Battery runtime estimates are usually simple in structure but easy to overstate. A useful first-pass formula is:
usable Wh = nominal Wh x depth of discharge x system efficiency
runtime hours = usable Wh / load W
Example:
nominal battery capacity = 1200 Wh
depth of discharge = 80%
system efficiency = 90%
load = 100 W
usable Wh = 1200 x 0.8 x 0.9 = 864 Wh
runtime = 864 / 100 = 8.64 hours
That is a planning estimate, not a guarantee. Real runtime can be lower because of battery age, temperature, voltage sag, BMS cutoff, inverter idle draw, surge loads, wiring losses, and load variation.
For AC loads through an inverter, the inverter's efficiency and idle consumption matter. A small always-on inverter load can materially reduce runtime, especially with low-power devices.
I made a free calculator that applies this same logic and shows the assumptions:
https://speccalchub.com/en/calculators/battery-runtime
Disclosure: I'm the creator of this free tool.
