To convert amp-hours to watt-hours, multiply the amp-hour capacity by the battery’s nominal voltage:
Watt-hours = amp-hours × nominal volts
For example, a battery rated 100Ah at 12.8V has a calculated nominal energy of:
100 × 12.8 = 1,280Wh
This conversion makes batteries with different voltages easier to compare. However, nominal watt-hours do not tell you exactly how much energy reaches your appliances or which appliances the battery can operate.
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Considering a DIY power guide? Review the Ultimate Off-Grid Generator offer and check whether its battery specifications identify voltage, capacity and usable energy. We have not independently verified its contents or performance.
What do Ah, Wh and kWh mean?
Amp-hours, or Ah, describe electrical charge capacity. They become useful for comparing energy only when voltage is also known.
Watt-hours, or Wh, describe energy. They allow a more direct comparison between batteries operating at different voltages.
Kilowatt-hours, or kWh, are a larger energy unit:
1 kWh = 1,000 Wh
These units are different from watts, which describe power.
| Unit | What it describes | Example |
|---|---|---|
| A | Current | Current flowing from a battery |
| Ah | Charge capacity | A battery labeled 100Ah |
| W | Power | An appliance drawing 100W |
| Wh | Energy | A 100W load operating for five hours uses 500Wh |
| kWh | Energy in thousands of watt-hours | 2,000Wh equals 2kWh |
For the complete battery selection process, see our off-grid battery bank guide.
The three useful conversion formulas
Convert amp-hours to watt-hours
Wh = Ah × V
Example:
100Ah × 25.6V = 2,560Wh
Convert watt-hours to amp-hours
Ah = Wh ÷ V
Example:
2,560Wh ÷ 25.6V = 100Ah
Convert watt-hours to kilowatt-hours
kWh = Wh ÷ 1,000
Example:
2,560Wh ÷ 1,000 = 2.56kWh
Use the manufacturer’s stated nominal voltage. If the product already lists nominal energy in Wh or kWh, compare that specification and its test conditions directly.
Why voltage changes the comparison
Two batteries rated 100Ah can store very different amounts of energy.
| Nominal voltage | Capacity | Calculated nominal energy |
|---|---|---|
| 12V | 100Ah | 1,200Wh |
| 12.8V | 100Ah | 1,280Wh |
| 24V | 100Ah | 2,400Wh |
| 25.6V | 100Ah | 2,560Wh |
| 48V | 100Ah | 4,800Wh |
| 51.2V | 100Ah | 5,120Wh |
These are arithmetic examples, not interchangeable product specifications.
A 51.2V, 100Ah battery has four times the calculated nominal energy of a 12.8V, 100Ah battery. It also requires equipment compatible with its voltage and operating characteristics.
Do not choose the higher-voltage battery simply because its energy figure is larger.
Use nominal voltage—not charging voltage
A battery’s terminal voltage changes during charging and discharging.
For the basic nameplate conversion, use its stated nominal voltage. Multiplying amp-hours by a temporary charging voltage can overstate nominal energy.
For example, if a battery is specified as 12.8V, 100Ah, use:
12.8 × 100 = 1,280Wh
Do not substitute a higher charger setting to make the capacity appear larger.
The simple calculation is a nominal estimate. Actual energy delivered during discharge depends on how voltage and current change over time.
Convert milliamp-hours before calculating
Portable batteries and power banks may advertise capacity in milliamp-hours, written as mAh.
1,000mAh = 1Ah
Therefore:
Wh = (mAh ÷ 1,000) × nominal V
For a hypothetical battery labeled 20,000mAh at 3.7V:
20,000 ÷ 1,000 = 20Ah
20 × 3.7 = 74Wh
Do not multiply by a USB output voltage unless the capacity rating explicitly applies at that voltage. The internal battery voltage and output voltage can differ.
If the label gives watt-hours, that is usually the clearer starting point for an energy comparison.
Nominal energy is different from usable energy
The Ah-to-Wh calculation does not establish how much energy your appliances will receive.
Relevant factors include:
- The permitted operating range.
- Any reserve you choose to retain.
- Battery condition and temperature.
- Discharge conditions.
- Conversion losses.
- Inverter and monitoring consumption.
A hypothetical usable-energy example
Start with a nominal 1,280Wh battery.
Assume, only for illustration:
- A planned usable fraction of 80%.
- A battery-to-appliance conversion factor of 90%.
Then:
1,280 × 0.80 × 0.90 = 921.6Wh
That is approximately 922Wh at the appliance boundary under those assumptions, before any separately excluded consumption.
Neither percentage is universal. If a manufacturer already provides usable AC energy under relevant conditions, do not subtract the same allowances again.
Our battery backup runtime guide explains how to turn usable energy into a preliminary operating-time estimate.
Before buying a guide or equipment, check the current offer’s specifications. A useful runtime claim should identify the load, battery configuration and test conditions.
Do not confuse amp-hours with maximum amps
A battery labeled 100Ah is not automatically permitted to deliver 100 amps continuously.
Capacity and current limits are separate specifications.
Check the manufacturer’s:
- Maximum continuous discharge current.
- Short-duration current limit and permitted duration.
- Charging-current limits.
- Battery-management restrictions.
Similarly, a 1,000Wh battery is not automatically capable of operating a 1,000W appliance.
Energy describes the available quantity. Power and current limits determine how quickly it can be delivered.
Compare chemistry using documented conditions
The arithmetic formula remains the same across battery chemistries, but usable performance can differ.
A capacity rating may be tied to a particular discharge rate, temperature and endpoint voltage. Compare the conditions as well as the headline number.
Our LiFePO4 versus lead-acid guide explains why equal nominal watt-hours may not provide equal service.
For an existing battery, the original label also does not establish its present capacity. Follow an appropriate assessment procedure rather than assuming age has had no effect.
How battery-bank arrangements affect the figures
For identical compatible batteries in an approved arrangement:
- Series connection adds voltage while amp-hour capacity remains the same.
- Parallel connection adds amp-hour capacity while voltage remains the same.
Consider two hypothetical 12.8V, 100Ah batteries:
| Conceptual arrangement | Bank rating | Nominal energy |
|---|---|---|
| Series | 25.6V, 100Ah | 2,560Wh |
| Parallel | 12.8V, 200Ah | 2,560Wh |
Both arrangements contain the same total calculated energy because both contain the same two batteries.
This is an explanation of ratings, not wiring instructions. Only use arrangements explicitly supported by the battery manufacturer and the complete system design.
If you are planning a new installation, our DIY battery backup for home guide covers the compatibility questions.
Watt-hours also help with charging calculations
Knowing the energy used helps you estimate what must be replenished.
However, energy taken from the charging source will generally exceed the energy restored to storage because the charging path has losses.
Do not calculate recharge time from amp-hours alone when the charger and battery operate at different voltages.
Our charging off-grid batteries guide explains how source power, ongoing loads and charging limits affect the result.
A battery-label worksheet
Complete this for each product you compare:
| Specification | Your entry |
|---|---|
| Exact model | _____ |
| Nominal voltage | _____ V |
| Capacity | _____ Ah |
| Calculated nominal energy | _____ Wh |
| Manufacturer-stated energy | _____ Wh |
| Rating conditions | _____ |
| Documented usable energy | _____ |
| Continuous output limit | _____ |
| Required charger and operating range | _____ |
If the calculated energy differs from the manufacturer’s stated figure, check rounding, nominal voltage and rating conditions. Ask the supplier to explain a material discrepancy.
Common conversion mistakes
Comparing amp-hours without voltage
“200Ah is bigger than 100Ah” does not establish which battery stores more energy.
Treating nominal energy as guaranteed runtime
Runtime depends on usable energy and actual demand.
Adding both voltage and amp-hours in a series calculation
For identical batteries in series, voltage adds; amp-hours do not also add.
Using charging voltage for the nameplate calculation
Use the specified nominal voltage.
Ignoring battery condition
The original rating describes specified performance, not a guarantee that an aging battery retains that capacity.
Use our battery maintenance checklist for routine care. If runtime changes unexpectedly, follow the battery backup troubleshooting guide.
Any system supplying household circuits also needs an appropriate installation; see connecting backup power to a home.
Considering the Ultimate Off-Grid Generator offer? Use these conversions to assess any documented battery claims. We cannot confirm its storage capacity or runtime from independently verified testing.
Frequently asked questions
How many watt-hours are in a 100Ah battery?
You need its nominal voltage. At 12V, the calculation is 1,200Wh. At 12.8V, it is 1,280Wh. At 25.6V, it is 2,560Wh.
How many amp-hours is 1kWh?
It depends on voltage. Since 1kWh is 1,000Wh, divide 1,000 by the nominal voltage. At 12.8V, the result is approximately 78.1Ah.
Is a 200Ah battery twice the capacity of a 100Ah battery?
At the same nominal voltage, its calculated nominal energy is twice as large. Usable performance still depends on the products and operating conditions.
Can I calculate watt-hours from amp-hours alone?
No. You also need voltage.
Does 1,000Wh mean a battery can run a 1,000W appliance for one hour?
That is an ideal energy relationship. Real operation also requires adequate output capability and sufficient usable energy after losses and operating restrictions.
Sources and further reading
- Victron Energy: Lithium NG Battery Voltage, Capacity and Energy Specifications
- Victron Energy: Battery Installation and Approved Connection Arrangements
Manufacturer specifications apply to their identified products. Examples in this article explain calculations and are not product recommendations.
This article provides general educational information, not battery wiring instructions. Follow equipment documentation and obtain qualified assistance for installation or changes to a battery bank.