What Size Solar Battery Do I Need?

Quick Guide: Battery Size by Home Type

Home Type	Daily Usage	Recommended Capacity	Battery Option
1–2 bed flat	6–10 kWh	5–6 kWh	Fox ESS EP5
3 bed house	10–15 kWh	8–10 kWh	Fox ESS 9.5kWh
4+ bed house	15–20 kWh	10–15 kWh	Tesla Powerwall (13.5kWh)
EV owner	+8–15 kWh for charging	13–15 kWh+	Tesla Powerwall or dual Fox ESS

How Battery Sizing Works

The goal of battery storage is to maximise self-consumption — the proportion of your solar energy that you use directly rather than exporting. Most households use most of their electricity in the mornings and evenings, while solar generates primarily during the middle of the day.

A correctly sized battery stores the excess daytime generation and releases it during the evening, overnight, and morning. The ideal battery size is one that:

1. Is large enough to store most of your daily surplus solar generation
2. Matches your evening and overnight electricity demand
3. Fits your budget

Your Solar System Size Matters

The more solar you generate, the more you need to store. A rough rule of thumb:

• 4kW solar system → 5–10 kWh battery is appropriate
• 6kW solar system → 10–15 kWh battery is appropriate
• 8kW+ solar system → 15kWh+ or multiple batteries

Installing a tiny battery on a large solar system wastes potential self-consumption. Installing a huge battery on a small system won't fill fully each day.

Consider Overnight Charging From Grid

With smart tariffs like Octopus Go (7.5p/kWh overnight), you can set your battery to charge from the grid at cheap overnight rates and discharge during expensive peak periods. This changes the optimal battery sizing calculation — larger batteries store more cheap overnight units for daytime use, increasing savings independent of solar generation.

If you're on or planning to switch to Octopus Go, a 9.5kWh or larger battery typically pays back faster than a smaller unit.

EV Owners Need More Capacity

If you're charging an electric vehicle at home, your effective daily energy demand increases significantly. A typical EV adds 8–15 kWh of daily demand. If you want to power EV charging from stored solar, you'll need a substantially larger battery — Tesla Powerwall at 13.5kWh or dual Fox ESS units are popular choices for EV-owning solar households.

The Minimum Viable Battery

The Fox ESS EP5 is the entry-point battery and perfectly adequate for smaller homes or as a starting point you can expand later. Fox ESS's modular design lets you add a second 5.2kWh unit later without replacing the inverter — a useful option if your needs or budget change.

Key Takeaways

• 3-bed homes typically suit a 9.5kWh battery best
• Match battery capacity to your solar system size and daily usage
• EV owners should consider 13.5kWh+ capacity
• Octopus Go tariff benefits increase with larger battery capacity
• Fox ESS is modular — start small and expand later if needed