Choosing a battery starts with one question: which consumption do you want to move through time? Solar panel capacity alone does not answer it. Two homes with identical solar roofs can need very different amounts of storage.
Separate kW from kWh
Kilowatts measure instantaneous power. Kilowatt-hours measure energy. A constant 1 kW demand over four hours requires 4 kWh. This is an arithmetic example; household demand varies throughout the day.
Ask for usable capacity rather than nominal capacity alone. Also check charging power, discharge power and inverter limits. A battery may hold enough energy for an evening without being able to power every appliance at once.
Model a day, then a year
Start with a representative day. Identify solar surplus after direct consumption, then examine demand when solar generation falls. The opportunity depends on both sides: surplus energy must be available to charge the battery, and later consumption must use it.
Repeat for other seasons, weekends and holidays. Sizing only around a sunny June day overlooks most of the year. Compare different battery capacities using exactly the same consumption data and efficiency assumptions.
Treat backup as a separate scenario
If you want to keep selected appliances running during a power cut, list their power requirements and the desired duration. Ask the installer to confirm off-grid operation and the equipment it requires. Owning a battery does not by itself prove that the home can operate during an outage.
A dedicated backup reserve can reduce the capacity available for daily optimisation. Show that reserve explicitly in the model rather than counting the full capacity twice.
Compare three sizes instead of one
- A smaller battery used frequently to cover part of evening demand.
- An intermediate size with a cost and usage balance to verify.
- A larger system justified by specific demand or backup requirements.
For each option, request annual energy actually delivered, losses and the installed price. Compare the outcome with and without appliance scheduling. Moving consumption into sunny hours can sometimes address part of the need without adding storage. ADEME highlights the value of aligning demand with solar production; that is a useful starting point before selecting a battery size.
Keep the inputs and assumptions attached to each quote. If a supplier changes both the proposed battery size and the assumed electricity price, you cannot tell which change created the apparent improvement. A consistent comparison makes that difference visible.
Sources and transparency
Sources document the principles discussed. Examples, checklists and calculations are educational tools from BatteryIncome, not quotes or performance promises. Contract terms and offers need to be checked when you assess a project.