Calculate battery energy, voltage, current, or time from any three values with unit conversions for V, A, s, J, Wh, and kWh as needed.
- All Physics Calculators
- All Energy Calculators
- Battery Energy Density Calculator
- Battery Reserve Capacity to Amp Hours Calculator
- Battery Run Time Calculator
- Battery Charge Calculator
- Battery Voltage Calculator
- Battery Voltage Percentage Calculator
Battery Energy Formula
The battery energy calculator uses voltage, current, and time to calculate electrical energy. The base 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)
If you leave voltage blank, the calculator rearranges the formula as:
V = E / (I * t)
If you leave current blank, the calculator uses:
I = E / (V * t)
If you leave time blank, 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 back to the unit you selected for the missing value.
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 |
Common Battery Energy Reference Values
| Battery or device example | Typical voltage | Typical energy range |
|---|---|---|
| AA alkaline battery | 1.5 V | About 2 to 4 Wh |
| Single lithium-ion cell | 3.6 to 3.7 V | About 7 to 15 Wh |
| Phone battery | 3.7 to 3.85 V | About 10 to 20 Wh |
| Laptop battery | 11.1 to 15.4 V | About 40 to 100 Wh |
| 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. Leave the time field empty, then enter voltage, current, and battery energy. The result gives the time based on a constant current draw. Real runtime can be lower because battery voltage changes during discharge and devices are not always 100% efficient.
Why do I need to leave exactly one field empty?
The formula has four variables: energy, voltage, current, and time. If you provide any three of them, the missing fourth value can be calculated. If more than one field is empty, there is not enough information to solve the equation.
