Battery Capacity Calculator
Estimate the battery capacity needed to run a device for your required amount of time. Enter the device power, desired runtime, battery voltage, and estimated efficiency to calculate the required battery capacity in Ah, mAh, and Wh.
Required Energy (Wh) = Power (W) × Runtime (hours) ÷ Efficiency
Required Capacity (Ah) = Required Energy (Wh) ÷ Battery Voltage
How Much Battery Capacity Do I Need?
The battery capacity you need depends on how much power your device uses and how long you want it to run. A device that consumes more power will require a larger battery. Similarly, a longer runtime requires more battery capacity.
Our Battery Capacity Calculator estimates the required battery size using your device's power consumption, required runtime, battery voltage, and estimated system efficiency.
How to Calculate Battery Capacity
First, determine how much power your device uses in watts (W) and how long you want the device to operate. The calculator then estimates the amount of energy the battery must provide.
Required Battery Energy (Wh)
Required Energy = Device Power × Runtime ÷ Efficiency
Once the required watt-hours are known, the required battery capacity in amp-hours can be calculated:
Required Capacity (Ah) = Required Energy (Wh) ÷ Battery Voltage
Battery Capacity Example
Suppose you need to run a 100W device for 4 hours using an 18V battery. If you estimate 85% system efficiency, the required energy is:
100W × 4 hours ÷ 0.85 = 470.6Wh
Converting that energy to amp-hours:
470.6Wh ÷ 18V = 26.1Ah
In this example, a battery of approximately 26Ah would be required under the stated conditions. In practical applications, choosing some additional capacity can provide a useful margin.
Ah, mAh and Wh: What's the Difference?
Battery capacity can be expressed in amp-hours (Ah), milliamp-hours (mAh), or watt-hours (Wh). These measurements are related but they do not mean exactly the same thing.
- mAh: Commonly used for small batteries, smartphones, power banks, and portable electronics.
- Ah: Commonly used for larger batteries such as automotive, solar, backup, and power-tool batteries.
- Wh: Measures energy and is useful when comparing batteries with different voltages.
To convert battery capacity from Ah to Wh:
Wh = Ah × Voltage
For example, an 18V 8Ah battery has approximately:
18 × 8 = 144Wh
Battery Capacity for Power Tools
Power-tool batteries are often rated in Ah, such as 2Ah, 4Ah, 5Ah, 8Ah, or 12Ah. A higher Ah rating generally means the battery can store more energy at the same voltage.
For example, an 18V 8Ah battery has approximately 144Wh of nominal energy. If a power tool or other device consumes 100W continuously, the theoretical runtime before efficiency losses would be about 1.44 hours. Real-world runtime can be lower because power tools do not always operate at a constant load.
Drills, grinders, saws, impact tools, and other motor-driven equipment can draw significantly more power during heavy work than during light operation.
How Battery Voltage Affects Capacity
Voltage is important when calculating battery capacity because the same Ah rating does not necessarily represent the same amount of stored energy at different voltages.
For example:
- 12V × 10Ah = 120Wh
- 18V × 10Ah = 180Wh
- 24V × 10Ah = 240Wh
This is why watt-hours are often more useful than Ah when comparing batteries with different voltage ratings.
Why Battery Efficiency Matters
A battery cannot normally deliver all of its stored energy to a device without losses. Energy can be lost through voltage conversion, wiring, electronics, battery resistance, heat, and other components.
For this reason, the calculator includes an efficiency setting. A typical estimate may use around 85%, but the actual value depends on the battery, device, power converter, and operating conditions.
How to Choose the Right Battery Capacity
Once you know the required capacity, consider the actual operating conditions before choosing a battery. If the device has a variable load, its average power consumption may be more useful than its maximum rating.
For applications where runtime is important, selecting a battery with some additional capacity can help compensate for efficiency losses, temperature changes, battery aging, and changes in load.
Battery Capacity Calculator FAQ
How do I calculate the battery capacity I need?
Multiply the device power consumption in watts by the required runtime in hours, divide by the estimated efficiency, and then divide by battery voltage to obtain the required capacity in Ah.
How many Ah do I need for a 100W device?
It depends on the required runtime and battery voltage. For example, a 100W device running for 4 hours at 18V and 85% efficiency requires about 26.1Ah.
How do I calculate battery capacity from watts?
You also need the desired runtime and battery voltage. Watts describe power consumption, while watt-hours describe the energy required over time. The calculator uses both values to estimate the required battery capacity.
Is a higher Ah battery always better?
A higher Ah rating generally provides more stored energy when comparing batteries at the same voltage. However, battery size, weight, charging requirements, current capability, compatibility, and cost should also be considered.
How do I calculate the capacity of an 18V battery?
For an 18V battery, multiply the capacity in Ah by 18 to estimate its energy in Wh. For example, an 18V 8Ah battery has approximately 144Wh of nominal energy.
Can I use this calculator for power-tool batteries?
Yes. The calculator can estimate required capacity for power-tool batteries, portable equipment, backup systems, and other battery-powered devices. Actual runtime for power tools can vary because motor loads change during use.
Battery Capacity Calculator
Estimate the battery capacity needed to run a device for your required amount of time. Enter the device power, desired runtime, battery voltage, and estimated efficiency to calculate the required battery capacity in Ah, mAh, and Wh.
Required Energy (Wh) = Power (W) × Runtime (hours) ÷ Efficiency
Required Capacity (Ah) = Required Energy (Wh) ÷ Battery Voltage
How Much Battery Capacity Do I Need?
The battery capacity you need depends on how much power your device uses and how long you want it to run. A device that consumes more power will require a larger battery. Similarly, a longer runtime requires more battery capacity.
Our Battery Capacity Calculator estimates the required battery size using your device's power consumption, required runtime, battery voltage, and estimated system efficiency.
How to Calculate Battery Capacity
First, determine how much power your device uses in watts (W) and how long you want the device to operate. The calculator then estimates the amount of energy the battery must provide.
Required Battery Energy (Wh)
Required Energy = Device Power × Runtime ÷ Efficiency
Once the required watt-hours are known, the required battery capacity in amp-hours can be calculated:
Required Capacity (Ah) = Required Energy (Wh) ÷ Battery Voltage
Battery Capacity Example
Suppose you need to run a 100W device for 4 hours using an 18V battery. If you estimate 85% system efficiency, the required energy is:
100W × 4 hours ÷ 0.85 = 470.6Wh
Converting that energy to amp-hours:
470.6Wh ÷ 18V = 26.1Ah
In this example, a battery of approximately 26Ah would be required under the stated conditions. In practical applications, choosing some additional capacity can provide a useful margin.
Ah, mAh and Wh: What's the Difference?
Battery capacity can be expressed in amp-hours (Ah), milliamp-hours (mAh), or watt-hours (Wh). These measurements are related but they do not mean exactly the same thing.
- mAh: Commonly used for small batteries, smartphones, power banks, and portable electronics.
- Ah: Commonly used for larger batteries such as automotive, solar, backup, and power-tool batteries.
- Wh: Measures energy and is useful when comparing batteries with different voltages.
To convert battery capacity from Ah to Wh:
Wh = Ah × Voltage
For example, an 18V 8Ah battery has approximately:
18 × 8 = 144Wh
Battery Capacity for Power Tools
Power-tool batteries are often rated in Ah, such as 2Ah, 4Ah, 5Ah, 8Ah, or 12Ah. A higher Ah rating generally means the battery can store more energy at the same voltage.
For example, an 18V 8Ah battery has approximately 144Wh of nominal energy. If a power tool or other device consumes 100W continuously, the theoretical runtime before efficiency losses would be about 1.44 hours. Real-world runtime can be lower because power tools do not always operate at a constant load.
Drills, grinders, saws, impact tools, and other motor-driven equipment can draw significantly more power during heavy work than during light operation.
How Battery Voltage Affects Capacity
Voltage is important when calculating battery capacity because the same Ah rating does not necessarily represent the same amount of stored energy at different voltages.
For example:
- 12V × 10Ah = 120Wh
- 18V × 10Ah = 180Wh
- 24V × 10Ah = 240Wh
This is why watt-hours are often more useful than Ah when comparing batteries with different voltage ratings.
Why Battery Efficiency Matters
A battery cannot normally deliver all of its stored energy to a device without losses. Energy can be lost through voltage conversion, wiring, electronics, battery resistance, heat, and other components.
For this reason, the calculator includes an efficiency setting. A typical estimate may use around 85%, but the actual value depends on the battery, device, power converter, and operating conditions.
How to Choose the Right Battery Capacity
Once you know the required capacity, consider the actual operating conditions before choosing a battery. If the device has a variable load, its average power consumption may be more useful than its maximum rating.
For applications where runtime is important, selecting a battery with some additional capacity can help compensate for efficiency losses, temperature changes, battery aging, and changes in load.
Battery Capacity Calculator FAQ
How do I calculate the battery capacity I need?
Multiply the device power consumption in watts by the required runtime in hours, divide by the estimated efficiency, and then divide by battery voltage to obtain the required capacity in Ah.
How many Ah do I need for a 100W device?
It depends on the required runtime and battery voltage. For example, a 100W device running for 4 hours at 18V and 85% efficiency requires about 26.1Ah.
How do I calculate battery capacity from watts?
You also need the desired runtime and battery voltage. Watts describe power consumption, while watt-hours describe the energy required over time. The calculator uses both values to estimate the required battery capacity.
Is a higher Ah battery always better?
A higher Ah rating generally provides more stored energy when comparing batteries at the same voltage. However, battery size, weight, charging requirements, current capability, compatibility, and cost should also be considered.
How do I calculate the capacity of an 18V battery?
For an 18V battery, multiply the capacity in Ah by 18 to estimate its energy in Wh. For example, an 18V 8Ah battery has approximately 144Wh of nominal energy.
Can I use this calculator for power-tool batteries?
Yes. The calculator can estimate required capacity for power-tool batteries, portable equipment, backup systems, and other battery-powered devices. Actual runtime for power tools can vary because motor loads change during use.