Battery usable capacity, depth of discharge and what “13.5 kWh” really gives you
The energy a battery can actually deliver. Home batteries usually quote usable kWh; portable power stations lose ~10–15% converting to AC.
Batteries keep some charge in reserve to protect their cells. Home batteries like the Powerwall 3 (13.5 kWh) quote usable capacity. Many lead-acid and some DIY lithium banks quote total capacity — you can’t use all of it.
Converting battery DC to household AC also costs energy: roughly 5% for home battery systems and 10–15% for portable power stations running AC appliances.
Utilities and apps often keep a backup reserve (e.g., 20%) so you are not left empty when an outage starts. Planning a backup scenario, assume you start from that reserve — not 100% — unless you pre-charge for a storm.
What this means for you: Size backup batteries on usable kWh, and on continuous kW for your largest simultaneous loads.