Select energy, voltage, current, or time to solve for, then enter the other three values and their units. The calculation assumes constant voltage and current.

Assumes constant voltage and current. Enter nonnegative magnitudes; actual usable battery energy depends on discharge conditions.

Battery Energy Formula

The battery energy calculator uses constant voltage, constant current, and elapsed time to calculate electrical energy. If voltage or current varies, integrate their product over time instead. The constant-value formula is:

E = V × I × t
  • E = battery energy in joules (J)
  • V = voltage in volts (V)
  • I = current in amperes (A)
  • t = time in seconds (s)

When you select voltage as the value to solve for, the calculator uses:

V = E / (I × t)

When you select current as the value to solve for, the calculator uses:

I = E / (V × t)

When you select time as the value to solve for, the calculator uses:

t = E / (V × I)

The calculator converts your selected units to base units before calculating. Voltage is converted to volts, current to amperes, time to seconds, and energy to joules. After the calculation, the result is converted to your selected result unit.

Battery Energy Unit Conversions

These are the unit conversions used for the inputs and results.

Quantity Unit Base unit conversion
Voltage mV 1 mV = 0.001 V
Voltage kV 1 kV = 1000 V
Current mA 1 mA = 0.001 A
Current kA 1 kA = 1000 A
Time min 1 min = 60 s
Time h 1 h = 3600 s
Energy Wh 1 Wh = 3600 J
Energy kWh 1 kWh = 3,600,000 J

Battery Energy Rating Notes

Battery or device example Voltage guidance Energy guidance
AA alkaline battery Use the cell rating Depends on load and cutoff voltage
Single lithium-ion cell Use the cell rating Use the cell Wh rating or specified discharge data
Phone battery Use the battery label Use the battery Wh rating
Laptop battery Use the pack rating Use the pack Wh rating
12 V lead-acid battery 12 V Varies by amp-hour rating

Example Calculations

Example 1: Calculate battery energy

You have a 12 V battery supplying 2 A for 3 hours. Find the energy in watt-hours.

First convert time to seconds if using the joule formula:

3 h = 10,800 s

E = 12 × 2 × 10800 = 259200 J

Convert joules to watt-hours:

Wh = 259200 / 3600 = 72 Wh

The battery energy is 72 Wh.

Example 2: Calculate runtime

A 24 V battery provides 5 A, and the available energy is 240 Wh. Find the time in hours.

For watt-hours, you can use power in watts:

P = V × I = 24 × 5 = 120 W
t = 240 Wh / 120 W = 2 h

The runtime is 2 hours.

FAQ

What is the difference between joules and watt-hours?

Joules and watt-hours both measure energy. Joules are the SI unit. Watt-hours are common for batteries because they are easier to read for electrical devices used over time. The conversion is:

1 Wh = 3600 J

Can this calculator estimate battery runtime?

Yes. Select Time, then enter constant voltage, constant current, and available battery energy. The result assumes constant power. Real runtime also depends on discharge conditions, conversion losses and cutoff limits.

How do I choose which value to calculate?

Choose a value in Solve for. Only the three required inputs and the result unit are shown. Zero factors can make inverse calculations nonunique or impossible; the calculator explains those cases rather than dividing by zero.