There is no honest way to size a battery from a photograph of a roof. There is a very quick way to size one from a meter, and the data is free.
Step 1 — get the half-hourly data
Your supplier holds half-hourly consumption for any smart meter in smart mode, and will give it to you on request. Ask for a full month, and ask for a winter month if you have one — that is the load you are actually buying for.
Step 2 — find the evening block
For each day, sum the half-hours between 16:00 and 22:00. Then take the median of those daily totals across the month.
The median matters. One evening with the oven, the dryer and a guest bathroom running is not the day to size capital equipment against, and a mean lets that day set your budget.
Step 3 — compare against usable, not rated
Two numbers, and only one of them is yours:
| Pack | Rated | Usable | | --- | --- | --- | | 5 kWh residential | 5.12 kWh | 4.5 kWh | | 16 kWh residential | 16.07 kWh | 14.46 kWh |
The difference is the 90 % depth of discharge the BMS holds back to keep the cells out of the floor of their range. Sizing against the rated figure is how a system ends up running flat at nine every night in January.
Add a little for round-trip losses — 97 % on the smaller pack, 98 % on the larger.
Step 4 — the subtraction most quotes skip
The saving per unit is not your import rate. It is:
import rate − export rate
A unit stored and used in the evening avoids the import price. Had you not stored it, you would still have been paid the export price for it. The battery earns you the gap, not the whole import bill.
Annual saving ≈ evening block (kWh) × 365 × (import − export)
Then divide installed cost by annual saving for a payback in years. Put your own rates in. Any quotation that presents a payback without showing this subtraction is overstating the case, and you should ask to see the working.
What moves the answer most
A cheap overnight window. The single biggest lever, and it costs nothing but a tariff switch. If the pack can fill overnight at a low rate, it earns in December as well as it does in June and the case stops depending on the weather.
Rising import prices. These help the arithmetic, because your avoided cost rises with them. Do not let anyone build an assumed escalation into a quoted payback figure, though — that is a forecast being sold as a calculation.
Starting small. Both residential packs parallel up to sixteen units on the same bus. If the numbers are marginal, buy one and add the second when a year of real data says so. That option is worth something, and it costs you very little to keep.
What we will do
Send us a month of half-hourly export and we will run this against your own data before anyone writes a quotation — including in the cases where the answer is that storage does not pay yet.