The runtime formula.
Runtime in hours = battery Wh × usable fraction × starting-charge fraction ÷ average load in watts.
Average load = running watts × time-in-use fraction + station idle draw. Usage reduces average energy demand; it does not reduce the running-output rating a station needs.
A worked example.
A 1,024 Wh battery at full charge, with 80% usable capacity, a constant 100 W load and 10 W of station idle draw gives 1,024 × 0.8 ÷ 110 = about 7.4 hours.
That same load needs about 1,100 Wh of nominal capacity to reach an eight-hour target under these assumptions.
What this number leaves open.
A battery can have enough energy and still lack the output to start a motor. These quick tools do not know your devices’ startup demand. The transferred plan uses running watts as a placeholder for starting watts; replace it or add individual appliances before relying on a match.
Battery condition, temperature, inverter behavior and real appliance use can change runtime. No solar charging, extra batteries or replenishment is included. For a fridge or mixed appliance load, start with the fridge guide or the full planner.