Battery storage. Should I or what?

battery-1-All I seem to be reading about at the moment is how battery storage will plug the gap from generation to consumption and make us all very rich or at least smug. I hear stories from neighbours with PV who are being targeted by battery storage sales people promising all sorts of claims. I was off last week and so decided to look into what this could mean to me if I went over to the storage side. At commercial, gird and such like there are other aspects such as dealing with power interruption, grid shortages, peak power, selling at peak times and such like. For Mr Jones its a mater of economics. Will it pay for itself in a reasonable time frame or should I keep looking at what to do with the excess power in summer when I don’t need it for heat or evening electricity and so on. I have a 4kw PV system in the back garden and then I realised I did not understand my daily, monthly and seasonal use of energy or my energy generation characteristics. I then spent a couple of evenings modelling, measuring and understanding what I make in PV energy and how much I need and when do I need it?

It got a bit silly and using a plug in monitor i went through my electrical use appliance by appliance and then looked at when i use it. My single heaviest energy use is the Sunday  lunch in the ovens. I don't produce enough energy at any time of the year for this meal!

It got a bit silly and I had to make informed guesses on some of it. Using a plug-in monitor I went through my electrical use appliance by appliance and then looked at when I use it. I did find that a washing machine being off and off in the wall there is a 4 Watt difference. My smoke detectors use a lot of energy and i need to look at more efficient ones.  My single heaviest energy use is the Sunday lunch in the ovens. I don’t produce enough energy at any time of the year for this meal!

But what does a battery do and how does it behave? What’s the difference between Lead Acid and the becoming cheaper Lithium battery. The difference seems to be efficiency, cost and the availability of energy. My main lesson has been the last bit. If you buy a 6.4 kwh Tesla Powerwall with all its hype and nice packaging. You don’t actually get 6.4 kwh but more like 75% of this because a battery never truly empties. A lead acid is more like 50% of the capacity is available. On the Powerwall the 6.4 kwh actually means 4.7 kwh in available energy. You also loose 7.5% in efficiency in terms of what you put in vs what you get out (Tesla efficiency rating, I will leave that out for now)

I then moddeled my energy generation and matched it to my consumption patern to see what the macth was like. I generate a lot in summer when i least need it and simple moving of more appliances to the daytime use through the use of time clocks or delayed start would sort out better match

I then modeled my energy generation and matched it to my consumption pattern to see what the match was like. I generate a lot in summer when I least need it (not rocket science) and simple moving of more appliances to the daytime use through the use of time clocks or delayed start would sort out better match. Energy generation on the left and energy need on the right

the comparison. what am i making and when through the PV and matching against how much is avaialble for storage, what can the battery hold or should i say release vs how much energy do i need in winter and summer

The comparison. How much energy am I making and when through the PV and matching against how much is available for storage, what can the battery hold or should I say release vs how much energy do I need in winter and summer nights

The results of all of this data crunching when tested against my initial objective which was economics is an estimated saving of £110 per annum (for credibility lets allow another 20% variance and push the saving to £132 pa.) This has to be matched against energy inflation (cost in theory goes up over time) but then balanced against the degradation of the battery over thousands of cycles as it gets older and wears out. Tesla Powerwall (the reason I have chosen the Powerwall for the comparison is that it seems competitively priced) seems to be costing around £2000 to buy plus let’s be generous and allow £500 for installation (one day, plus brackets, some cabling and such like). This presents me with a 19 year pay back. According to the BRE Batteries and Solar Power guidance the expected life is around 10 years ‘but is improving all the time’ The Powerwall would also need to be next to the PV for DC charging before the inverter (or a new trench for the 240ft of cable to and from the house to the inverter). Outside is ok since it has been designed to be inside or outside

Overall and for my situation it’s not quite (quite a bit) there yet. I am though looking to change my heating and hot water system and this may be a better option but as I said earlier, its sunny when I need the least amount of energy! Electric car? But I have no doubt the price is still reducing for storage and I need to look at better energy management in the house. I have a feeling its time will come. But not yet for me!

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13 Responses to Battery storage. Should I or what?

  1. Simon says:

    Great write up! Thanks for looking st this topic in so much depth. Do you reckon they’re worth considering for those of us who still suffer regular outages?

    • Keith Jones says:

      I am doing the very thing as an uninterrupted power supply arrangement. Depends how critical the energy is to your life but worth a cost estimate and see how you can adapt to fit it. Eg lighting, computer and boiler pump/controller for example

  2. Nick Allen says:

    Good article and that fits is the general opinion.
    One for early adopters etc.

    Battery prices are coming down around 12% pa so I would guess the economics would work in the UK in around 5 years.

    Thats not the case in other countries like Germany where they make sense today given high prices and Californa which has high peak time pricing due to A/C use in the midday.

  3. Me says:

    Actually, the Tesla or any lithium battery can be discharged 100% of their rated capacity. So the Tesla Powerwall really does provides 6.4KWh of energy. However lead acid batteries shouldn’t be discharged too much to prolong life of the battery. This specification is called ‘Depth of Discharge’. The powerwall is also warrantied for a specific amount of energy thru their life (decreases with age), but lithium batteries seem to be degrading less anticipated.

    • Keith Jones says:

      Thanks for this. As I hinted battery storage is new to me. Having read a bit more following your note the energy in vs energy out is around 87% efficient because of the battery efficiency and then the inverter efficiency. (I have a very good SMA inverter) the payback now looks around 17 years. But as I said this was for my situation. There are situations especially for off grid, ups and just generally wanting to be self sufficient where it’s worth doing. For me I need to focus on lowering use and shifting my use patterns and letting the price come down further after the first movers.

  4. Neil Lewis says:

    This is great “real-world” data Keith. The devil etc. I attended a Wattstor installers course recently. Customers approach for Li-ion and always end up with Pb acid. Lithium is £1000 per kW more expensive! Pb acid weighs 70kG so won’t work in the loft. You can recoup 30% cost at end of use with Lead. Worryingly Li-ion ends up in landfill. A nice option may be Sodium-ion-as it is non-toxic and recyclable. Personally, I am waiting for the 30kWh battery sitting on my drive to be usable as domestic and grid storage. This technology is currently in development and may only be 18 months away? Avoids duplication of non-recyclable resources.

  5. Nice article. Most of the growth in battery storage in the next few years will either be commercial scale, or aggregations of domestic. So I think you’re bang on with your analysis of your own position, but there are many interesting business models taking advantage of National Grid revenue streams (rapid power response, enhanced frequency response etc).

    As others have noted, the costs are plummeting, so domestic energy storage may be commonplace within a decade or so.

  6. andy@exeter says:

    Thank you Keith – its always good to see another man’s numbers!

    I recently did some analysis of my own, based on a theoretical 4 kWp installation and a 6.4 kWh storage unit. It may have been based assumptions and generalisations but I came up with similar conclusions. My ‘system’ without storage or any form of load shifting exported about 61 % of its energy to the grid (~2,600 kWh). With the battery exports were reduced to 23% (~970 kWh) and 38% (~1,600 kWh) was stored and consumed later.

    This sounds promising but the additional value of the 1,600 kWh at 15 p/kWh is about £240. The (installed) price of the storage system I was looking at, which included switchgear, was about £6,000 which would give simple payback in about 25 years. I think off-gridders will even budget to replace batteries after 8 years, depending on type, use etc. At this stage of the game there may well be more to be gained by switching stuff off and/or using simple timers to run our ovens and washing machines in the middle of the day. Casserole anyone?

    At the commercial scale offers are already being made to landowners to buy long term leases on ‘energy barns’ with capacity of up to 10 MW. With the drop in FITs I assume these are speculating on the next rounds of the Capacity Market auctions, although the Institute for Public Policy Research seems to think that government policy has currently ‘incapacitated’ this market (see

  7. Huw says:

    Have you considered your electric car as a battery source? Charge it up during the evening when electricity is cheaper, and then discharge to the grid during the day when prices are high?

  8. Alsbury, Sarah says:

    Hi, Keith – thanks for this – I’ve just asked the same question.

    And I guess the economics would be even less attractive for workplaces, where the energy is more likely to be used on site during the day anyway…


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