Battery calculators

Charging time, runtime, unit conversions and wiring — with the formula, the assumptions and the sources behind every result. Works offline.

Charge time

How long charging takes for a given charger current and state of charge.

Capacity & runtime

How long a battery will carry a given load.

Ah ↔ Wh

Convert between Ah, Wh and kWh at a known system voltage.

Backup planner

List your own appliances and get the energy, the continuous power, the starting surge and the pack size.

Series & parallel

Bank voltage and capacity for series and parallel wiring.

Voltage drop

Voltage drop along a cable and the required conductor size.

Reactive power

Reactive and apparent power, cos φ, and the theoretical capacitance needed to correct it.

Which calculator do you need?

Start with the situation you have. Each tool shows the assumptions used for its result.

Charge time

You have a charger and want to know when the battery will reach a chosen level. Enter Ah, charger current and the starting state of charge.

Capacity & runtime

You want to run equipment from a battery or size a bank for a required time. Enter power, voltage and runtime, then check inverter efficiency.

Ah ↔ Wh

You have Ah, mAh, Wh or kWh on a label and need to compare devices. Enter voltage, because Ah alone does not state stored energy.

Series & parallel

You are wiring identical batteries and need the figures for the complete bank. Choose series and parallel counts to see the result and connection diagram.

Voltage drop

You are running a DC cable and worry that the load will receive too little voltage. Enter current, one-way length and cable cross-section.

You have a list of appliances and want to know what pack will carry them — or whether yours is enough. Enter the appliances, powers and hours to get the energy, the continuous power and the starting peak.

Reactive power

You have the power and cos φ of a motor, or the figures from a rating plate, and want the real current and the powers. The calculator also gives the theoretical capacitance for a cos φ correction.