Solar / Battery Storage considerations

Continued to agonise about what size battery to fit to the solar system I’m having installed.

5.8kWh or 11.6.

Seemingly I’ll export 20% of the electricity and only use around 5% to charge the battery.

To my (simplistic) way of thinking, I export at around 5p per kWh and will be buying for around
30p. Most of the year the panels won’t be supplying enough electricity to feed the house and the 5.6kWh battery won’t run the house for much more than a single night (I assume it offers less than 5kWh of usable power).

So for each kWh of ‘consumption’ I can get from solar saved to the battery rather than exported, (reducing a purchased kWh) I save 25p.

If I assume that the additional 5.8kWh of storage gives me a usable additional 4kWh per day then that’s £1 a day saved. So ignoring the lost returns the £2,500 (or so) of investment would pay back in around 7 years- I think… Of course this assumed that every day there’s enough excess solar to fully charge the battery which is an error.

If I move to a suitable tariff I can then also charge the batteries off-peak at a greatly reduced rate and consume on-peak on days that the panels don’t produce enough (Octopus Go allows purchase from 00:30-4:00 at 7.5p per unit currently; potentially saving ‘close to’ the pound a day mentioned above).

I think I’m likely to go for the larger battery.

I suppose I could wait until the smaller battery is installed and see how it works out, but if I follow that route then I need to pay VAT at 20% on the second battery rather than at 5% when it’s installed as part of the system.


Will hopefully be able to respond when I have more time, but just to check, £2,500 for a 5kWh battery? Are you sure? Which make, as usual it’s about £1000 per kWh.
Also what is you solar panel max energy production?
Also do you have an immersion heater?

In answer:

It’s a Solax unit (Triple Power 5.8kWh) , and yes it’s around 2.5k. I’ve seen it a little cheaper from other suppliers. To be exact the ‘delta’ between 5.8 and 11.6 is about £2,500.

Solar panel max is 5kW but there is a degree of shade in the late afternoon/early evening (mounted in a field).

Yes, have an immersion heater which I’d planned to (somehow) wire in to take excess power when available.

Modern panels convert almost 22% of the incident energy into electrical energy. My weather station gives me some idea of the incident energy. You need more than 7kWp of panels to begin to make batteries pay. It then depends on being able to use the stored energy before the sun shines again. On a sunny summer day you will still be selling silly amounts of energy to the grid.

Tesla PowerWall is hard to beat but 12-15month wait is the norm. Solar Edge are getting there with the features for their Hub Inverter.


I think that there might be a case for buying off-peak electricity and using it at peak times using the battery.

As long as something like Octopus Go is available where you can buy at 7.5p (00:30-4:00) rather than (something like) 30p there’s a differential of around 20p per unit available. Looking at the data I’ve been accumulating using my (non smart) meter I seem to burn through something around 20-30 units a day. If I could offset just 8 of those using batteries that would ‘save’ around £1.60 a day. ish.

Maybe close to ten years to pay back… assuming I keep sufficient battery capacity for ten years.
And some addition from the solar generation should make this look better.

Sounds like I’m trying desperately to talk myself into doing it really…

Octopus Go requires you have EV charging and proof of EV ownership probably.

It’s very important to make sure the whole system is compatible with battery charging from the grid. If one had a Nissan Leaf then Vehicle to Grid (VTG) might make sense instead a battery. However, it is still experimental and is not widely supported by EV manufacturers.



I have a Nissan Leaf so clear on that hurdle!

Isn’t VTG all about selling back to the grid (with the car battery acting as temporary storage effectively) rather than using the car to power the house?

Functionally, I can’t see any difference as long as the House Energy Management is in control of what happens. If you have a battery with the Solar you would want to be able to charge from the grid at the most advantageous rate for the days when the solar is low just like charging the EV. The Tesla site is one of the best I have seen explaining what they can do. I must check again.


So if you have a EV, why don’t you use the batteries to power the house ( Vehicle-to-home V2H.). That will give you much more capacity, and without the cost of house batteries

I’ve had a look and can’t really see any real use of V2H in the UK. All I see are some V2G trials.

I guess it would make sense if possible, but I think it’s more aspirational technology at the moment than operational. What I’ve read about V2H is quite smart though, certainly one to keep an eye on.

I’ve been looking into the feasibility of battery + solar/wind, so probably not as far ahead as you - very much at the research stage. Things that I think I have learned:

  1. The battery isn’t covered by any schemes in the same was a solar panels are.
  2. The battery lasts about 10 years, so that’s how long is got to recover the costs from.
  3. A battery also provides other (potential) benefits - the good ones allow you to quickly isolate your system from the mains to enable you to run off grid in the event of a power cut.

So, the value of it comes down to a bet on what you think will happen over the next 10 years to energy supply.

What I think will happen:

  1. Surge pricing/load shifting will be very important. We’re well behind on nuclear capacity, solar and wind don’t happen when demand is greatest, so you have to switch demand to when you have the solar and wind energy. On the demand side, this can happen through lifestyle choices (running washing machine at odd times of the day, for example), or via a battery - charge the battery when power is clean/cheap, use whenever you want it. On the supply side, load shifting is done through surge pricing - charging different rates at 30 minute intervals. Octpus Agile is the pioneer of this - really needs Internet of things (smart devices), as well as smart meter doing 30 minute reading/billing. Before the current crisis, there were periods when energy was negatively priced - so you got paid for using it. This is where a battery really comes in to its own - you charge when it’s negatively priced, use it when it’s expensive. This year, we’re seeing an evolution of this - there’s a pilot scheme with Octopus Agile, where, if you agree to a capped usage during PEAK periods, you get that for free. Again, ideal with a battery - use your allowance for charging, then discharge to avoid costs. So there could be periods where it’s negatively priced, or free replacing when it is most expensive.

  2. The grid supply is likely to be less resilient. There’s a massive difference in running a grid where you can switch on generation when you need it - to match supply to demand, to trying to run a system where supply and demand are not connected. The grid tends to break when things get out of alignment (voltage drops or gets too high etc) - the massive loss of power across the east of England a couple of years ago was an example of this. As we start getting more storms, we’ll get more periods of outage at the local end with exposed power lines being hit by trees etc. (Storm Arwen was a good example of this). So a battery can help.

  3. Gas prices will never come down again to anything like the levels of last year (3p a unit). Hopefully, electricity will (at least, relatively, as Wind/Solar starts to get cheaper per unit as we deliver scale). I don’t fully understand this, but the way the electricy generation market works is a bit odd. My crude understanding is, at the moment, If electricity generated via gas becomes very high, then all providers get paid the same price, so even the cheap stuff is dragged up. Which is why electricty has gone up at a rate similar to the increase in gas prices - where you’d expect it to be a lot less as gas is only 40% of generation these days. I expect the market to be reformed to break this relationship - so that the consumers don’t end up being vicitimised by costly gas increasing the prices of other sources. Wind energy is cheap to generate, and the costs are, sort of, in building it. Except you don’t pay to build a wind farm, you pay the interest on a loan to borrow the money to build a wind farm. The loan tends to come from the government, and that has an odd way of recouping the costs. Your wind farm gets paid, sort of, a fixed price. If cost of generation is lower than that, your wind farm makes a profit. But if the price of electricity is much higher than this fixed price, the government gets to keep that. Which isn’t much of an incentive to bring prices down. This has to be reformed. So, I’m betting that electricity prices might drop so that we can get back to tariffs like Octopus Agile having negative/zero cost/low cost again.

So, whether you get a battery and solar is about making a bet over what happens over the next 10 years (for the batter) and long for the panels. It’s not about looking at current prices. There are also non-monetary aspects. Switching from an ICE on the car to a charge at home EV is fine if there are no interruptions to the grid. ICE is currently independent - your car still works during a power cut. So there are lots of personal resilience situations that might mean the value of the battery is more than a simple time to repay/breakeven.

Personally, during the crisis, there are no “agile” tariffs. I also have an east/west aligned roof which isn’t good for solar, so my plan is to spend money on getting the house more thermally efficient and keep gas for a bit longer. Then next year look at a battery and solar (and possibly wind - I’m high up and very exposed so a small wind turbine might be practical). I’ll be accounting for the solar battery as an exercising in reducing gas dependency - so I’ll do the maths using that as a basis. Which probably means you could pay off the battery in a couple of years… Obviously, you have to have the cash to afford the battery/panels in the first place, depending on what grants/schemes/loans are available.


I wouldn’t disagree with much of what you said but would comment on the battery replacement.

The things that will degrade in the battery are the cells themselves, gradually losing capacity and these are replaceable so the cost of ‘rejuvenating’ a battery whose cells have lost capacity will likely be significantly less in ten (or so) years time than the cost of the whole assembly. (I used to be a little involved around lithium ion batteries for phones and the pricing of those were slowly going down even after a lot of manufacturing capacity was lost to EV use).

My back of an envelope guesses would have the battery paying for itself in around 8-10 years but that’s based on today’s electricity pricing and a tariff like Octopus Go. Should the cheaper overnight tariff go, or electricity pricing reduce then it probably won’t pay for itself.

1 Like

You have done some good investigations there. Just to add that a battery back system doesn’t necessarily work in a power-cut scenario. I think the Tesla does now, but may require an adaptor. So worth checking before assuming.

Yes it does seem to be slow off the ground, but I expect the electricity companies would be quite keen, while Car companies probably aren’t as keen for the batteries to be over used, and therefore not able to keep their warranty commitments.

Found this statement
Only a few EVs currently feature V2H technology, including the later model Nissan Leaf and Mitsubishi Outlander PHEV, which both use the older-style CHAdeMO connector.

From the link below:

Basically you need a Bidirectional Charger, and an EVs with bidirectional charging capability.

This is just me thinking out loud.

For the safety of staff working on the network, operating off grid requires disconnection from the grid. Equally if no off grid support is provided, the local inverter must shut down when the grid off.

Normally the inverter monitors grid voltage because only if the inverter voltage stays at the grid voltage will no energy be drawn from the grid through the AC cycle. To export to the grid the inverter voltage must exceed the grid voltage. I don’t have a clue how all this is done.

I have heard it claimed that inverters can limit the energy they support to the 3.6kW that does not require approval from your local distribution network operator.

There is a need for regulatory approval assuming the grid is willing to even allow off grid operating.


probably safer to bring out the candles, like we did in the 70’s

Hm… but they must be LED, rechargeable candles - we don’t want more fires

VW Buzz is supposed to support it.

Vehicle to home and grid does not exist. Yet - I agree strongly with the view that this is going to change but who knows how quickly. Currently there are trials involving Nissan leafs but you can buy a bi-directional charger. For £6500! Widespread use is years away

Kia EV6 and Hyundai ioniq5 have (well some have as standard, not base models) vehicle to load which is not the same as to home or grid - you can attach a single item with a regular home plug to the car with the provided adaptor

I am getting quotes for battery and solar at the moment - we have a share in a wind turbine which means that adding solar should mean batteries will pay faster for me than most. Am hoping solar from spring to summer and wind in rest of year will combine quite well if not perfectly

Also agree with the comment above that it’s a guess what will happen with electricity prices - so payback on solar and batteries is not really known. The quote I have received with projections assumes a 50% rise later in the year and then price going up at 9% pa - which I don’t believe and won’t count on and gives predicted 8 year payback - although that is without factoring in better use of wind generation. But the pylontech batteries have 6000x discharge warranty which would be around 16-17 years at once a day, so won’t be dead after 10 years. NB if solar only there will be winter days where there is no charge/discharge cycle as the batteries won’t do anything much - solar gen will just be used in the house. Unless you use an overnight tariff to buy cheap

Our electricity usage is pretty big at over 10,000kWh pa excluding the EV and phev (an extra 4000-5000 pa). ASHP is around 4000-4500 pa, plus range oven around 5000 pa plus general household using up whatever we manage to grab from the wind turbine which may be another 3000-4000 but isn’t measured.

The peakiness of wind is much worse than solar - eg, in feb half term in the storms our turbine was generating 140-150 kWh per day. If you were going to try to store that for one day you would need 11-12 Tesla power walls at a cost of around £100k. So I am unconvinced that onshore wind generation is the answer to all the country’s electricity generation problems. In a cold snap there can be days if not weeks with zero wind generation


@TimOopNorth , Out of interest,

  1. what heat pump do you have?
  2. what is the maximum water temperature circulating to the rads?
  3. what oversize factor are the rads?
  4. what SCOP does your system achieve?

Many thanks


Interesting Tim…I have been looking at solar/battery storage, we have similar usage, circa 15K p.a. + 3K (Hybrid).

Even using my solar powered calculator I cannot see the economic case for progressing on this…considering cost of capital/opportunity loss, depreciation, life-span & battery degradation, electricity costs (projected), minimal stored power hrs (in case of grid outage), etc, the payback period at best is either very extended or likely never.
Looks to me like a poor investment and not something I would do - am I missing something ?

1 Like

Shame the Chancellor seems to have excluded batteries from the ‘VAT reduction scheme’ announced today.

I think I’m sticking with the smaller battery. The larger one only makes sense if I can offset consumption using Octopus’s tariff and there’s no guarantee that continues.

Of course, if after a couple of month’s of fiddling around with the system I change my mind due to summer ‘own generated’ storage or winter ‘consumption offset’ I can always have the larger battery retro-fitted albeit at a price and VAT premium.