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.
Under load means real watts—not idle sleep
Sleep current and active draw differ by orders of magnitude. Estimating duration under load requires the watt figure while the work happens: transmit on a radio, heat on a 12 V blanket, pump on a bilge cycle. Idle mAh charts from phone reviews do not transfer to continuous field loads. Measure or sum nameplate W before dividing into Wh.
Variable loads need an average watt
A fridge compressor that cycles 40% of the time at 120 W averages 48 W—not 120 W for the full clock. Estimate battery duration under load by time-weighting each operating state. For worst-case planning, run the calculation at peak W to bracket the shortest duration, then at average W for expected trip length.
Document duration assumptions on the run sheet
Field teams should record pack Wh, measured W, derate factor, and calculated hours in one line—so a swap from 18.5 Wh to 37 Wh doubles duration only if load W stays fixed. When load rises (faster fan, brighter LED), duration falls proportionally. Re-run the estimate whenever the load profile changes.