Three quantities not to confuse
Sizing a battery for solar panels means telling three things apart:
- the panel power (in kWp, kilowatts-peak): the maximum they can produce;
- the energy produced (in kWh/day): what they actually produce over a day, depending on sunlight;
- the battery capacity (in kWh): the amount of energy it can store.
The classic mistake is sizing the battery on panel power. In reality, the battery should be set on what you consume when the panels aren't producing — that is, evening and night.
The three-step method
- Estimate your evening and night consumption. It's the share of your daily use that falls outside solar production. For a household it's often between 3 and 8 kWh (lighting, cooking, evening appliances, standby, possibly EV charging or a heat pump).
- Check that your panels produce enough surplus to fill it. No point having a big battery if the daytime surplus can't charge it most days. Surplus = production − consumption during sunlight hours.
- Account for usable capacity. A battery isn't discharged to 100%: the allowable depth of discharge (DoD) limits the energy actually usable. An LFP battery tolerates 80–90% DoD, often more than other chemistries.
A worked example
Take a house with 3 kWp of panels, producing on average 12 kWh/day over the year. During the day the house uses about 4 kWh in sunlight hours; the surplus is therefore about 8 kWh. Evening and night consumption is estimated at 6 kWh.
So the battery must deliver ~6 kWh usable. With 85% DoD, the required nominal capacity is:
6 kWh ÷ 0.85 ≈ 7 kWh nominal capacity.
The daytime surplus (8 kWh) is enough to recharge those 7 kWh on most sunny days. A 6.5 to 8 kWh pack is therefore well sized. Installing a 12 kWh battery, however, would be wasteful: the surplus would never fully fill it, and you'd have paid for unused capacity.
| Profile | Evening/night use | Battery (nominal) |
|---|---|---|
| Small home | 2–3 kWh | ~3–5 kWh |
| Family home | 4–6 kWh | ~5–8 kWh |
| Large home / heat pump | 7–12 kWh | ~8–12 kWh |
Power, not just energy
Capacity (kWh) says how much energy the battery stores; power (kW) says how fast it can deliver it. Check that the pack's discharge power covers your simultaneous demands (hob, oven, heat pump…). A good supplier states both. This is also where stationary use is forgiving: the power drawn is moderate and steady, which suits second-life cells perfectly.
Seasonal variation and common sense
Solar production varies strongly between summer and winter. Sizing "just right" on summer output leads to a battery underused in winter. Most households aim for a good annual compromise rather than full year-round autonomy, which would demand an oversized, costly battery. When in doubt, start from a reasonable size and adjust: at cirBATT, second-life packs are exactly what let you reach the right capacity at a controlled cost.