Battery capacity describes how much charge or energy a battery can provide, depending on how its capacity is expressed.
A battery may be rated in amp-hours (Ah), watt-hours (Wh), or milliamp-hours (mAh), depending on its size and application. To compare batteries accurately or determine how much capacity you need, you also need to consider voltage, load, usable capacity, and system efficiency.
A Battery Capacity Calculator simplifies these calculations by converting between Ah and Wh and helping estimate the battery capacity required for a specific electrical load.
Whether you are planning backup power for essential electronics, sizing a battery for an RV or camping setup, or comparing batteries for an off-grid system, understanding the relationship between Ah, Wh, and voltage is the foundation of accurate battery sizing.
Table of Contents
Quick Answer
Battery capacity can be expressed in either amp-hours (Ah) or watt-hours (Wh).
The basic relationship is:
Wh = Ah × V
To convert watt-hours into amp-hours:
Ah = Wh ÷ V
For example, a 12V 100Ah battery has:
100Ah × 12V = 1,200Wh
So its nominal energy capacity is 1,200Wh, or 1.2kWh.
However, nominal capacity is not necessarily the same as the amount of energy you can actually use. Depth of discharge, battery chemistry, age, temperature, and system efficiency can all affect usable energy.
For battery sizing, the general process is:
Load × Runtime = Required Energy
Then:
Required Ah = Required Wh ÷ Battery Voltage
The final battery size should account for the portion of the rated capacity that is actually usable and for losses between the battery and the connected load.
Battery Capacity Calculator
A Battery Capacity Calculator can help with two related but different calculations.
First, it can convert a battery’s rated capacity between **amp-hours (Ah) and watt-hours (Wh)** when the voltage is known.
Second, it can estimate the battery capacity required to operate a particular load for a target amount of time, while accounting for usable battery capacity and system efficiency.
These calculations should be kept separate because **Ah and Wh describe different aspects of a battery’s capacity**. Amp-hours measure electrical charge, while watt-hours account for both charge and voltage and therefore provide a more useful measure of nominal energy.
For example, consider two batteries:
– 100Ah at 12V
– 100Ah at 24V
Both batteries have the same amp-hour rating, but they do not have the same nominal energy capacity.
The 12V battery contains:
**100Ah × 12V = 1,200Wh**
The 24V battery contains:
**100Ah × 24V = 2,400Wh**
The second battery therefore has twice the nominal watt-hour capacity even though both batteries are rated at 100Ah.
This is why **voltage must be included when comparing battery capacity in Ah**.
Use the calculator below to convert Ah to Wh, convert Wh to Ah, or estimate the battery capacity needed for a specific load and runtime.
Use the calculator below to convert Ah to Wh, convert Wh to Ah, or estimate the battery capacity needed for a specific load and runtime.
Use the Battery Capacity Calculator
Calculate the battery capacity you need based on load, runtime, battery voltage, usable capacity, and system efficiency.
Calculation
Ah to Wh
Wh to Ah
Battery Capacity Formula
The fundamental battery-capacity formula is:
Wh = Ah × V
Where:
- Wh = watt-hours of nominal energy
- Ah = amp-hours of battery capacity
- V = nominal battery voltage
The formula can also be rearranged depending on what you need to find.
To Calculate Watt-Hours
Wh = Ah × V
Example:
100Ah × 12V = 1,200Wh
To Calculate Amp-Hours
Ah = Wh ÷ V
Example:
If you need a battery with 2,400Wh at 24V:
2,400Wh ÷ 24V = 100Ah
To Calculate Voltage
V = Wh ÷ Ah
This can be useful when the energy rating and amp-hour rating are known but the nominal voltage needs to be determined.
Why Watt-Hours Are Often Better for Battery Comparisons
Amp-hours describe electrical charge capacity, but they do not account for voltage.
Watt-hours include both variables and therefore provide a more useful way to compare batteries with different voltage ratings.
For example:
- 100Ah at 12V = 1,200Wh
- 50Ah at 24V = 1,200Wh
- 25Ah at 48V = 1,200Wh
The Ah ratings are different, but the nominal energy is the same.
This distinction becomes especially important when comparing portable power stations, RV batteries, home backup batteries, and battery banks operating at different system voltages.
Ah vs. Wh: What's the Difference?
Ah and Wh describe different aspects of a battery.
Amp-hours (Ah) measure charge capacity.
Watt-hours (Wh) measure energy.
A battery rated at 100Ah does not tell you exactly how much energy it stores until its voltage is known.
For this reason, comparing batteries solely by Ah can produce misleading results.
Amp-Hours (Ah)
Amp-hours indicate how much electrical charge a battery can deliver over time under specified conditions.
A simplified interpretation is that a 100Ah battery could theoretically deliver:
- 100A for 1 hour;
- 10A for 10 hours;
- 5A for 20 hours.
In practice, actual delivered capacity can differ because battery capacity depends on discharge conditions, temperature, chemistry, age, and other factors.
The Ah rating is therefore a useful battery specification, but it should not be treated as a universal measure of stored energy.
Watt-Hours (Wh)
Watt-hours express energy.
They combine the battery’s amp-hour rating with its voltage:
Wh = Ah × V
This makes Wh particularly useful when comparing batteries with different voltage ratings.
For example, a 100Ah 12V battery and a 100Ah 24V battery have very different nominal energy capacities.
That is why watt-hours are generally the more useful unit when determining how much energy a battery can provide to a load.
Why Ah Alone Can Be Misleading
Suppose you are comparing:
- Battery A: 100Ah, 12V
- Battery B: 80Ah, 24V
Battery A:
100 × 12 = 1,200Wh
Battery B:
80 × 24 = 1,920Wh
Although Battery B has a lower Ah rating, it contains substantially more nominal energy.
The voltage changes the energy represented by each amp-hour.
Quick Unit Conversions
A few simple unit conversions can make battery specifications easier to compare:
**1 Ah = 1,000 mAh**
**1 kWh = 1,000 Wh**
When a battery’s capacity is listed in milliamp-hours, you can convert it to watt-hours using:
**Wh = mAh × V ÷ 1,000**
How to Calculate Battery Capacity Step by Step
Calculating the required battery capacity is straightforward when the load, runtime, and battery voltage are known.
Step 1: Determine the Load
Start by identifying the device or appliances the battery needs to power.
For a single constant load, use its power rating in watts.
For multiple devices, add the loads that will operate at the same time.
For example:
- Router: 20W
- Laptop: 60W
- LED lighting: 40W
Total load:
20W + 60W + 40W = 120W
For variable or cycling appliances, use a realistic average power demand rather than automatically using the highest possible wattage.
If you need a more detailed explanation of measuring and calculating appliance consumption, our How to Calculate Power Consumption guide covers that process in greater detail.
Step 2: Determine the Required Runtime
Next, determine how long the battery needs to supply the load.
For example:
120W × 8 hours = 960Wh
The load therefore requires approximately 960Wh of energy over eight hours before accounting for efficiency losses and usable-capacity limits.
This is the point where battery capacity and battery runtime become related but distinct calculations.
The capacity calculation determines how much energy the battery needs to provide.
A runtime calculation determines how long a battery with a known capacity can support a given load.
We will keep those two intents separate so this article does not duplicate our dedicated Battery Runtime Calculator.
Step 3: Calculate Required Energy
The basic energy requirement is:
Required Wh = Load W × Runtime h
For a 120W load operating for eight hours:
120W × 8h = 960Wh
This 960Wh figure represents the theoretical energy requirement at the load.
It does not automatically mean that a 960Wh-rated battery is sufficient.
The battery may need additional nominal capacity because some of its rated energy may be unavailable for normal use and some energy may be lost through the inverter, converter, wiring, or other system components.
Step 4: Convert Required Wh to Ah
Once the required watt-hours are known, divide by the battery voltage:
Required Ah = Required Wh ÷ Battery Voltage
For a 960Wh requirement using a 12V battery:
960Wh ÷ 12V = 80Ah
The theoretical requirement is therefore 80Ah.
However, this is not necessarily the final battery size you should purchase.
The calculation has not yet accounted for usable depth of discharge or system efficiency.
Step 5: Account for Usable Capacity and Efficiency
A battery’s rated capacity represents its nominal specification, not necessarily the amount of energy that can be delivered to the load under every operating condition.
A simplified planning calculation can be expressed as:
Required Battery Wh = Load Wh ÷ (Usable Capacity Fraction × System Efficiency)
For example, suppose the load requires 960Wh, the usable capacity fraction is 80%, and system efficiency is 90%:
960 ÷ (0.80 × 0.90) = 1,333Wh
At 12V:
1,333Wh ÷ 12V ≈ 111Ah
So a simple 80Ah calculation would underestimate the nominal battery capacity needed under these assumptions.
The exact values should be based on the battery manufacturer’s specifications and the actual system configuration rather than generic assumptions.
Battery Capacity Example
Consider a backup setup that needs to operate a combined 200W load for 6 hours.
First calculate the energy requirement:
200W × 6h = 1,200Wh
The load therefore requires approximately 1,200Wh of energy.
If the system uses a 12V battery, the theoretical capacity requirement is:
1,200Wh ÷ 12V = 100Ah
So the basic calculation gives:
100Ah at 12V = 1,200Wh
But again, 100Ah is the theoretical minimum based on nominal capacity. If the system cannot use 100% of the battery’s rated capacity or has conversion losses, the actual battery should have a larger nominal capacity.
For example, if the planning assumptions are 80% usable capacity and 90% system efficiency:
1,200 ÷ (0.80 × 0.90) = 1,667Wh
At 12V:
1,667 ÷ 12 ≈ 139Ah
Under those assumptions, a battery around 140Ah at 12V would provide substantially more appropriate nominal capacity than a 100Ah battery.
Rated vs. Usable Battery Capacity
A battery’s nameplate capacity is not always equal to the amount of energy available to the load.
A battery rated at 1,200Wh may have less than 1,200Wh available for practical use because of depth-of-discharge limits, conversion losses, temperature, battery age, and operating conditions.
For battery sizing, it is therefore important to distinguish between rated capacity and usable capacity.
Depth of Discharge
Depth of discharge (DoD) describes how much of a battery’s capacity has been used.
For example, if a battery has a 1,000Wh rated capacity and 80% of that capacity is considered usable:
1,000Wh × 0.80 = 800Wh usable energy
This means the battery may provide approximately 800Wh under the specified conditions before reaching the manufacturer’s recommended limit.
The appropriate usable-capacity assumption depends on the battery chemistry and manufacturer specifications.
For lithium batteries, the permitted DoD may be substantially greater than for some lead-acid applications. However, the manufacturer’s stated operating limits should always take priority over a generic percentage.
Battery Efficiency and System Losses
The energy stored in a battery is not necessarily delivered to an AC appliance without losses.
An inverter, DC-DC converter, wiring, and other components can consume some of the available energy.
For example, if a battery provides 1,000Wh and the overall conversion efficiency is 90%:
1,000Wh × 0.90 = 900Wh
Approximately 900Wh would reach the load under that simplified assumption.
This is why a battery capacity calculation based only on Ah × V can overestimate the energy available to an actual appliance.
Battery Age and Temperature
Battery capacity can also change with operating conditions.
Cold temperatures can reduce available capacity, while high temperatures can accelerate battery degradation. As a battery ages and accumulates cycles, its maximum capacity can gradually decline.
For systems used outdoors, in RVs, camping setups, or emergency backup applications, these factors can become important when estimating real-world capacity.
Battery Capacity by Voltage
Battery voltage has a direct effect on the watt-hours represented by a given amp-hour rating.
The basic relationship remains:
Wh = Ah × V
This means that the same Ah rating represents different amounts of energy at different voltages.
12V Battery Systems
A 12V 100Ah battery has a nominal capacity of:
12V × 100Ah = 1,200Wh
A 12V system is common in smaller backup, RV, marine, and off-grid applications.
24V Battery Systems
A 24V 100Ah battery has:
24V × 100Ah = 2,400Wh
The Ah rating is identical to the 12V example, but the nominal energy capacity is twice as high.
48V Battery Systems
A 48V 100Ah battery has:
48V × 100Ah = 4,800Wh
Higher-voltage battery systems are commonly used where larger amounts of power need to be transferred efficiently.
The important point is that Ah should always be interpreted together with voltage when comparing battery capacity.
Battery Chemistry and Capacity
Battery chemistry affects how much of the rated capacity can be practically used and how the battery behaves under different loads.
The most common chemistries encountered in backup and portable power applications include lead-acid, AGM, and lithium iron phosphate (LiFePO4).
Lead-Acid Batteries
Lead-acid batteries have long been used for backup power, vehicles, RVs, and other applications.
Their usable capacity can depend significantly on discharge rate. At higher loads, the amount of energy that can actually be delivered may be lower than a simple nameplate calculation suggests.
AGM Batteries
AGM (absorbed glass mat) batteries are a type of valve-regulated lead-acid battery.
They are sealed and commonly used in backup power, RV, marine, and other applications where maintenance requirements and installation conditions matter.
Like other lead-acid batteries, AGM capacity can be affected by discharge rate and operating temperature.
LiFePO4 Batteries
Lithium iron phosphate batteries are widely used in portable power stations, RV systems, and off-grid applications.
They generally support deeper discharge than many traditional lead-acid systems and can provide a high usable proportion of their rated capacity.
However, the actual usable capacity still depends on the battery manufacturer’s specifications, temperature, discharge rate, and battery-management system.
Discharge Rate and Peukert’s Law
The rate at which a battery is discharged can affect how much of its rated capacity is actually available. This effect is particularly important with lead-acid batteries.
Peukert’s Law describes how higher discharge rates can reduce the usable capacity of certain batteries. In practical terms, a battery may deliver less usable energy when a high load is applied than its nameplate rating suggests.
For lithium batteries, including LiFePO4, the effect is generally much smaller, but actual performance still depends on the battery’s specifications, discharge rate, temperature, and battery management system.
For accurate battery sizing, use the manufacturer’s specified usable capacity rather than assuming that the full rated Ah or Wh capacity will always be available.
Battery Bank Capacity
A single battery is not always sufficient for a larger backup or off-grid system.
Multiple batteries can be combined into a battery bank to increase voltage, capacity, or both.
Batteries in Series
Connecting batteries in series increases voltage while keeping the Ah rating approximately the same, assuming identical batteries.
For example, two 12V 100Ah batteries connected in series produce approximately:
24V × 100Ah = 2,400Wh
The voltage doubles, while the amp-hour rating remains approximately 100Ah.
Batteries in Parallel
Connecting batteries in parallel increases the Ah capacity while maintaining approximately the same voltage.
Two 12V 100Ah batteries in parallel provide approximately:
12V × 200Ah = 2,400Wh
The voltage remains approximately 12V, while the capacity increases to 200Ah.
Series and Parallel Combinations
Larger battery banks can combine series and parallel connections to achieve the required system voltage and capacity.
For practical installations, batteries should be compatible and the manufacturer’s requirements for configuration, charging, protection, and balancing should be followed.
Common Battery Capacity Calculation Mistakes
Even simple battery formulas can produce misleading results when important variables are omitted.
Mistake 1: Comparing Batteries Only by Ah
A 100Ah battery does not automatically contain more energy than a 50Ah battery.
Voltage must also be considered.
For example:
100Ah × 12V = 1,200Wh
50Ah × 24V = 1,200Wh
Both batteries have the same nominal energy capacity.
Mistake 2: Assuming Rated Capacity Equals Usable Capacity
A battery rated at 1,000Wh does not necessarily provide 1,000Wh to the appliance.
Depth-of-discharge limits and system losses can reduce usable energy.
Mistake 3: Ignoring Inverter or Conversion Losses
If a battery powers AC appliances through an inverter, some stored energy is consumed by the conversion process.
This becomes especially important for long runtimes or relatively large loads.
Mistake 4: Using Maximum Appliance Wattage Without Context
A device’s maximum or startup demand is not always the same as its average operating consumption.
For variable loads, realistic energy consumption over the required period can produce a more useful battery-capacity estimate.
Mistake 5: Treating Battery Capacity as a Fixed Number
Capacity depends on operating conditions.
Temperature, discharge rate, battery age, chemistry, and system configuration can all affect the amount of energy actually available.
How Much Battery Capacity Do You Need?
The required battery capacity depends primarily on four inputs:
- Load in watts
- Required runtime
- Battery voltage
- Usable capacity and system efficiency
Start by calculating the energy required by the load:
Required Wh = Load W × Runtime h
Then convert that energy requirement into amp-hours:
Required Ah = Required Wh ÷ Battery Voltage
Finally, account for usable capacity and system losses.
A simplified sizing formula is:
Required Battery Wh = Load Wh ÷ (Usable Capacity × System Efficiency)
For example, suppose you need to operate a 300W load for 5 hours.
The load requires:
300W × 5h = 1,500Wh
Assume 80% usable battery capacity and 90% system efficiency:
1,500 ÷ (0.80 × 0.90) = 2,083Wh
At 12V:
2,083Wh ÷ 12V ≈ 174Ah
The theoretical battery requirement under these assumptions is therefore approximately 2,083Wh or 174Ah at 12V.
The final battery selection should then be checked against the manufacturer’s specifications, maximum continuous discharge rating, inverter requirements, and intended operating conditions.
Battery Capacity Calculator vs. Battery Runtime Calculator
These two calculations are closely related but answer different questions.
A Battery Capacity Calculator answers:
How much battery capacity do I have or need?
A Battery Runtime Calculator answers:
How long will a battery with a known capacity run my load?
For example, if you know that a system requires approximately 2,000Wh for a particular application, the Battery Capacity Calculator helps determine the appropriate battery size.
Once you know the battery’s usable capacity and the load, a runtime calculation can estimate how long that battery can operate the equipment.
This distinction is important because battery capacity and runtime are not interchangeable measurements
Frequently Asked Questions (FAQ)
How do you calculate battery capacity in Ah?
Battery capacity in amp-hours can be calculated from the required energy and battery voltage:
Ah = Wh ÷ V
For example, a 1,200Wh requirement at 12V is:
1,200Wh ÷ 12V = 100Ah
The actual battery may need a higher rated capacity after accounting for usable capacity and system losses.
How do you convert Ah to Wh?
Multiply the battery’s amp-hour rating by its nominal voltage:
Wh = Ah × V
For example, a 100Ah 12V battery has:
100 × 12 = 1,200Wh
This is the nominal energy capacity before accounting for usable-capacity limits and system losses.
How many Wh are in a 100Ah battery?
It depends on the battery voltage.
A 100Ah battery provides approximately:
- 1,200Wh at 12V
- 2,400Wh at 24V
- 4,800Wh at 48V
Therefore, Ah should not be compared without also considering voltage.
How many Ah do I need for 1,000Wh?
The answer depends on voltage.
At 12V:
1,000Wh ÷ 12V = 83.3Ah
At 24V:
1,000Wh ÷ 24V = 41.7Ah
At 48V:
1,000Wh ÷ 48V = 20.8Ah
These figures represent nominal capacity and do not include additional capacity needed for usable-capacity limits or system losses.
What is the difference between Ah and Wh in a battery?
Ah (amp-hours) measures electrical charge capacity, while Wh (watt-hours) measures energy.
Voltage connects the two:
Wh = Ah × V
Wh is generally more useful when comparing batteries with different voltage ratings because it accounts for both charge capacity and voltage.
Why does battery voltage matter when calculating capacity?
Voltage determines how much energy each amp-hour represents.
For example, 100Ah at 12V equals 1,200Wh, while 100Ah at 24V equals 2,400Wh.
This is why two batteries with the same Ah rating can have substantially different energy capacities.
How do you convert mAh to Wh?
Convert milliamp-hours to amp-hours first, or use the combined formula:
Wh = mAh × V ÷ 1,000
For example, a 5,000mAh battery at 3.7V has approximately:
5,000 × 3.7 ÷ 1,000 = 18.5Wh
This is particularly useful for comparing phone batteries, power banks, and other small battery-powered devices.
Is a 100Ah battery always the same size in terms of energy?
No.
A 100Ah battery’s nominal energy depends on its voltage.
A 100Ah 12V battery has approximately 1,200Wh, while a 100Ah 48V battery has approximately 4,800Wh.
The physical size, chemistry, and construction can also differ substantially between batteries.
What is the difference between rated and usable battery capacity?
Rated capacity is the battery’s nominal capacity under specified testing conditions.
Usable capacity is the amount of energy that can actually be extracted under the relevant operating limits.
Usable capacity can be reduced by depth-of-discharge limits, efficiency losses, temperature, battery age, and other operating conditions
How does depth of discharge affect battery capacity?
Depth of discharge determines what portion of the rated battery capacity is used.
For example, if a 1,000Wh battery is limited to 80% usable depth of discharge:
1,000Wh × 0.80 = 800Wh
The battery therefore provides approximately 800Wh within that operating window before considering additional system losses.
The appropriate limit depends on the battery chemistry and manufacturer’s specifications.
How much battery capacity do I need for a specific load?
First calculate the energy requirement:
Required Wh = Load W × Runtime h
Then convert the result into Ah:
Required Ah = Required Wh ÷ Battery Voltage
Finally, increase the nominal battery capacity as necessary to account for usable capacity and system efficiency.
For example, a 200W load running for 5 hours requires:
200W × 5h = 1,000Wh
At 12V, the theoretical requirement is:
1,000Wh ÷ 12V = 83.3Ah
The final battery should normally be larger than this theoretical minimum if the full rated capacity cannot be used or the system has conversion losses.
Can two batteries with different Ah ratings have the same energy capacity?
Yes.
For example:
100Ah × 12V = 1,200Wh
50Ah × 24V = 1,200Wh
Both batteries have the same nominal energy capacity even though their Ah ratings are different.
This is one reason Wh is often a better comparison metric than Ah when battery voltages differ.
Final Takeaway
A battery’s capacity is best understood through the relationship between amp-hours, voltage, and watt-hours.
The basic conversion is:
Wh = Ah × V
When sizing a battery for a specific application, start with the energy requirement:
Load W × Runtime h = Required Wh
Then convert the required energy into Ah using the battery voltage and allow for usable capacity and system losses.
The most important distinction is between nominal battery capacity and the amount of energy that can actually reach the load.
A Battery Capacity Calculator provides a useful starting point for these calculations, but the final battery selection should also be checked against the manufacturer’s specifications, discharge limits, operating conditions, and the requirements of the complete power system.
For permanent or higher-power installations, battery sizing should be verified against the actual electrical configuration and applicable installation requirements.