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Could new batteries take you off grid?

Posted by Gabby Mallett on 5 May 2015 at 11:05 am

For many years we have wondered about the problem of storing our PV generation.  In fact many customers install PV, thinking that they may be protecting themselves from future power cuts, without realising that the grid is required for their system to function.

A new development from Tesla Motors, the electric car manufacturers, may be about to solve that problem.  They have now developed a home battery, based on the ones used in their cars and claim it could revolutionise the ‘entire energy infrastructure of the world’. 

The new system, called Powerwall, is likely to be on sale in America this summer, so it will still be some time before these become common place in the UK.  With two sizes planned, a 7kWh and a 10kWh system and prices from around £2,000 there is much to look forward to, but these sizes are still very small compared to our current energy use which DECC calculated to be around 4,192 kWhs for the average UK household in 2013 (1).  However, with a decent PV system and sensible use of electricity, including staggering appliance use, they could be perfectly adequate for an average UK house.

See this BBC article for more about the product launch.

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5 comments - read them below or add one

NEF Gabby

NEF GabbyComment left on: 29 May 2015 at 8:14 am

It might be worth looking at the blog we did after this one which is about someone who actually uses a battery system.

Its called 'new lithium battery to store energy from solar PV panels' and was posted on 13th May.

This shows that this isn't theoretical anymore.  Some people are actually doing it already.


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GreenlandsComment left on: 28 May 2015 at 10:31 pm

The real question is not how much energy you need to store per year it is can I store enough energy during the day in a battery to last me overnight?  In my small house I find that during the month of May I was typically using 5- 7 Kw Hours each night during hours of darkness  - so an 8 Kw-hour battery system would be a minimal requirement.   During a reasonably bright day the solar panels generate enough power for most of our requirements - but we still draw some from the Grid - I expect this is when it is cloudy or if we run more appliances at the same time  - for example an electric cooker, kettle, dishwasher etc.  These can exceed the capacity of the solar panels and so draw power from the Grid.  If we had batteries backing up the solar panels I wonder if we would need a much larger inverter as batteries could deliver much greater surges than the panels can. 

At the moment it seems totally uneconomical to install £4000 plus worth of battery system in order to save 7 kw hours maximum per day

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RobertPalgraveComment left on: 7 May 2015 at 10:30 am

Francis - I agree with your comments. I am sceptical of an announcement like this from an elitist electric car manufacturer who is still making financial losses.

How does the battery system benefit a house with solar?

The average 4kWp solar panel array on a UK roof top will generate about 3.8MWh in a year.

About half of this is exported in most cases.

The usual household electricity consumption is around 4MWh per year unless they use electric heating.

So with solar panels a household would typically import (4-1.9) = 2.1 MWh per year. Cost of this at 15p per unit = £315pa

To save all of this £315pa I'd have to invest at least £2275 plus installation cost for a Tesla battery, and hope that the energy stored in the battery would always be enough to deal with my peaks in consumption. Possible in summer, but far less likely in winter.

I doubt if the payback would be much under 10 years at best. And how long do the batteries last?

For a non-solar house, the idea is that you would charge it up on a Economy 7 tariff and use that to get cheaper power during the day. Fine - but if millions of people actually did this it would re-balance the grid and power generation so that power retailers might revise their tariffs so that the night time prices creep up towards day time prices.

And I very much doubt that for £2k you're going to get a system which will run your house when there is a power cut. That would involve much more complex switching and safety arrangements. 'Standard' solar PV installations in the UK have to be configured so they shut down if the grid goes off. This is to avoid power being injected back into the grid when there may be a fault condition on which engineers are working.

If the objective is to reduce carbon emissions from power consumption it is  much better to be looking at grid-level storage to allow the grid to run with a  higher proportion of intermittent renewables like wind and solar. For example - more pumped storageliquid air and interconnections with Norway

There are not of course unlimited supplies of the raw materials used to make these batteries, and there definitely are environmental impacts in extracting Lithium by the millions of tonnes. see  from 2008 where car manufacturers commented:

Mitsubishi, which plans to release its own electric car soon, estimates that the demand for lithium will outstrip supply in less than 10 years unless new sources are found.
And they have ended up in Bolivia.
"The demand for lithium won't double but increase by five times," according to Eichi Maeyama Mitsubishi's general manager in La Paz.
"We will need more lithium sources - and 50% of the world's reserves of lithium exist in Bolivia, in the Salar de Uyuni," he adds, pointing out that without new production, the price of lithium will rise prohibitively.

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FrancisMcNComment left on: 7 May 2015 at 8:28 am

I suggest reading Chris Goodall's analysis at Carbon Commentary before getting too excited about the Tesla announcement.  The installed cost will higher than  £2000 - more like £4000.  No data on likely battery life is available but 10 years might be realistic and if that is right you would not get back what you spent in the first place.

I would suggest that it needs a break through in a different battery chemistry than Lithium based schemes.  The announcement in early April by Stanford University of an Aluminium/graphite system that is robust and very long lasting has much more promise and deserves watching for commercial development.

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Mand.TComment left on: 5 May 2015 at 4:29 pm

Yes its a product backed by a large multi national, but a few questions still remain.

1). The average house uses about 11.5 kw per day, i use about 18kw, and the average 4kw system generates about 20kw on a good day.

2). how many of these am i really gonna need as 1 10kwh unit doesn't seem to be enough. Surely we need a 100kwh solution atleast?

3) how does this product differ from what's already available to the UK market in the form of smart inverters/battery solutions; such as SMA's battery inverter combo, PowerRouter etc.


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