Battery runtime calculator: from mAh or Wh to hours
This battery runtime calculator answers "how long will this battery last?" Enter the capacity in mAh, Ah or Wh, the nominal voltage and the load in watts, amps or milliamps to get the runtime in hours and minutes, with the energy, current and usable capacity behind it.
It goes beyond capacity divided by current: an efficiency field covers converter or inverter losses, depth of discharge keeps a reserve in the battery, and an optional Peukert exponent models how lead-acid capacity shrinks at high currents. Two more modes work backwards — the capacity needed for a target runtime — or estimate charge time.
Presets cover a power bank, a laptop and 12 V lead-acid and LiFePO4 banks, plus home inverter batteries (150 Ah and 200 Ah tubular, two 150 Ah on 24 V) and UPS batteries for a PC or CCTV. Build the load from a list of typical appliances. Everything is calculated in your browser.
How to use the Battery Runtime Calculator
- Choose Runtime, Capacity needed or Charge time, or press a preset.
- Enter the capacity in mAh, Ah or Wh and the nominal voltage: 3.7 V for a Li-ion cell, 12 V per lead-acid battery, 12.8 V for LiFePO4.
- Enter the load in W, A or mA, or add appliances from the list.
- Adjust efficiency and depth of discharge, and open Peukert correction for lead-acid batteries. The result updates as you type.
Battery Runtime Calculator features
Any capacity unit
mAh, Ah or Wh, converted through the nominal voltage.
Inverter and UPS presets
Tubular 150 Ah and 200 Ah, a 24 V pair, 7 Ah and 9 Ah UPS batteries and a CCTV backup.
Peukert correction
Uses the rated hour rate (C20, C10) to show how lead-acid runtime drops at high currents.
Reverse and charge modes
Battery size for a target runtime, and time to full from a charger.
The formulas behind the calculator
Energy in watt-hours is capacity in amp-hours multiplied by voltage: Wh = Ah × V, and Ah = mAh ÷ 1000. The battery current for a load of P watts is I = P ÷ (V × η), where η is the system efficiency. Runtime is then t = (Ah × DoD) ÷ I.
With the Peukert correction the runtime becomes t = H × (C ÷ (I × H))^k, where C is the rated capacity at the H-hour rate (typically 20 h) and k is the Peukert exponent. For k = 1 this collapses to the simple formula; above 1, a battery drained faster than its rated hour rate delivers fewer amp-hours.
| Example | Inputs | Runtime |
|---|---|---|
| Phone from a power bank | 20,000 mAh × 3.7 V = 74 Wh, 10 W load, 85 % efficiency | ≈ 6.3 h |
| 12 V fridge on lead-acid | 100 Ah × 12 V, 60 W (5 A), 50 % DoD, k = 1.25 | ≈ 10 h |
| Router on LiFePO4 | 100 Ah × 12.8 V, 12 W, 90 % DoD, 90 % efficiency | ≈ 86 h |
| Laptop | 56 Wh, 15 W, 95 % DoD, 92 % efficiency | ≈ 3.3 h |
Inverter and UPS backup time
Inverter battery backup: hours = Ah × V × DoD × efficiency ÷ load W. A 150 Ah tubular battery at 12 V, 300 W, 80 % depth of discharge and an 80 % efficient inverter gives 150 × 12 × 0.8 × 0.8 ÷ 300 = 3.84 h; 200 Ah gives 5.12 h. Because the Ah is rated at the C10 or C20 rate, the presets add Peukert k = 1.15, which gives about 3 h for 150 Ah. Two 12 V batteries in series make 24 V with the same Ah, so 2 × 150 Ah runs 600 W for the same 3 h.
UPS backup time: a 600 VA UPS usually holds one 12 V 7 Ah sealed battery. A 120 W PC gives 7 × 12 × 0.8 ÷ 120 = 34 minutes by the simple formula, or about 23 minutes with Peukert 1.1; a 30 W CCTV recorder with four cameras runs about 1 h 55 min. Ageing batteries deliver much less. To size a whole home system with surge, use the inverter size calculator.
Why power banks deliver less than their label
A 20,000 mAh power bank is rated at its cells’ 3.7 V, about 74 Wh. After the 5 V boost converter (85–90 % efficient) and the phone’s own charging losses, roughly 11,000–13,000 mAh reaches the phone, so a 5,000 mAh phone gets about two charges. Airlines count Wh (usually 100 Wh without approval), about 27,000 mAh at 3.7 V.
Choosing depth of discharge and efficiency
Size cables for the current shown with the wire gauge calculator; the watt calculator and Ohm's law calculator help when you only know volts and amps.
- Lead-acid: 50 % depth of discharge for long life (deep-cycle tubular inverter batteries are often run to 80 %); Peukert 1.2–1.4 flooded, 1.05–1.15 AGM or sealed.
- LiFePO4: 80–90 %, with almost no Peukert effect (k ≈ 1.02–1.05).
- Li-ion: the protection circuit limits discharge, so use 95–100 %.
- Efficiency: DC–DC converters 85–95 %, inverters and UPS units 80–92 %, worse at light load.
Battery Runtime Calculator FAQ
How do I calculate battery runtime from mAh?
Convert to amp-hours (mAh ÷ 1000), multiply by the voltage to get watt-hours, then divide by the load in watts. A 10,000 mAh, 3.7 V battery holds 37 Wh and runs a 5 W device for about 7.4 hours before losses.
What voltage should I use for a power bank?
Use 3.7 V: the label’s mAh is quoted at the cells’ voltage, not the 5 V USB output.
What is the Peukert exponent?
It describes how a lead-acid battery’s usable capacity falls as the current rises. Leave it at 1 for lithium.
Why is depth of discharge 50 % for lead-acid but 80 % in the inverter presets?
Lead-acid cycle life drops sharply when the battery is routinely drained below half. Home inverters cut off near 10.5 V, which is close to 80 % for a tubular battery, so the presets use that for backup time.
How accurate is the charge-time estimate?
It assumes a constant charger output. Chargers slow down near full, so the breakdown adds a 20–30 % allowance.
How do I calculate UPS or inverter backup time?
Multiply the battery Ah by its voltage, depth of discharge and the UPS or inverter efficiency, then divide by the load in watts. Press a UPS or inverter preset and enter your load; for a 24 V or 48 V bank use that voltage with one battery's Ah. To size a new bank, switch to Capacity needed.