep beneath our basements so that it didn't come out into our habitable space too early when we didn't want it to.

If you have already dug 3.5m down the only safe way to go further might be with piles or a borehole. If you hit water then any energy you tried to store would be washed away instead.

I'm not sure storing energy underground could be viable.

About 20 years ago, the Building Research Establishment, BRE, produced a paper about the U value of a basement without insulation.

The U Value of an average domestic basement, just because it is buried, is about 0.16 before you add any insulation.

The point, therefore, is that a basement neither needs much insulation nor much heating.

Unfortunately, you cannot include a figure for 'cave' in your SAP calculation, but you can include a figure for thermal mass.
ICF basement U Values ICF basement U Values ICF basement U Values

Finally, my recommendation. (Slightly amended March 2019).

I would
  • Maximise thermal mass: all the floors, all the walls and the flat roof all built with waterproof, always completely crack free, reinforced concrete.

                        - Decorating some walls in cork and soft furnishings will reduce echo.

  • Heat Recovery Ventilation.

  • Excellent air tightness. Windows, doors and service entries as well as the concrete structure.

  • Continuous insulation under the basement, all over the walls and over the roof outside the thermal mass preventing any cold bridging.

                        - Just the above, I am led to understand, is likely to exceed Passivhaus Standard, depending on your windows.

  • I might still cover the roof in solar panels, perhaps a mix of photo voltaic and water heating. Limited to what I could use or store. Best of all would be solar to charge up your electric car.

seasonal heating cooling envelope

I envisage heat recovery ventilation changing the air slowly during winter, except when the air gets particularly hot during cooking or washing when air changes per hour can be increased; and the air being changed slowly most of the time during summer but air changes per hour increased to maximum all night long on the 2 or 3 hottest nights of the year. This could all be automatically controlled.
Continuous insulation outside the thermal mass walls and floors. Shown here in yellow. Minimum SAP requirement under the basement.

A Chinese manufacturer, Himin, can make a roof entirely of solar panels. You needn't have a roof with panels fixed to it. Just one roof of panels. Other manufacturers make photo voltaic roof tiles.

I am making the assumption that solar panels might not be completely weatherproof so my waterproof roof is the flat roof above the bedrooms.

I have shown extra footings to provide a foundation for brickwork, shown in brown. A careful choice of wall tie will avoid thermal bridging.

I have shown this sketch to heat pump suppliers at exhibitions and the major positives were:

  1. The thermal mass would help hold the temperature inside constant. Your heating source should be set to put heat in at the actual desired temperature, and allowed to be thermostatically controlled 24/7/52. Always on.

  2. The thermal mass would not be shocked by a sudden temperature drop outside. Therefore, there should never be a sudden demand for heat.
You might only need a couple of electric radiators. 1 KW might be all you need to slowly bring the temperature back up to 20oC after it dropped to 19oC after the weather outside turned to freezing.

At other times, a rise in temperature from solar gain, cooking, heating and body heat is stored in the concrete until released during the night resulting in less heating required first thing in the morning.

You could vault your ceiling / loft; in which case you have to find a way to fix your solar panels and / or roof tiles without causing a thermal bridge and you need to bring your heat down to your actual living space.

Can a concrete house be zero carbon over its lifetime? Without generating energy on site from wind or solar probably not. But concrete can have the lowest carbon cost needing less equipment, less equipment changes and less energy than any other structural fabric.

The benefit of the concrete house is its air-tightness and thermal mass. Once up to temperature, you hardly need anything to keep it there because of cooking, washing and your own body heat. The savings are:
  1. Hardly requiring any energy.

  2. No need to invest in any expensive (and carbon-rich) GSHP, ASHP or biomass equipment for space heating.
You still have to choose your domestic hot water provision. The cheapest might be electric hot water heated partially by solar energy and brought up to temperature on demand by electricity. London is going to forbid immersion heaters soon. Storing hot water is wasteful because stored hot water soon goes cold.

The environmental benefit of concrete is not in how much renewable energy you get from an investment of tens of thousands of pounds, but how much less energy you use during its incredibly long life time than building any other way.

I am convinced that once built and lived in, my proposed home would require the least energy to keep it warm and cool compared to any other scheme, unless that scheme cost several times as much.

I have said you might not need GSHP, ASHP or biomass. These things are sold because salesmen explain how the RHI makes them virtually free. But you get more RHI the worse your Energy Performance Certificate rating. My proposed dwelling would probably have a very good EPC and not use enough energy to get mu RHI.

You cannot profitably sell excess solar energy to the grid when the sun is bright and warm. If you store solar energy you might soon be able to sell it profitably months later when the sun is dark. But much better would be storing solar energy and using it in your own car. Less emissions, less particulates, lower running costs, and so on.

You might not like any of my thinking. Fair enough. I'm not selling it, I'm only sharing it.

Phil Sacre
2018 except where obviously added or changed March 2019.

A little coup for me recently (2018).

A client mentioned that a good friend of his is a Professor of Environmental Engineering.

I asked whether this page reflected the professor's conclusions. I was told he "very much supports" my points.

Then, shortly after, I received this paragraph, presumably aimed at the client but with my web page in mind.

"Putting the concrete on the room side of the insulation (ie ground-insulation-concrete-room) it will retain heat (like a storage heater) and reduce the rate (speed) at which the overall room cools down and heats up. It will make no difference (over a season) how much heat you use to heat the basement (if its kept at a reasonably constant temperature). It will take longer to heat if you allow the basement to cool down so if you wanted to use the basement infrequently for short periods (eg as a spare bedroom) and then leave it empty (and cool) then it may not be the best solution (although that could be overcome by adding more insulation internally)."

It seems to be saying three things.
  1. Concrete inside, insulation outside: will help keep the accommodation constantly comfortable;
  2. It would cost no more to keep your basement at a comfortable temperature, if insulated as I suggest.
  3. But if you wanted a particular space to get cool and only warm it occasionally, you might choose insulation inside (whether timber frame, SIPs, ICF, concrete or masonry), so that it cooled fairly quickly and could be warmed temporarily with less energy.

* 21st September 2018. I just returned from the London Homebuilding & Renovating Show 2018.


One of the speakers was Tim Pullen. He consults, sells his book and writes for the HB&R magazine. His talk included "how to choose the right heating system"

I photographed a few of his slides and I will try to share with you some of his more interesting observations.

He warned that neither electric or LPG would figure in his choices because they are too expensive. That made me think about how GSHP and ASHP people talk about their systems giving you 3 or 4 times the energy back for the ELECTRICITY you put in to run them. And before you could boil an egg Mr Pullen made a similar point. Heat from gas is cheapest by far. Can heat from alternative energies compete with gas on price?

HB&R Tim Pullen 1.jpg

This slide tells us that a 200m² house insulated to meet only Building Regulations needs 11,000 kWh p.a.

Oil, cheaper than electricity would have cost, when this slide was produced, £750 to heat the space and £1,150 to heat Domestic Hot Water.

However, the same house to Passivhaus requires only 3,000 kWh p.a.

The purpose of these slides was, as you can see, to compel us to design a lower energy requirement into our proposed project before we choose a relevant heating system.

HB&R Tim Pullen 2.jpg

Here he compared different energies. Presumably mains electricity on a standard tariff would be in the region of £2066. And if GS or AS heat pumps were a third the cost they would be about £680 - which he says they are. But getting your investment back on GSHP or ASHP is only viable, it seems, compared to standard rate electricity. They aren't much cheaper than gas, or even oil.

HB&R Tim Pullen 3.jpg

I cannot remember exactly what Mr Pullen said about this slide. But, I believe he said that the RHI is no longer paid for solar.

HB&R Tim Pullen 4.jpg

Next I went to this stand

HB&R usethesun.jpg

It caught my eye because a poster said that a PV panel that heats water as well as generate electricity is more efficient. This man fully agreed. If a PV panel is cooled during hot weather it will generate more electricity. Even if the heat energy collected is wasted it makes more money.

So, I asked him, what should an ordinary couple in an average house with a roof facing East and West buy?

Without any hesitation he said this air source heat pump because the RHI and the cost of energy saved equal or slightly exceed the credit payments and after 7 years you stop paying for your credit, you stop receiving the RHI and you still get cheaper heat.

But if you go back 2 slides, I have gas so I would only save £100 or 12% or so a year.

They displayed a Tesla Powerwall 2 at £7,000. Their poster explained how solar power could charge it up, so could economy 7 mains electricity, and both would save compared to standard tariff electricity. You could charge your electric car from the Powerwall with economy 7 electricity downloaded the night before, and so on.

But the easier sell would seem to be the ASHP paid for by RHI. He didn't really seem interested in reducing fossil fuel.

(This is still what I wrote in 2018).

My recent copy of "The Economist" has an article saying that at mid-day during our Summers coal and nuclear power stations need to switch off, which puts up their costs, because solar panels on houses produce much of the very low level of power needed at those times.

So, it now seems more reasonable to me that electricity from the grid costs 14p but we can only sell it to the grid for 4p. When we want to sell electricity the grid doesn't need it as much as when we want to it buy from the grid in Winter at night. Simple supply and demand.

Indeed, if we have many more solar panels, then perhaps at very sunny moments panels will try to sell more to the grid than the nation is using at that moment. What happens then? Does stuff explode? Perhaps the grid will have to refuse to buy solar electricity and we will have to do something else with our excess power.

This brings me to the unanswered points in my graphic at the top of the page.

We should generate as much solar power as we can. But we should not be selling it. We should be storing it until we need it ourselves.

The next big emergency has to be air pollution and the particles from petrol engines and diesel engines but, I read, twice as much again from log burning and coal burning, which we choose for the theatre of a real fire in our living rooms. Fine particles from these three sources are now being blamed for causing stunted lung growth in our children and dementia in our elderly.

Twice as many damaging particulates are produced by fires as from traffic though, of course, in most areas fires aren't as concentrated as cars in cities, particularly at school gates dropping off and picking up the very children the particulates are harming most.

I have spent today looking into charging electric cars from solar.

I found a useful starter guide on Youtube by the Red Dwarf and Scrapheap Challenge star: Robert Llewellyn, here.

With all the kit: electric car, Tesla Powerwall 2 and solar panel photovoltaics - and the grid, you charge the car first with solar electricity, next with Tesla stored electricity, third with economy 7 electricity at night and, only if you must do you use standard tariff electricity. He says he hasn't used any standard tariff electricity despite having two electric cars to charge.

Economy 7 costs about 8p a unit whereas standard tariff might be 15p.

If the Tesla Powerwall wasn't fully charged by solar you can, or soon will, charge it with economy 7.

    I think that puts it all fairly simply.

    In the video, RL said that over previous weeks he had charged the car with 320 miles directly from his solar panels. My petrol car would cost £50 for that much petrol. He has also charged the car from the Tesla Powerwall that was itself charged from his solar panels. All that saved petrol has effectively gone into his pocket to repay his investment. Financially, it looks like you could save money over the lifetime of the kit.

    Air pollution and carbon emission-wise, with no petrol and no log burner he is saving our planet and our children while many of the rest of us are causing them both harm.

    It might be that over 10 years some of us would save money having a Tesla Powerwall and charging it at night with economy 7 electricity even if we did not have any solar panels. That would benefit the power generators because you would use your battery power during standard tariff periods, daytime, and the generators would have a more even demand over 24 hours. All your grid electricity could be 7p a unit cheaper. Your car could cost 10p a mile less to power.

    I found talk that in future we might be paid to use the battery storage we own. We might be paid to sell stored electricity to the grid, at the times the grid is trying to avoid firing up another power station, more than the electricity cost us the night before on economy 7.

    Everyone is different. As far as I can tell, back of an envelope savings might be:

  1. Not owning an electric car. Charging your Tesla Powerwall 2 half each with your own solar and economy 7 but only using 10kWh a day and not the 13.5kWh capacity of the batteries: Save £400 a year toward your investment of £10,000 or so.

  2. Driving 300 miles a week in your electric car (50 kWh) and charging it 50:50 with free solar power and economy 7: Save £1,700 a year on petrol and another £100 or more on domestic electricity.
The film says batteries came down in price 40% last year. If they do that again and electric cars don't cost too much more than petrol, it seems that the full kit described here will soon be economically viable as well as excellent for the health of our planet and children.

From DIRECTIVE 2010/31/EU

Article 9
Nearly zero-energy buildings

1. Member States shall ensure that:
(a) by 31 December 2020, all new buildings are nearly zero-energy buildings;

2. 'nearly zero-energy building' means a building that has a very high energy performance, as determined in accordance with Annex I. The nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or nearby;

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