Estimating battery savings from solar PV data and domestic usage

I’d like to try and work out whether it makes financial sense to install a battery. We have solar PV already installed (generating ~4400 Khw / year), and an air source heat pump, and our annual electricity consumption is ~9000Kwh (including onsite solar electricity usage).
I’ve got an EmonPi and EmonTX measuring solar generation, electricity import and export, with around 6 months of data.
The online calculators for working out battery return on investment don’t seem very realistic. I’m not quite sure how to go about it, but I’m sure it would be possible to look at real historic electricty usage data and solar PV, and calculate what we would have saved had we had a battery installed.
Has anyone done this, or have any tips on how I would go about it?

I think I decided it was too complicated!

What we did do was to look at our typical overnight consumption which we found was generally 8-12kWh (we also use about 9.5mWh, but generate about 9 and use about 4.5).

On pretty much any sunny day we would also have exported at least that much so then we looked at how much for a battery of that kind of capacity. About a year ago the best I found was ~£8k for a Tesla which I think was about 13kWh?

We did some sums on whether it would pay back or not and at the time decided it was about break even so were going to go ahead because it was green and found a company who would do us both a Heat Pump and Tesla so happy days. Unfortunately it seemed they had some commercial business that was worth more to them than us and luckily we decided not to go with them before we paid the deposit.

We learned 2 valuable things from this company:

  1. it wasn’t worth paying the extra for islanding to be able to use the battery in a power cut.
  2. the battery is limited in how much energy it can give back by it’s inverter, typically 3.68kW, so don’t think you can not import any electricity at all as cooking an evening meal creates peaks the battery can’t handle. So does the heat pump!

I had been looking into getting the Victron kit with Pylontech batteries, googled for something like “cheap battery storage” and found a brilliant offer from EDF for an 8kWh Powervault battery for £4k, half price! It was a no-brainer and we went ahead pretty much straight away. The offer ended ages ago unfortunately.

The battery went live at the end of September last year and I remember thinking we hadn’t had much luck with the weather right through from then until we had some sunny days earlier this year. We were charging it on Octopus Go overnight for 5p/kWh so about 40p and saving about £1.20 so net saving from that during those dreary months of 80p/day plus some more, but not really much, from what the Solar put into the battery.

Since the weather has been better and the days longer, it’s been amazing, the battery has still had maybe 25% charge at 00:30 so we stop it discharging and put maybe a couple of kWh into it so it gets through until the sun comes up and has enough to let the heatpump run with minimal import.

Each day from April 17-21 we imported 2.3kWh of cheap rate elec and 0.5-0.9kWh of daytime, that’s 87p for the 4 days.

Dunno if this helps, I hope so!

thanks Christian. That is helpful.
I’ve already got stuck trying to download our energy data from the EmonCMS in the right format. I tried downloading feed data but the file sizes are too big and it throws error messages. If i use larger time intervals (e.g. 30 mins) it doesn’t look like it is suming the data, just showing the individual value at each interval and ignoring all the data point in between.

On batteries, I signed up to the county wide group purchasing deal (Solar Together Kent), and the deal they offer is to fit a 3.6 kwh AC coupled inverter with 4.2 khw batteries for £3600 or 7.2 kwh batteries for £4600. Seems like a reasonable deal.

That reminds me, we were first signed up with Solar Together via Tonbridge & Malling Borough Council. Their surveyor clearly didn’t have experience adding batteries to existing PV setups and said they couldn’t do it; silly man!
I also remember I found that I could buy the same stuff (Growatt) from EBay installed for less but of course that contract wouldn’t have the potential backup of the council deal.

Can I suggest you do a daily bar graph of your Import for say a year?
In Graphs choose Year right at the top then below the chart set frequency to daily, below that in Feeds In View, choose bars, fill and delta then press reload.

If the graph doesn’t show you enough then copy the CSV data from the bottom of the page and pop it into excel then do something to get the various types of average.

I think you’ll want the 7.2kWh though! Much cheaper per kWh and you will use it.

After a bit more investigating, the offer from Solar Together is installation from Senergy Direct, who have mixed reviews.
Their offer is for a LuxPower 3.6 kWh AC Coupled Inverter (LXP series) with Pylontech batteries (the US2000 2.4 kwh ones). Those batteries can be bought for £882 incl VAT each, but i can’t find how much that particular inverter sells for. It seems similar 3.0 or 3.6 kwh AC inverters cost around £800 - £1200 incl VAT, so total cost for 2 batteries and inverter would be (£882 x 2 + £1200) £2964, plus any sundries (battery cabinet, cables, etc.), plus labour, so Senergy Direct’s offer of £3600 incl VAT for 4.8 kwh of batteries seems ok.
My plan is to order the 4.8 kwh, then add in 1 or 2 more pylontech batteries myself to give a total storage of 7.2 or 9.6 kwh.
If i had more time on my hands or knew what i was doing I’d try and do the whole installation myself. We have cables in place already running from consumer unit to utility room.

Thanks for the graphing setup but it didn’t seem to work for me. As soon as i set ‘type’ to daily it reports very low values for the daily bars of import or use (e.g. only a few hundred watts per day on most days and then occasional jumps to 2-3 kw) but our daily usage is much higher 10 - 20 kwh. it looks like when i set the interval to daily, it is just reporting a single value for the feed at midnight, rather than sum of the whole 24 h period.

Maybe you are using the import feed, rather than import_kwh (the Power to kWh feed)?

yep, you’re right! I was using the wrong feed, should have used the import_kwh

so I’ve downloaded daily import and export kwh. I only have data from 18th Dec 2021 to today (0.37 of a year) as i had a problem with my EmonPi last year and lost all the data.
I ran some calculations in Excel, to calculate the potential savings per day with either a 4.8, 7.2, or 9.6 kwh battery, with 85% DOD. The calculations workout how much would be saved by filling the battery to capacity from daily exported solar generation only. I didn’t correct for round trip efficiency.
What i found is that from 18th Dec to today we would have saved 306.64 kwh with a 4.8 battery, 413.27 kwh with a 7.2, and 480.56 kwh with a 9.6 kwh battery (all with 85% DOD).
If i extrapolate from the data i have to cover a whole year, and even at energy prices at ~£0.3/kwh i don’t think batteries are worth it in our case. It is almost worth it at £0.4 per kwh.

During the winter months we can use 20 - 40 kwh per day, so even using the Octopus Go or similar market rate tarrifs, I don’t think batteries would help because we wouldn’t be able to store enough during the cheap periods and would get stung during the expensive periods.

It’s been a very useful exercise, and now i know how to do it, I’ll re-run it in 6 months to see if things have changed. I’d be happy to share my data and calculations if its of interest to you.

You’re probably right although if you’re using your export figures from a fairly dreary winter, you are probably not filling your battery every day which you probably will for the next 5-6 months.

Before price increases, our annual electricity bill was £450-£600, the battery doesn’t make that go away so we knew that for a £4k spend we would probably not make our money back in 10 years. We also added the heat pump so our usage has increased dramatically.

It’s really nice though, going to bed and the battery is still working, knowing that we’ve drawn teeny amounts from the grid.

I know where you’re coming from. I’d also really like to be able to use more the electricity we generate, and during the summer we’d essentially not import anything if we did have batteries.
The problem is that our highest consumption period is during winter (average 36 kwh/day in Dec and Jan) with the lowest solar generation (average 4.5 kwh/day). And because we are working from home and we have an ASHP our solar generation usage during Dec - May averages 70%.
I haven’t completely ruled getting a battery. I haven’t properly looked into the market rate tarrifs yet. If we could charge the battery overnight at a fraction of normal fixed rates, it might make more economic sense.

I think Octopus Go is 6.5p/kWh for 4 hours 00:30-04:30 now but you have to have a smart meter.

I installed a pulse counter from Openenergy two years ago now to look at our usage.
Using an ESP32 it records to an onsite mySQL database.

We used this info to work out average background energy usage and we seem to use 300w a hour day or night. Approx

The solar system I installed was a 1.28KW and we use as much as we can during the day as we are at home. i.e. washing machine/dishwasher on during height of solar production not in the evenings any more.

We are not getting paid for excess energy as its a DIY install. So everything above 5 watts a minute is going back to the grid (unless an extra load is on washing machine etc.). So I have ordered a Victron Multiplus II and a Pylontech battery. As its a small investment that can be expanded.

But I agree its not easy to work out the cost savings so just I am basing it on every time its sunny I am losing 75% production to the grid. As you can see in the attached image.

hi adam,
I think our average baseload is around 250 - 400 w/hr too. However, we have an ASHP for all our heating and hot water, so when that kicks in it can ramp up to near 3Kwh. In the winter we can use >40 kwh per day (the majority being used by the ASHP).

I’ve also been looking at the vicron multiplus II (GX version) with pylontech batteries as it seems a good combo and expandable. I’m interested to know how difficult it is to DIY retrofit this. I’m reasonably handy with these things but don’t want to mess about with electrical stuff unless I’m sure I know what I’m doing, and I’ve never done anything as big as installing an inverter.

How much did the victron and sundries set you back?

So it is and has been more complicated to install the Inverter than the solar panels were to do.
I do have an electrical background but am not a rated electrcian. The hardest bit was to install the energy meter ET112 in the consumer unit. But it is not fully installed yet, still waiting for the battery.

I paid just approx £2,800 from Voltconsolar
Thats the
Victron MultiPlus-II 48/3000/35-32
Energy Meter ET112 Single Phase 100A Victron ESS
Pylontech US3000C 3.55kWh Lithium Battery 48V | 95% D.O.D
Pylontech Pair of Long Battery Cables 2meters
Victron Cerbo GX
Current Sensor Transformer for Multiplus-II

But it does require cables which voltconsolar dont do which I found weird. So I used Battery mega store for those.

cerbo gx to Pylontech BMS
Victron Energy VE.Can to CAN-bus BMS type A Cable 1.8
cerbo gx to Et112
Victron Energy RS485 to USB interface cable 1.8m
for ve.bus connection to cerbo gx and mutiplus ii
Victron Energy RJ45 UTP Cable 0.9m

Prices and availability have changed since I ordered mine. I.e. higher prices and no availability.

Hope that helps.

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Just got my Emon Pi and trying to work out how to connect it all into the system. However, in response to you main question (Does it make financial sense to install a battery?) my response would be an unequivocal YES. The rider to that answer is you have to take everything into consideration.

We have solar PV producing around 3600Kwh per annum, solar thermal HW (which was the first but we installed over 7 years ago), a Tesla Powerwall 2 and replaced our old oil boiler with an ASHP 6 months ago. Our electricity consumption is around 7500Kwh per year (that’s historical still waiting for the first full year of ASHP costs, but we previously ran an electric car for 6 years before deciding it really doesn’t work out here in rural Wales (few public charge points, won’t tow 2 tones of sheep trailer etc! and less expensive to use a but more diesel than run a second vehicle).

We still maintain our Octopus Go tariff (4.5p/unit for 4 hours overnight) and that effectively charges the powerwall (unless Tesla in their infinite wisdom decide it is going to be sunny tomorrow and rely of solar PV instead). This time shifting has a huge impact of the average price per unit - last year (prior to ASHP and the huge energy price rises we averaged about 7.5p per unit over the year. Taking into account the FIT (and allowing for the fact that we export less than 10% of the solar production) our nett energy cost for the year was practically zero - and that was all down to the battery.

This year, trying to work things out is more complex, with greater demand from the ASHP and huge tariff rises makes like for like tough.
With the ASHP working out more effective that anticipated ( we used an average of 20 odd Kwh per month between November and April but that has now fallen to just a few as we hit summer. The Solar thermal feeds into the 300L heat store so provides piping hot water most days meaning the ASHP doesn’t need to, and it is now effectively free energy, long since paid for after 7 years of RHI payments. Electric bills have soared so far, but the time shifting impact of the Powerwall is even more critical now. We had a problem with our solar inverter so lost almost 3 months of production in spring, but that is pushing out up to 25Kwh on a good day so most days we use no peak hours import from the grid. Just had my latest Octopus bill and daily power cost was down below £1 for a good proportion of days. Now you also have to look at the plus side arising from the switch - we have RHI payments amounting to over £1500 per annum, we no longer use oil for heating (previously about £1500 per annum, but on current rates it would have been £4500+ per annum to heat for the six winter months. That saving alone more than pays the electricity bill even with todays rates.

Now of course the elephant in the room is the capital cost of doing all this, but that isn’t always a simple decision. The oil boiler and system needed replacing, so that partly offset the ASHP capital cost and although the RHI payments will barely cover the rest the overall running cost of the system is distinctly lower and much more environmentally sound. The Powerwall 2, set against todays high tariffs looks set to pay for itself fairly quickly and in conversations with our installer they have a massive backlog of orders and a shortage of batteries, solar panels, inverters etc so good luck trying to get anything installed now, which kind of implies people generally think it is a good thing. On top of that the Powerwall/Gateway 2 provides emergency backup power (limited as it may be) in these days of recurrent power cuts.

Bottom line - look at the pluses and minuses, grants, FIT/RHI payments. Choose the right tariff to support time switching. Solar thermal seems to be a low cost way to generate heat for heating and hot water as long as your heat store is big enough. Get the biggest battery storage you can afford (we are seriously considering adding a second Powerwall battery pack if our installer can find one). Having a backup heating option (IMHO) is critical once you run everything from the electricity supply, but we have the log burners to handle that so I am comfortable that we are set fair for the winter to come. Looking forward to having a full years data to make better comparisons in about six months time.

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Get a tariff like Octopus Go (needs a smart meter) and you get 4 hours overnite at around 5p/unit. That is more than enough to fully charge our Tesla Powerwall 2 with 13.5 kwh of power. During the day as it discharges your 4.5kwh of solar will put charge back into the battery. By late afternoon you may be having to revert to importing grid power but the average cost of your power will be slashed. We use 7000kwh per annum, with ASHP and average cost so far this year (on current Octopus Go tariff) is about 16p/unit (5p off peak, 23p peak). In the summer months we use practically zero grid power, so average cost falls down to under 7p/unit. IMHO the most valuable spect of a domestic battery is the ability to time shift and use low tariff power during the rest of the day.

hi steve,
Many thanks for your reply and details on your system and performance. It’ll be interesting to see the results of your data collection from the emonPi.
In our case, we use around 9000kwh/ year (more than half by the ASHP). Our solar PV array produces around 4400 kwh per year. From the data i’ve got so far we use more than half of our daily solar generation directly, and during the winter months it’s much higher. So we wouldn’t get that much ‘free charge’ into the battery during the winter. Our daily winter usage average is around 30 kwh per day but can be as high as 50 kwh (i think due to the ASHP defrost cycling a lot when its damp and cold!).
Here is some data of our solar production (yellow), export (green), import (red). You can see from Jan to mid-March we’re exporting only a few kwh per day, and using the majority of our solar energy directly. It’s not until the last half of march that we start exporting >10kwh per day fairly consistently. Unfortunately I don’t have data from Nov and Dec, but i suspect its similar to the Jan and Feb data.

If we go for Octopus Go or similar tarrif, i think we’d need a very large battery and large inverter to get enough cheap electricity stored to last us through the day. In our case I don’t think it’s worth the substantial investment.
Instead, we’ve invested in a wood burning stove to take some pressure off the ASHP during the coldest periods, and i’ve bought a solar diverter to dump excess solar energy into the hot water tank. Total cost of that is around £2000. I won’t know how much it’s helped until next year though.

Thanks for this, it is interesting to see what others are finding. I agree with many of your points, and totally accept that with the usage levels of an ASHP you will never get a big enough battery to last all day. Ours is a big unit (largest single phase unit available) and it is wired up with the ability to pull 60 amps (!) but with detailed monitoring I have never seen it take more than 12 (3kw). It was commissioned in October last year and used an average of 23 units per day in the six months to end of April. May usage is down to just a few units since the solar thermal installation we installed 8 years ago keeps the water cylinder hotter than the ASHP will! Taking into account the off peak and peak usage (which used to be around 70%/30% but is now around 30%/70% in Dec thru Feb the average cost per unit was 17p (4.7p off peak and 23p peak on Octopus GO). In the last three months the proportion of off peak usage has risen to parity and then 60/40 as the ASHP does not cycle so much during the day so average price per unit is down to 12p. We also have wood burners that will heat the house in winter.

The killer for us is that the ASHP was installed to replace our not so old oil boiler, so on top of the £400 per quarter RHI payment we have also not needed the 3000 litres of heating oil we previously needed for the winter period , a further saving of over £4000 at current prices so the £680 or so in electricity the ASHP has used to date seems like a clear win to which the battery has contributed. At that rate payback on the ASHP will be under 4 years.

I think the battery is a leap of faith in many ways, but with energy prices bound to stay high the savings from time shifting (assuming the tariffs remain available) will continue to mount and the protection from power cuts (the Powerwall 2 provide backup power when the grid goes and will allow the solar PV system to remain operational) is a valuable benefit here in the middle of nowhere.

I am still struggling to get the EmonPi system to monitor everything I want (house usage, ASHP usage, solar production, grid input and exports) so will have more data in time for the winter!



I need to look at on-peak and off-peak prices again. I think last time i looked the on-peak prices where so high it made me think a battery wouldn’t work for us as we’d get stung by high on-peak usage during the winter.
I also suspect our ASHP is slightly too small for our demand in winter. My feeling is that it’s needing to work too hard in winter and this is causing excessive ice build up on the fins and consequently the defrost cycle kicking in too often.
I haven’t ruled out getting a battery in the future. It sounds like it’s working out well in your situation.

I found the EmonPi installation and EmonCMS a bit of a learning curve, but the online support is great - if you get stuck, just ask in the forums.

p.s. our ASHP replaced an old LPG boiler. We were spending around £1500-£2000 per year on LPG and electricity back in 2017-18.
Since installing the ASHP our annual electricty bill (no gas anymore!) is around £1000, plus we get another £350 or so in payments. However, its not a fair comparison as we extended the house, put in loads of insulation, and UFH.
We were also able to get rid of the unsightly lpg tank sat in the driveway.