Duration under load is energy divided by draw—no mystery once you fix the watt number. This guide shows how to estimate battery duration under load from pack mAh, voltage, and the watts your device actually pulls.
Benefits
- Duration (h) = Wh ÷ load_W with Wh from mAh and voltage.
- Use nameplate or metered watts for honest under-load estimates.
- Compare scenarios by changing only the load column.
How it works
- Sum or measure load watts while the device runs normally.
- Convert battery mAh and nominal V to watt-hours.
- Divide Wh by load W—read duration before BMS or inverter cutoff.
FAQ
How do I estimate battery duration under load?
Wh = (mAh ÷ 1,000) × V; duration h = Wh ÷ W. Example: 12 V 100 Ah (100,000 mAh equivalent at 12 V = 1,200 Wh) at 60 W load → 1,200 ÷ 60 = 20 hours. Use average watts if load cycles on and off.
What load value should I use?
Steady DC loads: use measured watts from a meter. Inverter-fed AC: use output watts plus ~10% for conversion loss, or input watts at the battery. For intermittent loads, multiply on-time fraction by peak watts for average W.
Why is duration under load shorter than the label suggests?
Labels assume light test currents; heavy loads cause voltage sag and earlier BMS shutdown. Heat and cycle age shrink usable Wh. Derate 15–25% when the estimate must survive cold weather or an aged pack.
Technical specifications
- Duration_h = ((mAh ÷ 1,000) × V) ÷ load_W.
- Average load_W = duty_cycle × peak_W (intermittent).
- Planning margin: 0.8–0.85 × calculated hours (typical).
- Related: battery-runtime-calculator, ups-runtime, watts-to-amps.