Capacity in mAh alone does not tell you runtime—you need voltage and load watts. This guide walks through the battery runtime calculator: mAh, nominal V, and power draw for hours of operation.
Benefits
- Wh = (mAh ÷ 1,000) × V, then runtime h = Wh ÷ W.
- Works for Li-ion cells, power banks, and small DC loads.
- Shows energy in Wh alongside duration for fair pack comparisons.
How it works
- Enter battery capacity in mAh from the label or spec sheet.
- Add nominal voltage (3.7 V cell, 12 V AGM, etc.).
- Enter steady load in watts—read estimated runtime in hours or minutes.
FAQ
How does the battery runtime calculator work?
Convert mAh to Wh: Wh = (mAh ÷ 1,000) × V. Divide Wh by load watts for hours. Example: 5,000 mAh at 3.7 V → 18.5 Wh; at 10 W draw → ~1.85 hours. Lower watts extend runtime linearly.
Why convert mAh to Wh before dividing by watts?
mAh is charge, not energy—voltage completes the picture. A 10,000 mAh 3.7 V phone pack (37 Wh) and a 5,000 mAh 7.4 V pack (37 Wh) deliver the same runtime at the same watt load despite different mAh ratings.
Should I derate the calculated runtime?
Yes for planning—heat, age, and high discharge rates reduce usable capacity. Derate 15–20% for outdoor gear, aged cells, or continuous max draw. Pair with Battery Charging Time to plan recharge after the discharge window.
Technical specifications
- Wh = (mAh ÷ 1,000) × V.
- Runtime (h) = Wh ÷ load_W.
- Assumes near-constant DC or inverter load in watts.
- Related: battery-runtime, ah-to-wh, battery-charging-time, ups-runtime.
mAh ratings need voltage to mean runtime
Marketing highlights milliamp-hours; engineers plan in watt-hours. The battery runtime calculator bridges both: multiply mAh by voltage, divide by 1,000 for Wh, then divide by your load watts. A 20,000 mAh power bank at 3.7 V is 74 Wh—not 20,000 mAh hours. Comparing packs on mAh alone mis-ranks runtime when voltages differ.
Load watts drive the denominator
Runtime scales inversely with draw. Halving load doubles hours—until BMS cutoff or voltage sag under heavy current. Use measured or nameplate watts for routers, LED strips, CPAP inverters, and ham-radio rigs. For pulsed loads, use average watts over the duty cycle, not peak surge.
From runtime estimate to system design
After hours are known, check whether depth-of-discharge and recharge time fit your use case—camp weekends, field work, or outage windows. Chain to Ah to Wh when sizing from amp-hour banks at 12 V or 48 V, or UPS Runtime when the load is on an AC inverter with efficiency loss to add mentally.