A home battery has two different headline ratings. Energy capacity in kilowatt-hours (kWh) describes how much it can store. Power in kilowatts (kW) describes how quickly its inverter/charger can charge or supply loads. A large kWh number with a low kW limit may run for longer but still be unable to support several high-power appliances at once.
Nominal capacity is not usable capacity
Manufacturers may state nominal capacity and usable capacity separately. Depth of discharge, reserve settings and battery management controls determine how much of the nominal energy is normally available. Compare products on usable AC-delivered energy where possible, and keep charge and discharge efficiency explicit.
For example, 10 kWh nominal at a 90% usable fraction provides 9 kWh before discharge-path losses. The energy that reaches household circuits can be lower after inverter conversion.
Size from the energy you need to shift
Start with an ordinary day, not the largest day of the year. Identify daytime solar production, direct daytime consumption and the evening or overnight demand you want the battery to cover. Half-hourly smart-meter data is much better than annual kWh for this task because it reveals when electricity is actually imported.
- Solar shifting: store genuine daytime surplus for use after generation falls.
- Tariff shifting: charge from the grid during an eligible off-peak window for use at a more expensive time.
- Backup: decide which essential loads must run and for how long; backup operation needs compatible equipment and design.
- Export: treat battery export as a separate, opt-in assumption and confirm supplier eligibility.
Energy Saving Trust similarly notes that battery size depends on how much electricity you want to store, excess generation and whether grid charging on a time-of-use tariff is planned.
Check charge and discharge power
The effective power ceiling is the lowest relevant constraint: battery/BMS current, battery voltage, inverter/charger rating and configured charge or discharge limit. Hybrid inverters may share one AC rating between PV and battery activity. An AC-coupled system has a PV inverter and a separate battery inverter/charger.
This raw electrical value is not permission to operate at that level. Datasheet conditions, temperature, state of charge, cabling, protection and inverter compatibility all matter.
Do not size storage from solar array kWp alone
A 5 kWp array does not automatically need a 5 kWh or 10 kWh battery. Array kWp is power under standard test conditions; battery kWh is energy. Estimate annual and seasonal surplus after direct household consumption. Oversized storage may spend long periods partly empty in winter or unused in summer if there is insufficient evening demand.
Compare solar array sizing first, then test more than one battery capacity. A good calculator must be allowed to show that solar only has better economics.
Model off-peak charging carefully
Grid charging only creates a saving when the avoided peak-rate cost exceeds off-peak input cost after charging and discharging losses. The available window and charge power limit constrain how much energy can enter the battery. Standing charges, tariff rules and changes over time sit outside a simple per-kWh comparison.
Paid export from a grid-charged battery is not automatically available under every export contract. Ofgem’s Smart Export Guarantee guidance explains the generator application framework, while the chosen supplier determines tariff and metering terms.
Battery payback needs a separate calculation
Separate battery benefit from solar benefit. Include avoided peak imports, any qualifying export income, extra grid charging cost and losses. Then divide the additional installed battery cost—not the whole solar project cost—by incremental annual battery benefit. See the cost and payback guide for a reconciliation checklist.
Compare storage honestly.
Test solar only, battery only, solar plus battery and tariff optimisation.