Calculate Battery Energy Capacity
Enter your battery specifications and expected load below.
What Is a Battery Energy Calculator?
A Battery Energy Calculator converts battery voltage and amp-hour capacity into stored electrical energy. The result is commonly expressed in watt-hours (Wh) or kilowatt-hours (kWh).
The calculator can also account for battery quantity, usable capacity and system efficiency to estimate how much energy may actually be available to a connected load.
Battery Energy Formula
Battery energy can be estimated by multiplying battery voltage by amp-hour capacity.
Battery Energy (kWh) = Voltage × Ah ÷ 1,000
For multiple identical batteries, the nominal stored energy is multiplied by the number of batteries, assuming the bank configuration preserves the combined total energy.
How to Use the Battery Energy Calculator
- Enter the nominal voltage of one battery or the applicable battery-bank voltage.
- Enter battery capacity in amp-hours or milliamp-hours.
- Enter the number of identical batteries.
- Enter the percentage of nominal energy you intend to use.
- Enter estimated system efficiency.
- Optionally enter a connected load in watts or kilowatts.
- Click Calculate Battery Energy.
The calculator displays nominal energy, usable battery energy, delivered energy after losses, total amp-hour capacity and an estimated runtime.
Battery Energy Calculation Example
Suppose you have one 12-volt battery rated at 100 Ah. The nominal stored energy is:
= 1,200 Wh
= 1.2 kWh
If only 80% of the nominal capacity is intended to be used, the usable battery energy before additional system losses is approximately 960 Wh.
What Is the Difference Between Ah and Wh?
Amp-Hours
Amp-hours measure electric charge capacity. Ah alone does not fully describe stored energy because battery voltage also matters.
Watt-Hours
Watt-hours measure electrical energy and include both battery voltage and amp-hour capacity.
Kilowatt-Hours
One kilowatt-hour equals 1,000 watt-hours. Larger battery banks are commonly described using kWh.
How to Convert Ah to Wh
To convert amp-hours into watt-hours, multiply Ah by the battery voltage.
For example, 200 Ah at 12 volts equals 2,400 Wh, while 200 Ah at 48 volts equals 9,600 Wh. This shows why voltage must be known when converting Ah to energy.
How to Convert Wh to kWh
Converting watt-hours to kilowatt-hours is straightforward. Divide watt-hours by 1,000.
For example, 4,800 Wh is equal to 4.8 kWh.
What Is Usable Battery Capacity?
The full nameplate capacity of a battery is not always intended to be used during every discharge cycle. Battery chemistry, manufacturer recommendations, reserve requirements and desired battery life can affect the usable energy.
For example, a 10 kWh battery used to 80% of its nominal capacity provides approximately 8 kWh before considering additional system losses.
Battery Efficiency and Delivered Energy
Power conversion and battery-system losses can reduce the energy that ultimately reaches a load. The calculator includes an optional efficiency factor for a simplified delivered energy estimate.
For example, 8 kWh of usable stored energy at 90% system efficiency gives approximately 7.2 kWh of estimated delivered energy.
Battery Runtime Formula
If the connected load is known, battery runtime can be estimated by dividing delivered battery energy by the load power.
For example, 1,000 Wh of usable delivered energy powering a 250-watt load gives a theoretical runtime of about four hours.
Common Battery Energy Examples
| Battery | Nominal Energy |
|---|---|
| 12 V, 50 Ah | 600 Wh / 0.6 kWh |
| 12 V, 100 Ah | 1,200 Wh / 1.2 kWh |
| 12 V, 200 Ah | 2,400 Wh / 2.4 kWh |
| 24 V, 100 Ah | 2,400 Wh / 2.4 kWh |
| 48 V, 100 Ah | 4,800 Wh / 4.8 kWh |
| 48 V, 200 Ah | 9,600 Wh / 9.6 kWh |
Batteries in Series and Parallel
Connecting identical batteries in series increases the total bank voltage while the amp-hour capacity remains the same. Connecting identical batteries in parallel keeps the voltage the same while increasing the total amp-hour capacity.
In either case, the total ideal stored energy is the sum of the energy stored in the individual batteries, assuming identical batteries and an appropriate configuration.
12V vs 24V vs 48V Battery Energy
For the same amp-hour capacity, a higher-voltage battery bank stores more energy. For example, a 100 Ah battery bank at 48 V stores four times the nominal energy of a 100 Ah bank at 12 V.
| Voltage | Capacity | Energy |
|---|---|---|
| 12 V | 100 Ah | 1.2 kWh |
| 24 V | 100 Ah | 2.4 kWh |
| 48 V | 100 Ah | 4.8 kWh |
Factors That Affect Real Battery Energy
Temperature
Very high or low temperatures can change battery performance, available capacity and overall efficiency.
Discharge Rate
The usable capacity of some battery chemistries changes with the rate at which current is drawn.
Battery Age
Battery capacity generally declines with cycling, age and operating conditions.
Battery Energy for Solar Systems
Battery energy calculations are useful when sizing energy storage for solar PV systems, backup power systems and off-grid installations. The required storage capacity depends on load energy, backup duration, usable battery capacity and system losses.
Battery energy sizing should be considered separately from maximum power output. A battery may have enough kWh for the required runtime but still need sufficient current and power capability to operate high-power loads.
Frequently Asked Questions
How do I calculate battery energy?
Multiply the battery voltage by its amp-hour capacity. For example, a 12 V 100 Ah battery stores approximately 1,200 Wh or 1.2 kWh of nominal energy.
How many kWh is a 12V 100Ah battery?
A 12 V 100 Ah battery has approximately 1,200 Wh of nominal energy, which equals 1.2 kWh.
How many kWh is a 48V 100Ah battery?
A 48 V 100 Ah battery stores approximately 4,800 Wh or 4.8 kWh of nominal energy.
What is the difference between Ah and kWh?
Ah measures electric charge capacity, while kWh measures energy. Battery voltage is required to convert amp-hours into watt-hours or kilowatt-hours.
How do I calculate battery runtime?
Divide usable delivered battery energy in watt-hours by the connected load in watts. This gives an approximate runtime in hours.
Does a 100Ah battery always provide the same energy?
No. A 100 Ah battery at 12 volts stores less energy than a 100 Ah battery bank at 24 or 48 volts. Voltage must be included when calculating energy.
Is calculated battery runtime exact?
No. The calculation is an estimate. Actual runtime depends on battery chemistry, discharge current, temperature, battery condition, inverter losses and other operating conditions.