How big a battery do you actually need?
Most homes are sold too much battery. Here is the maths we use, with a worked example.
by Southwind team
Battery size is night-time consumption plus the outages you want to ride through, not a round number the salesman likes.
The calculation
Take your monthly kW·h from the bill. Divide by 30. Roughly 40 per cent of that is used after sunset in a typical home. That is your nightly draw. A 600 kW·h household uses about 8 kW·h a night, so a 10 kW·h battery covers it with margin. Twenty would be money on the wall.
The outage question
If you want to ride through a two-hour outage in the evening, you already have it. If you want to ride through a full night with no sun the next day, add another 5 kW·h. Beyond that you are buying insurance against events that rarely happen.