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?