Bigger isn't better. The right battery capacity matches your evening usage and solar surplus.
Size to your evening use
A battery's job is to cover the energy you use after the sun goes down. The right capacity is roughly the amount you consume between sunset and the next morning — not the biggest battery you can afford.
The basic maths
Look at your bill for your average daily usage. If you use, say, 20 kWh a day and roughly half of that is evening/overnight, a 10–13 kWh battery would cover most of it. Oversizing means paying for capacity you rarely fill.
Size to your surplus too
A battery can only store what your solar produces above your daytime use. If your solar surplus is small, a huge battery sits half empty. Match battery size to both your evening load and your daytime surplus.
Modular batteries help
Many modern batteries are modular — you can start with one unit and add more later as your needs grow (for example, after buying an EV). This avoids over-investing upfront.
The takeaway
Right-sizing a battery to your actual usage and surplus is the single biggest factor in whether it pays back. That's why we model your consumption before recommending a capacity.