The rule of thumb
The useful battery size is the smaller of two numbers: the surplus your panels export on a typical day, and the electricity you use between sunset and morning. A battery bigger than both rarely fills or empties.
Step 1: your evening and night use
From your bills or monitoring, we estimate how much you use after the sun goes down: cooking, lighting, TV, fridge, evening air conditioning. For many occupied homes that is 4 to 10 kWh a day.
Step 2: your daily surplus
From the solar design we estimate how much is exported on ordinary days in each season.
Step 3: seasonal occupancy
A home occupied all year cycles the battery daily. A holiday home may only use it for a few months.
Step 4: backup
Backup is a separate requirement. If you want power cuts covered, the battery must handle the peak load of the backed-up circuits as well as store enough energy.
Examples
| Property | Typical battery |
|---|---|
| Permanently occupied apartment | Often none, or around 5 kWh |
| Occupied townhouse or small villa | 5–10 kWh |
| Occupied villa with evening air conditioning | 10–15 kWh |
| Large villa with backup | 15 kWh or more |
| Holiday home used a few months a year | Often none; monitoring instead |
Questions we are often asked
Is a bigger battery always better?
No. Unused capacity costs money and saves nothing.
Can I add more capacity later?
With modular batteries like Enphase and SolaX, yes.