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Battery life calculator

Estimate how long a battery lasts from its capacity and the load current.

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The Battery life calculator turns Capacity (mAh), Load current (mA), Efficiency (%) into Runtime (hours), Runtime (minutes), instantly and for free. For instance, with Capacity (mAh) = 5,000, Load current (mA) = 500 and Efficiency (%) = 85 it returns Runtime (hours) = 8.5 and Runtime (minutes) = 510.

How to use it

  1. Enter your values: Capacity (mAh), Load current (mA), Efficiency (%).
  2. Read the result instantly: Runtime (hours), Runtime (minutes).

Frequently asked questions

What does the Battery life calculator actually compute?

It takes Capacity (mAh), Load current (mA) and Efficiency (%) and derives Runtime (hours) and Runtime (minutes) from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

3 values: Capacity (mAh), Load current (mA) and Efficiency (%). Nothing else is required — no account, no file upload.

Can you show a worked example?

With Capacity (mAh) = 5,000, Load current (mA) = 500 and Efficiency (%) = 85, the calculator returns Runtime (hours) = 8.5 and Runtime (minutes) = 510. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What happens if I enter larger values?

It moves a lot. Using Capacity (mAh) = 10,000, Load current (mA) = 1,000 and Efficiency (%) = 170 instead, Runtime (hours) goes from 8.5 to 17 — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to Capacity (mAh) = 2,500, Load current (mA) = 250 and Efficiency (%) = 43, Runtime (hours) comes out at 4.3. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Putting a number on a bill: what an appliance costs to run, whether a solar array covers the load, and how much heating a room actually needs.

What is the most common mistake?

Confusing power with energy. A kilowatt is a rate and a kilowatt-hour is a quantity; a 2 kW heater on for half an hour uses 1 kWh, and only the second figure appears on a bill.

What is the difference between the Battery life calculator and the Solar battery size calculator?

This one returns Runtime (hours) and Runtime (minutes); the Solar battery size calculator returns Battery capacity (kWh). That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Amperage Calculator is the closest one after this: Convert a load in watts and a supply voltage into the current drawn in amps.

What else is worth having open alongside it?

Heat pump savings calculator and Power dissipation calculator — they come up in the same task often enough to be worth a second tab.

Further reading

All guides →
How-toWhat Size Solar Battery Do I Need? Sizing by Daily UseSize a solar battery from your daily energy use, the days of autonomy you want, and the battery's usable depth of discharge.ComparisonSolar Panels: the Self-Consumption Rate Decides the PaybackSince June 2026 a French rooftop is paid 1.1 cents for an exported kilowatt-hour and avoids 25.61 cents by consuming the same one. That ratio of 23 to 1 makes the self-consumption rate — the variable almost every quote omits — worth more than the amount of sun falling on the roof.How-toHow to Convert kWh to CO₂: Grid Emission Factors ExplainedConvert electricity in kWh into CO₂ emissions using your grid's emission factor — and understand why that factor differs so much by country and time.How-toHow Many Solar Panels Do I Need? A Simple Sizing MethodSize a solar array from your annual electricity use, panel wattage, and local sun hours — plus a quick payback estimate in dollars.How-toHow Much Does a Space Heater Cost to Run?Multiply the heater's kilowatts by hours used and your price per kWh — a 1.5 kW heater for 5 hours a day adds up fast at typical tariffs.How-toHow Many Amps Does an Appliance Draw?Divide the appliance wattage by the supply voltage to get the current in amps — the number that decides breaker size, wire, and what can share a circuit.