Battery Savings Calculator

I have been cold called a few times recently by companies trying to sell me battery storage to go with my PV panels. You may like this idea for other reasons than simply saving money but costs are also important. Unfortunately it is very hard to calculate how much you will save from these because it depends hugely on your pattern of use as well as the battery system installed. Your salesman may make extravagant claims, but no guarantees. This tool will give you a rough guide as to the maximum savings you could get, using some very simple rules.

Basic parameters

Update the following parameters to calculate your maximum savings from adding battery storage to your PV system. The values entered already are plausible defaults for a 4 kWp system in Cambridge and average household electricity use.

The total amount you generate your year depends on your PV panels and the location. If you have kept your meter readings that you send off for FiT payments you can get it from them. Also your FiT registration document and your original quote will have an estimate of your predicted annual yield. 3600 kWh/year is reasonable for a 4kWp system (usually 16 panels) in an ideal location in Cambridge.
PV Generation (kWh/year)

This parameter is how much electricity you take from the grid each year. You should find this on your bill somewhere. Households vary. For a medium household your 'true usage' (see below) could be 3200 kWh/year. However, since you probably use at least some of the electricity from your panels already your actual bill should be a bit less than this.

The price you pay for electricity should also be on your bill. 14p/kWh is what I am paying at the moment on a green tariff.
Electricity bill (kWh/year)
Electricity price (p/kWh)

Your self consumption rate is how much of your own power you are using already. You will almost certainly be using some. If you are in during the day and trying hard to use your own power it could be 40% or even more. If you are mostly out of the house it would be much less. The average is probably 25% to 30%.
Self consumption (%)

Your 'true usage' is how much electricity you actually use, including your self consumption as well as the amount you take from the grid. You can enter either this or your self consumption rate and this tool will adjust the other one.
True usage (kWh/year)

Calculate maximum savings by summer/winter

Your bill savings are limited both by the amount of electricity generated and the amount of electricity you use. In the winter you are likely to be limited by how much you generate and in the summer you are likely to be limited by how much you use. Here is a very rough calculation of what that could mean for you. It assumes that your true electricity usage is the same throughout the year. In practice it is likely to be higher in winter than in summer so this will tend to overestimate your savings.

This rough calculation splits the year into two. 2/3 of your PV electricity is generated during April to September.

In practice the size of the battery is also important. This calculation assumes that your battery is 'big enough'.

  Generation (kWh) Usage (kWh) Max. savings (kWh)
Apr-Sep (67%) {{summerGeneration|number:0}} {{summerUsage|number:0}} {{maxSummerSavings|number:0}}
Jan-Mar, Oct-Dec (33%) {{winterGeneration|number:0}} {{winterUsage|number:0}} {{maxWinterSavings|number:0}}
Total {{generation|number:0}} {{trueBill|number:0}} {{totalSavings|number:0}}
Maximum savings from PV (kWh/year) {{totalSavings|number:0}}
Current savings from self consumption (kWh/year) {{currentSavings|number:0}}
Maximum additional savings from battery (kWh/year) {{netSavings|number:0}}
Maximum self consumption with battery {{finalSelfConsumption|PC}}
Maximum additional savings from battery (£/year) {{netSavingsGBP|GBP}}

Adjustments for reduced Feed in Tariffs payment

If the battery is going to be DC connected (i.e. to your inverter) then when the battery is used it reduces the amount of electricity recorded by your generation meter slightly, depending on the efficiency of the charge/discharge cycle. This is normally between 80% and 95%.

If the battery is going to be AC connected, enter 100% for no loss.

The Feed in Tariff comes in two parts. The generation tariff is paid for everything you generate. The rate you get depends on when you installed the system. Check your most recent Feed in Tariff payment information for your current rates. The export tariff is paid on half of what you generate - it is assumed that you use half and export half. So the total Feed in Tariff is the generation tariff plus half the export tariff.

Total feed in tariff (p/kWh)
Generation tariff + half the export tariff.
Battery Efficiency (%)
Typically 80% to 95%.
Reduction in Feed in Tariffs payment (p/kWh) {{FiTsLoss|number:1}}
Reduction factor {{FiTsFactor|PC}}
Adjusted maximum savings £/year {{FinalSavingsGBP |GBP}}

Calculate minumum payback time

Calculate the overall minimum payback time based on the cost of your battery system. Ideally your battery should be guaranteed for this long. System costs vary. A DC connected system may be cheaper then an AC connected system but even then £3200 is probably at the low end. Always get at least two quotes, preferably three.

Cost of system £
Payback time (years){{paybackTime|number:0}}

I hope this has been helpful. Remember it is only a rough guide to your maximum savings - the assumptions are optimistic and the size of the battery is not taken into account at all. If you play with the self consumption parameter you will see how important this is.

If you find that battery storage is not viable for you, do not despair. When we all have smart meters and time of use varying tariffs are available, that will very likely make storage much more economic.