What the tool does
A battery is sized by what it changes over a whole year, not by one good day. The tool simulates every hour or every 15 minutes: PV first supplies the consumption, surplus charges the battery within its power and capacity, and the battery supplies the consumption when PV is short. It then reports what changed.
- Self-consumption, with or without a reserve kept for outages: self-sufficiency, share of PV used on site, grid import and export, battery cycles and losses, month by month.
- Capacity: the same year for a range of capacities, with power growing at the ratio you set, savings, cost and payback for each.
- Peak shaving: for a range of import limits, the largest energy above the limit in one day and the largest power above it, so the battery is sized by the worst day; a simulation with a limit shows the import that remains.
- Off-grid: the rule of thumb (daily use x days of autonomy / (depth of discharge x efficiency)) and a simulated year that counts consumption not served and PV spilled.
How to use it
- Enter the consumption: twelve monthly totals with a daily pattern (household, office, shop), or a meter file with 15-minute, 30-minute or hourly values in kWh, kW or W.
- Add the PV: open a project file saved by the PV string sizing and energy yield tool (with its weather data) or an hourly CSV with a column ac_w. A factor scales it to another array size.
- Set the battery: usable capacity, power, round-trip efficiency and lowest state of charge, and choose the operation.
- Enter your tariffs and battery prices to see savings and payback.
- Read Results and Days, compare capacities, check peak shaving or off-grid, and export the report, the workbook or the time series.
How it was checked
The simulation reproduces hand calculations exactly. With 1 kW of constant consumption and 3 kW of PV for four hours a day, a 10 kWh battery without losses cuts import from 20 to 12 kWh a day and raises self-sufficiency from 16.7 % to 50 %, with 292 full cycles a year; at 81 % round-trip efficiency the 8 kWh charged gives 6.48 kWh back. A load of 10 kW with two hours at 20 kW every day needs 10 kWh and 5 kW to stay under 15 kW, and the simulation holds the import at 15 kW with 12 kWh but not with 8 kWh. Fourteen such cases run before every release.
Limits
Monthly totals with a daily pattern are an estimate; peaks in particular need a meter file. Ageing, temperature, standby losses, time-of-use tariffs and grid charging for arbitrage are not modelled. Warranties, cycle limits, fire and installation rules and the utility's conditions decide what can be installed.
Last checked 2026-10-19