TSHE: Time for a solar battery?

It’s definitely a pretty piece of kit, but is it worth the price tag? (credit)

This post is part of an ongoing series in The Solar House Experiment where I review and investigate different options available to drag my house into the 21st century and play with some cool technology along the way. Other posts in the series are included below:
The Solar House Experiment: An Overview
TSHE: Solar Panels + 1 year review

I finally did it. I broke down and started calculating out whether I could justify a solar battery or not for the house. I wish I could say it was due to boredom or having a long weekend of little to do, but it was actually because of this picture I took of the system output:

Yep, it’s early afternoon and both the electric car and hot water are maxed out. That’s 2.6 kWh of power leaving the house, every hour…

As we head into the summer months I have an increasing ‘problem’ that my solar panels are producing way more than I can consume during the day. And then at night we fire up the oven / TV / computers / lights and none of it was saved and I have to pay (more) to buy the electricity I sold to the grid (for way less). It just seems wasteful and inefficient to me!

Doing the maths

I usually work out how much something will cost and then work out the Return On Investment (ROI) in years but I’m going to do something a bit different today and work forwards instead. Currently we use about 1500 kWh per year in the house, excluding the hot water solar diverter (which only uses excess solar) and the electric car. Currently having the solar panels reduces our usage from the grid by about ~35% meaning we’re paying to use about 975 kWh of electricity for the house and (in non-Coronavirus times) I estimated the car would need about 2,500 kWh of electricity a year for my driving patterns. However the car is charged at a much cheaper rate due to the tariff I’m on (Octopus Go).

So in summary the maximum savings I could achieve in an ideal scenario where all solar energy is produced and consumed by myself (aka pay nothing for grid electricity) is:

Usage TypePower Usage (kWh)Cost Per kWhTotal
Table showing max potential savings with a solar battery (assumes all solar power generated is consumed instead of exported)

This solar battery would need to be able to store about 4 kWh a day on average (3000 kWh produced a year, minus 500 kWh consumed by the house already, minus 1000 kWh consumed by the hot water, and then divided by 365 days a year). Or to simplify:

(3000 kWh – 1500 kWh) / 365 days = 4 kWh/day

This would probably allow me to cover 90% of the house’s needs through the year… but would barely reduce my electric car charging usage. Hmm. We’ll come back to this.

I also decided that a 12 year ROI is not a bad thing for a solar battery. I think they will easily last up to 15 years and beyond but will of course degrade over time. As electricity bills have generally been increasing by 3% each year, the total I should be prepared to spend for a battery and receive a 12 year ROI that would cover only the house usage would be about £2,000 overall.

Checking out the options

After speaking with several companies and getting some quotes I narrowed my options down to three main choices (plus a wild card!). Spoiler alert: none of them meet my above requirements, but it’s an interesting investigation into the market right now.

Moixa BatteryTesla PowerwallEnphase BatterY
Product InfoLinkLinkLink
(2x 1.2)
Usable Capacity (kWh)3.8413.52.4
(2x 1.2)
Max Sustained Output (kWh)
Warranty (Years)10 / Lifetime1010
Notes12 months 0% payment plan available
Extra £50/year for signing up to GridShare program
Mountable inside or outsideModular design allows easy expansion at later date
(with installation)
Table highlighting the different options available I would consider

Straight out of the gate you can see that even the cheapest usable option, the 2x Enphase batteries, are too expensive and don’t have enough capacity for me to make a big enough saving to justify it. It also outputs a very paltry amount of power (500 Watts an hour).

At the other end, the Tesla Powerwall has more than enough capacity but is over 4x my 12 year ROI budget. It could power a kettle / oven / dishwasher too which is nice. The problem is that the ROI greatly exceeds the potential lifetime of the battery, even if I used it to charge the car as well as power the house. This is only for people who have money to burn.

The Moixa battery is actually the most interesting to me for two reasons. One, they offer a 12 month 0% finance option which would mean the maths on the ROI changes a little and I wouldn’t have to stump up all the cash straight away. Second, they offer an ‘unlimited’ warranty on the battery and £50 a year (for 3 years) if you sign up to their GridShare scheme which basically makes money for them by using a percentage of your battery to deal with high demand on the grid, when electricity costs the most. What annoys me is that 20% of the battery capacity is locked away to, I suspect, fulfil that lifetime warranty. If there was a 6 kWh usable capacity at that price point, I think I would take the dive.

The wildcard option

This is an 8 year old electric car with a 24 kWh battery available for around £5,300.

A 2012 Nissan Leaf on the AutoTrader website

This is a Vehicle to Grid (V2G) charger that works with the Leaf’s CHAdeMO connector that is currently being tested by OVO Energy in the wild.

The prototype V2G charger by OVO Energy

V2G is where you store power in your car which is connected to your house through a charging cable, like above. When the house needs energy, the car’s battery is used to supply that energy and no power is taken from the grid. In the future it is expected that more and more of the national grid in the UK will be managed in this way, helping reduce the need for coal / oil / gas power plants which are mainly used to deal with high load on the grid (such as when everyone gets home and starts cooking dinner).

Aside from space issues (I happen to have an empty garage I could utilise), there is nothing to stop me buying an old electric car like the one above, whose battery is likely to still have about 70-80% of it’s original capacity (let’s say ~17 kWh for that Leaf above), install a V2G charger when they become more wildly available and then plug it in and never touch the thing again. I could declare the car SORN (not legal to drive on UK roads) and it then literally becomes a battery on wheels with more capacity than a Tesla Powerwall at a cheaper price.

And I can easily sell the battery (car) on at a later date. A lot harder to sell a fixed installation battery on to someone else. I could even potentially take it with me to a new property if I decide to move!

But you have an electric car already? Use that?

I do, but alas Tesla have basically said they will not allow their cars to be used as V2G batteries. The fact they have a separate product (the Powerwall) to sell you is more likely to be the reason than the car is unable to do it, but I don’t know for sure.

Current V2G chargers only work with the CHAdeMO connector, whereas my Tesla Model 3 has a CSS connector and I don’t know of any in development right now. Maybe in the future…

In summary

I am optimistic that the costs for solar batteries will be coming down over the next 5 years or so. But right now I think they either offer too little capacity for the cost or they are simply way too expensive for your average user to ever really make their money back on them. I am still interested in the Moixa battery personally and will be keeping an eye on their future offerings, but right now? I’m going back to watching someone else make use of my excess solar power. Oh well.

3 thoughts on “TSHE: Time for a solar battery?

  1. Interesting stuff – thanks. We recently installed solar panels and an air source heat pump. Came to the same conclusion about the battery – not yet financially viable. I’m hoping the government incentivise batteries at some point but we may need a better government first.

    It’s frustrating exporting excess power for 5p per kWh only to buy it back later for about 14p per kWh!


    • Yeah, it’s so close for the lower capacity batteries – if the Moixa battery was 6 kWh usable capacity for £4,500 I’d probably jump on it.

      How much did you pay for your air sourced heat pump and installation if you don’t mind me asking? It’s not feasible to do in our house unfortunately but I’d be interested in how efficient they are compared to a 85% efficient gas central heating system. I think you can use them in reverse to air condition as well? Great combination with solar panels! 😀


      • The ASHP and hot water cylinder combined were £11k. We’ll get more than £9k of that back with the Renewable Heat Incentive – quarterly payments for seven years. Other than the cashflow challenge that poses, it puts the ASHP pretty much on a par with the cost of a new gas boiler. We use no gas at all now which is nice.

        The running cost is said to be similar to a gas boiler but we haven’t had it long enough to know. It has sat conpletely idle since sometime in March. The solar panels will of course reduce the running cost even in winter.

        I believe they can work as air conditioners. Ours doesn’t but we don’t have the need.

        Liked by 1 person

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