Balcony Battery Size Calculator: How Much Storage Do You Need?

A battery only earns money when it stores solar you would otherwise export, and releases it when you would otherwise buy power. This calculator estimates your surplus from a plug-in kit, suggests a usable capacity and compares the yearly saving of common battery sizes.

How it works

• Surplus: solar production above your daytime use, hour by hour, on a smooth daily curve. The year mixes clear, mixed and dull days, as in our appliance calculator.
• Energy moved to the evening: each day, the smallest of the surplus, the battery’s usable capacity and your evening and night use, multiplied by the round-trip efficiency.
• Net saving: that energy times your electricity price, minus the battery’s own standby consumption over the year.
• Suggested capacity: the clear-day surplus or your evening use, whichever is smaller. Beyond that, extra capacity sits empty most days.

What usually changes the answer

• Evening prices. On a time-of-use plan, use your evening peak price: that is the power the battery replaces. See time-of-use tariffs.
• Standby use. A battery drawing 10-20 W all year can cancel a large part of its savings. Check the spec sheet.
• Daytime use. If someone is home all day, there is less surplus to store.
• Backup. The calculator does not put a value on keeping the fridge running in an outage. If that matters to you, see plug-in solar in power outages.

Before you buy

Run your kit without a battery for a few sunny weeks and compare your exports with the calculator’s surplus. If the payback is longer than the warranty, the battery is a comfort purchase, not an investment. Read do you need a battery?, how big a battery for 800 W and the batteries compared.

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