The relationships
Amp-hours, watts, voltage, and runtime are all the same physics expressed four different ways. Each calculation is just an algebraic rearrangement of the same equation.
energy_Wh = capacity_Ah × voltage
current_A = power_W ÷ voltage
runtime_h = capacity_Ah ÷ current_A
Every real-world correction - efficiency, usable depth-of-discharge, Peukert effects - shows up as a multiplier on one of those three numbers.
Frequently asked
Do I need to apply efficiency twice?
Once is usually right. If your load is a DC device on the battery directly (no inverter), set efficiency to 100% - there’s no AC conversion happening. If it’s an AC appliance through an inverter, 85–92% is realistic.
Why does the Watts → Ah tab give a different number than Amps → Ah?
Because Watts → Ah assumes AC loads through an inverter (with inverter losses), and Amps → Ah assumes a DC load drawing current directly. For the same energy, the AC path needs a bigger battery to cover the inverter overhead.
What’s a sensible usable DoD?
LiFePO₄ - 80–95% (most BMS protect at 10%). AGM - 60% comfortable. Flooded lead-acid - 50% for daily cycling; 80% for occasional deep discharge if you accept shorter life.