Battery Backup Calculator

Find out how long your battery can power your essential devices during a power outage.

Select your battery type and voltage, enter its capacity in ampere hours (Ah), and add the total running load in watts. The calculator will estimate the total battery energy, usable energy, and expected backup time.

Actual backup time may vary depending on battery condition, inverter losses, temperature, and changes in equipment load.

What does the battery backup calculator show?

The calculator provides three main results: estimated backup time, total battery energy, and usable battery energy.

Estimated backup time

Backup time indicates approximately how many hours your battery can support the connected load listed in the calculator. A larger battery typically provides longer backup. A larger electrical load uses up the available battery energy more quickly. The calculation assumes that the load remains constant throughout the backup period.

Total battery energy

The total energy of a battery is calculated from the battery voltage and ampere-hour capacity.

The formula is:

Total energy (kWh) = Battery voltage × Battery Ah ÷ 1,000

For example, a 12V 200Ah battery is:

12 × 200 ÷ 1,000 = 2.4 kWh

This is the nominal energy of the battery. The amount available for normal use may be less.

Usable battery energy

Usable energy is the portion of the total battery energy available after applying the Depth of Discharge (DoD) used by the calculator.

The calculator currently uses:

•  Lithium-ion battery: 85% DoD
•  Tubular lead-acid battery: 55% DoD

For example, a 2.4 kWh lithium battery using 85% DoD would provide:

2.4 × 0.85 = 2.04 kWh usable energy

These percentages are used for calculations and may not match every battery on the market. Before deciding how deeply to discharge a particular battery, check the battery manufacturer’s specifications.

How to use the battery backup calculator?

1. Select your battery type.

Select the battery technology you are using:

•  Lithium ion
•  Tubular lead acid

The battery type matters because the calculator uses a different depth of discharge for each option.

2. Select battery voltage.

Choose your battery or battery bank voltage:

•  12V
•  24V
•  48V

Voltage alone does not determine backup time. It must be considered in conjunction with the battery capacity in Ah.

3. Enter the battery capacity in Ah.

Enter the ampere-hour rating shown on the specifications of your battery or battery bank.

For example:

12V 200Ah

This means that the battery has a nominal voltage of 12 volts and a rated capacity of 200 ampere hours.

4. Enter your attached load.

Add up the running wattage of all the devices you want to run on battery.

Common backup loads

These may include:

•  Fan
•  LED lights
•  WiFi router
•  Television
•  Refrigerator
•  Computer
•  Other necessary equipment

The combined running load in watts, not the maximum power rating of your inverter. For example, if your appliances use 500W when running, enter 500W even if your inverter is rated at 1,000W or more. The higher the connected load, the faster the battery will discharge.

5. Calculate your battery backup.

Select Calculate Backup :

•  Estimated backup time
•  Usable battery energy
•  Total battery energy

You can then change the battery capacity or load to compare different setups.

How is battery backup time calculated?

The calculator uses battery voltage, Ah capacity, depth of discharge and connected load.

Step 1: Calculate the total battery energy.

The first calculation converts battery voltage and Ah to kilowatt hours:

Battery voltage × Ah ÷ 1,000 = Total energy (kWh)

Step 2: Calculate usable battery energy.

The corresponding depth of discharge is then applied:

Total energy × DoD = Usable energy

The calculator uses:

For 85% DoD lithium ion

And

55% DoD for tubular lead acid

Step 3: Calculate the backup time.

The connected load is converted from watts to kilowatts.

For example:

500W = 0.5 kW

Backup time is calculated as follows:

Backup time = Usable battery energy ÷ Load in kWh

If a battery has 2 kWh of usable energy and the connected load is 500W:

2 ÷ 0.5 = 4 hours

Battery backup example: 12V 200Ah lithium battery

Suppose you have:

• Battery type: Lithium-ion
• Voltage: 12V
• Capacity: 200Ah
• Connected load: 500W
• Depth of discharge: 85%

Calculate the total battery energy.

12 × 200 ÷ 1,000 = 2.4 kWh

Calculate usable energy.

2.4 × 0.85 = 2.04 kWh

Calculate backup time.

A 500W load is equivalent to:

500 ÷ 1,000 = 0.5 kW

So:

2.04 ÷ 0.5 = 4.08 hours

The estimated backup time is approximately 4.08 hours. Real-world runtime may be lower because the calculation does not separately account for inverter losses or adjust for battery age, temperature, and changing equipment loads.

Lithium vs. Tubular Battery Backup

Let’s compare lithium-ion and tubular lead-acid batteries using the same voltage, Ah capacity, and load. Both examples use:

• Voltage: 12V
• Capacity: 200Ah
• Load: 500W

Lithium-ion battery

85% DoD usage:

Total energy: 2.4 kWh

2.4 × 0.85 = 2.04 kWh usable

Estimated backup:

2.04 ÷ 0.5 = 4.08 hours

Tubular lead acid battery

Using 55% DoD:

Total energy: 2.4 kWh

2.4 × 0.55 = 1.32 kWh usable

Estimated backup:

1.32 ÷ 0.5 = 2.64 hours

Under these calculator settings, a lithium battery delivers more usable energy than the same nominal voltage and Ah capacity. This comparison reflects the 85% and 55% DoD values used by the calculator . Individual battery specifications may vary.

What do V, Ah and kWh mean?

Understanding these three measurements makes calculating battery backup much easier.

Battery voltage (V)

Voltage describes the electrical potential of a battery system. Typical battery backup systems can operate on:

•  12V
•  24V
•  48V

Voltage alone doesn’t tell you how much energy a battery can store.

Ampere hours (Ah)

Ampere hours indicate the rated electrical charge capacity of a battery under specific test conditions. When the voltage remains the same, a higher Ah rating generally means more stored energy. 

For example:

A 12V 200Ah battery has more nominal energy than a 12V 100Ah battery.

Kilowatt hours (kWh)

Kilowatt hours measure energy.

The basic conversion used in the calculator is:

Voltage × Ah ÷ 1,000 = kWh

For example:

12V × 200Ah ÷ 1,000 = 2.4 kWh

Converting battery capacity to kWh makes it easier to compare the stored energy to the power your devices need.

What is the depth of discharge of a battery?

Depth of discharge, usually written as DoD , indicates how much of the battery’s nominal capacity is used during discharge.

Suppose a battery stores:

2.4 kWh nominal energy

If the calculation allows for 85% DoD :

2.4 × 0.85 = 2.04 kWh

Therefore, the usable energy will be 2.04 kWh .

The calculator selects a DoD according to the battery type, but the actual recommended discharge limits vary between battery models. Always check the manufacturer’s specifications when working with a particular battery size or function.

How does connected load affect battery backup?

The amount of power your devices use has a direct impact on backup time. A large load drains the available battery energy more quickly.

For example, suppose a battery provides approximately:

4 hours of backup at 500W

If the load increases:

1,000W

A usable battery will provide approximately the same amount of energy:

2 hour backup

This simple relationship shows why turning off unnecessary devices can significantly extend battery runtime.

Running Load vs. Startup Surge

Battery backup time is primarily determined by the continuous or running load. However, some devices require a lot of power briefly when starting up.

Devices that may have a high startup load.

These may include:

•  Refrigerators
•  Water pumps
•  Air conditioner
•  Electric motors

This short burst of extra power is known as startup or surge power. An inverter should be able to handle surges even if the normal operating wattage of the appliances is very low. For example, your battery may have enough energy to run an appliance for several hours, but a small inverter may still be unable to start it.

Water pumps are commonly used for agricultural and solar-powered irrigation. Since pumps require both operating power and additional startup power, it is essential to understand energy consumption and backup requirements when planning a solar irrigation system. You can use our solar irrigation calculator to estimate the energy needs for irrigation purposes.

Why is my actual battery backup time different?

Real battery systems do not operate under perfectly defined conditions. Several factors can cause your actual runtime to differ from the calculated data.

Inverter performance

An inverter converts DC electricity from a battery into AC electricity for home appliances. Some energy is lost during this transformation. The calculator does not apply a separate inverter efficiency factor, so the actual AC runtime may be less than the results shown.

Battery age and condition

Battery capacity may decrease as the battery ages. An old or degraded 200Ah battery can no longer store as much usable energy as a new 200Ah battery. The calculator uses the Ah capacity you enter and does not make additional adjustments for battery health.

Temperature

Battery performance may change at higher or lower operating temperatures. Temperature correction is not included in the calculation.

Changing the load of the equipment

Not every device uses the same amount of electricity consistently. Refrigerators, inverter air conditioners, and water pumps can turn on and off or change their power consumption while operating. The calculator uses the constant wattage entered instead of modeling these changes over time.

Lead acid discharge rate

The capacity of a lead-acid battery can be affected by how quickly the battery is discharged. Therefore, heavy loads may result in different runtimes based on primary voltage × Ah.

Battery and Inverter Cutoff Settings

Battery management systems and inverter low-voltage protection can prevent battery discharge before all theoretical usable energy is used. These protection settings can also affect the actual backup time.

How to calculate backup with multiple batteries.

The capacity of a battery-bank depends on whether the batteries are connected in series or parallel. The calculator does not have a separate field for the number of batteries, so you must first determine the effective battery bank voltage and Ah capacity.

Batteries connected in series

A series connection increases the voltage.

For example, two identical 12V 200Ah batteries in series produce: 24V 200Ah.

Batteries connected in parallel

A parallel connection increases the Ah capacity.

Two identical 12V 200Ah batteries in production: 12V 400Ah.

Do not multiply both voltage and Ah by the number of batteries. For more complex battery banks, calculate the effective bank specifications before entering values into the calculator.

Does the calculator include inverter efficiency?

No. The calculation uses:

•  Battery voltage
•  Battery Ah
•  Battery type DoD
•  Attached load

A separate inverter does not have a performance loss. Since real inverters are not 100% efficient, your actual AC backup time may be less than the calculated figure.

Does the calculator include solar charging?

No. The calculator estimates how long the energy already stored in the battery can support your connected load.

This does not include:

•  Solar charging during shutdowns
•  Battery recharge time
•  Transforming solar energy generation
•  Charge controller performance

If your solar panels continue to charge the battery while appliances are running, your actual operating time may be longer than the battery-only estimate.

What is not included in the battery backup calculation?

The calculator is intentionally focused on basic battery runtime. It is not calculated separately:

•  Inverter performance
•  Battery health status
•  The battery is aging.
•  Temperature correction
•  Lead-acid pecukart effect
•  Variable equipment load
•  Increase in startup
•  Battery management cutoff
•  Battery recharge time
•  Solar charging during discharge
•  Detailed series/parallel battery-bank configuration

These factors explain why the actual backup time may differ from the results shown online.

Who should use a battery backup calculator?

This calculator can help:

•  Domestic UPS users
•  Solar battery owners
•  Homeowners are planning backups for essential appliances.
•  Shops and offices
•  People comparing lithium and tubular batteries
•  Users considering a larger battery
•  People are deciding which devices to run during the shutdown.

This is especially useful when you already know the specifications of your battery and want a quick estimate of how long it can support a particular load.