What the battery runtime calculator estimates
This battery runtime calculator estimates how long a battery may power a connected device or group of devices. It can be used for routers, laptops, LED lighting, televisions, heating controls, circulation pumps, chargers and small backup systems operating through an inverter or uninterruptible power supply.
The calculation goes beyond simply multiplying battery voltage by amp-hour capacity. It also accounts for inverter or UPS losses, the portion of the battery capacity that can reasonably be used, and an optional safety margin for loads that may consume slightly more power than their stated rating.
For example, a 12 V 100 Ah battery contains a nominal 1,200 Wh of energy. That does not mean a 100 W appliance will always operate for 12 hours. Conversion losses, battery discharge limits, inverter self-consumption and the condition of the battery can reduce the available runtime considerably.
- Convert battery voltage and amp-hours into nominal energy in watt-hours
- Estimate usable energy after inverter losses and depth-of-discharge limits
- Add a safety margin to the connected load
- Estimate battery runtime in hours and minutes
- Calculate the battery capacity required for a target runtime
- Compare batteries and battery banks with different voltages and amp-hour ratings
Information you need for the calculation
To estimate runtime, enter the total connected load in watts, battery system voltage, battery capacity in amp-hours, expected inverter or UPS efficiency, usable depth of discharge and a load safety margin. You can also enter a target runtime to estimate the battery capacity that may be required.
Battery runtime calculator inputs
| Input | What it means | What to check |
|---|---|---|
| Connected load | The combined power demand of the devices in watts | Use measured consumption where possible, or add a reasonable margin to rated values |
| Battery system voltage | The nominal voltage of the battery or battery bank, such as 12 V, 24 V or 48 V | Two 12 V batteries connected in series form a 24 V bank while the amp-hour capacity remains unchanged |
| Battery capacity | The rated capacity of the battery in amp-hours | For batteries connected in parallel, amp-hour capacities are added while voltage remains the same |
| Inverter or UPS efficiency | The proportion of battery energy expected to reach the connected equipment | Some energy is lost through conversion, heat and the inverter's own electronics |
| Usable depth of discharge | The percentage of nominal capacity you intend to use | Lead-acid batteries are often operated at a lower depth of discharge than lithium batteries, subject to manufacturer guidance |
| Load safety margin | An additional percentage added to the entered load | This helps cover measurement uncertainty, standby consumption and small load variations |
| Target runtime | The number of hours the system should support the load | The calculator uses this value to estimate the required amp-hour capacity |
How battery runtime is calculated
The calculator first converts the battery rating into nominal energy by multiplying voltage by amp-hour capacity. It then applies inverter efficiency and usable depth of discharge to estimate the energy available to the load. Finally, usable energy is divided by the connected load after the safety margin has been added.
- Nominal battery energy = battery voltage × battery capacity
- Usable energy = nominal energy × inverter efficiency × usable depth of discharge
- Adjusted load = connected load × (1 + safety margin)
- Estimated runtime = usable energy ÷ adjusted load
- For a target runtime, the calculation is reversed to estimate the required battery capacity
Battery runtime calculation example
How long can a 12 V 100 Ah battery support a 100 W load?
Answer: The nominal battery energy is 12 × 100 = 1,200 Wh. With 85% inverter efficiency and 80% usable depth of discharge, estimated usable energy is 1,200 × 0.85 × 0.80 = 816 Wh. Adding a 10% safety margin increases the calculated load to 110 W. Estimated runtime is therefore 816 ÷ 110 = approximately 7.42 hours.
Explanation: A calculation based only on nominal capacity would suggest 12 hours. The more realistic estimate is lower because it includes inverter losses, a discharge limit and a load safety margin.
Battery capacity calculation example
What battery capacity is needed to power a 150 W load for 6 hours?
Answer: With a 10% safety margin, the adjusted load is 165 W. Six hours requires 165 × 6 = 990 Wh of usable energy. At 85% inverter efficiency and 80% usable depth of discharge, the nominal energy required is 990 ÷ 0.85 ÷ 0.80 = approximately 1,456 Wh. For a 12 V system, this is 1,456 ÷ 12 = approximately 121 Ah.
Explanation: The required capacity must cover both conversion losses and the part of the battery that should remain unused. Selecting a battery from watts and hours alone can produce an undersized system.
Why nominal battery capacity is not the same as usable runtime
A 12 V 100 Ah battery has a nominal energy rating of approximately 1,200 Wh, but not all of that energy will be available to connected appliances.
An inverter or UPS uses part of the stored energy while converting battery DC power into the output required by the equipment.
Battery life can also be shortened by repeated deep discharge, particularly with some lead-acid batteries, so the safe usable portion may be lower than the full rated capacity.
Why actual battery runtime may be different
The calculator provides a planning estimate based on the values entered. Real equipment rarely operates under perfectly fixed conditions. Appliance consumption may change, the battery may no longer deliver its rated capacity, and inverter efficiency can vary with load.
- An older battery may provide less capacity than its label states
- Low temperatures can reduce available battery energy
- Inverters and UPS units consume power even when the connected load is small
- Motors, compressors and pumps may require high starting power
- Cycling appliances may switch on and off rather than draw constant power
- Actual inverter efficiency may change at very low or very high loads
- Loose connections, undersized cables and voltage drop can reduce system performance
Common battery runtime calculation mistakes
- Multiplying volts by amp-hours and treating all nominal energy as usable
- Assuming the battery can be repeatedly discharged to zero without consequences
- Ignoring inverter efficiency and standby consumption
- Entering the power of only one device when several devices will operate together
- Confusing watts, watt-hours and amp-hours
- Expecting an aged battery to deliver its original rated capacity
- Ignoring motor or compressor starting power
- Comparing batteries by amp-hours without considering system voltage
How to compare batteries and battery banks
Amp-hour capacity alone is not enough for a fair comparison. Battery voltage must also be considered. A 12 V 100 Ah battery stores approximately 1,200 Wh of nominal energy, while a 24 V 100 Ah battery bank stores approximately 2,400 Wh.
Examples of nominal battery energy
| Battery system | Capacity | Nominal energy |
|---|---|---|
| 12 V | 100 Ah | 1,200 Wh |
| 24 V | 100 Ah | 2,400 Wh |
| 48 V | 100 Ah | 4,800 Wh |
| 12 V | 200 Ah | 2,400 Wh |
How to interpret the calculator result
Use the result as an initial comparison between battery sizes, loads and target operating times. It can show how much nominal and usable energy the battery provides, how long the selected load may operate and what capacity may be required for a planned runtime. Before buying or installing equipment, also check battery chemistry, manufacturer discharge limits, inverter surge rating, cable size, protection, ventilation and the measured consumption of the connected devices.
Frequently asked questions
How do I calculate how long a battery will last?
Convert the battery rating into watt-hours by multiplying voltage by amp-hours. Reduce that figure for inverter losses and the usable depth of discharge, then divide the remaining energy by the connected load in watts. The calculator performs these steps automatically.
What is the difference between Ah and Wh?
Amp-hours describe electric charge, while watt-hours describe stored energy. To compare battery systems with different voltages, watt-hours are more useful. A battery's nominal watt-hours are calculated by multiplying volts by amp-hours.
Can this calculator be used for lithium and lead-acid batteries?
Yes, provided that you enter an appropriate usable depth of discharge and realistic efficiency. Different battery chemistries have different voltage behaviour, discharge limits and ageing characteristics, so manufacturer specifications should always take priority.
Does the calculator include inverter standby power?
Standby power is not entered as a separate field. For a more conservative estimate, add the inverter or UPS self-consumption to the connected load, or increase the load safety margin.