Solar Battery Size by Excess Energy Calculator — Size Storage From Surplus PV
Estimate a battery's usable capacity from the amount of solar surplus you repeatedly want to capture, rather than starting with an arbitrary battery size.
Excess-energy sizing inputs
Why size a battery from excess energy?
Solar batteries are often oversized because a user starts from a desired "whole-home" battery number without checking how much surplus PV is actually available to charge it. An excess-energy method ties battery capacity to the energy that the solar system can realistically provide for storage.
Why peak and typical days should both be checked
A battery that handles an average day may still be undersized on high-production days. The peak-day scenario provides a second sizing signal so the result is not based on one average number alone.
Interpret the result as a planning range
Use the typical-day requirement as a recurring-energy reference and the peak-day requirement as a stress case. A final battery size should also consider backup needs, battery operating windows, degradation over time and available charging power.