Battery Backup Time Calculator
Estimate how long a battery will power your load - enter voltage, capacity, efficiency and load watts.
What this tool does
A battery backup time calculator estimates how long a battery will power a given load. Enter the battery voltage, the capacity in amp-hours, the depth of discharge (how much of the battery you are willing to use), the inverter efficiency if the load is AC, and the power of the load in watts. The calculator converts amp-hours to watt-hours, applies the depth of discharge and inverter efficiency to find usable energy, then divides by the load power to get runtime in hours.
Converts battery capacity in amp-hours to watt-hours using voltage.
Applies depth of discharge to find the usable portion of the battery.
Applies inverter efficiency if the load is AC.
Divides usable energy by load power to find runtime in hours.
How to use it
- Enter the battery voltage (commonly 12 V, 24 V or 48 V).
- Enter the battery capacity in amp-hours.
- Enter the depth of discharge you are willing to use (50% is typical for lead-acid, 80% or more for lithium).
- Enter the inverter efficiency if the load is AC (typically 85% to 95%).
- Enter the total power of the load in watts.
- Read the estimated runtime in hours.
Why use this calculator
Shows how depth of discharge and inverter efficiency affect real-world runtime, because the amp-hour rating on the battery label is not the energy you can actually use. A 100 Ah lead-acid battery might be rated at 100 Ah, but using only half of that capacity extends its life, and an AC load through an inverter loses another 10% to 15%. Ignoring these factors overestimates runtime by 60% or more.
Why we built it
We built the battery backup time calculator to include depth of discharge and inverter efficiency as explicit inputs, because they are the two biggest gaps between the label rating and the real runtime. The watts to amps converter helps size the wiring between the battery and the inverter, and the solar panel system size calculator can size a solar array to recharge the battery.
If your task goes further, try the energy consumption calculator to total up the load you want to run, and the electricity cost calculator to see what that load costs from the grid.
Who this tool is for
- Owners of off-grid or backup battery systems
- Campers and van-lifers sizing a leisure battery
- Solar homeowners checking how long a battery covers a home's night-time load
- Anyone sizing a battery for a specific appliance or set of appliances
When to use it
- Sizing a battery for a known load and desired runtime.
- Checking whether an existing battery will last through a power cut.
- Comparing battery types - lead-acid versus lithium - on usable runtime.
- Planning an off-grid system's battery bank.
How it is calculated
Watt-hours = voltage × amp-hours. Usable watt-hours = watt-hours × depth of discharge × inverter efficiency. Runtime in hours = usable watt-hours ÷ load power in watts.
Worked example: a 12 V, 100 Ah lead-acid battery at 50% depth of discharge powering a 200 W AC load through a 90% efficient inverter. Watt-hours = 12 × 100 = 1,200. Usable = 1,200 × 0.50 × 0.90 = 540 Wh. Runtime = 540 ÷ 200 = 2.7 hours. The same battery as lithium at 80% depth of discharge would give 1,200 × 0.80 × 0.90 = 864 Wh, or 4.3 hours.
Lead-acid batteries are typically limited to 50% depth of discharge to avoid shortening their life. Lithium iron phosphate (LiFePO4) batteries can safely use 80% to 90% of their rated capacity.
What you enter, and what you get back
| Item | Details |
|---|---|
| Battery voltage | The nominal voltage: 12 V, 24 V or 48 V. |
| Battery capacity | The rated capacity in amp-hours. |
| Depth of discharge | The usable fraction of the capacity. |
| Inverter efficiency | The efficiency of the inverter for AC loads. |
| Load power | The total power of the load in watts. |
| Result | The estimated runtime in hours. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| Runtime is shorter than expected | Depth of discharge and inverter losses were not included. | Enter the correct values for your battery type. |
| Battery drains faster under heavy load | Batteries deliver less capacity at high discharge rates. | Use a larger battery or reduce the load. |
| Ah and Wh confused | Amp-hours and watt-hours are different units. | This calculator converts between them. |
A worked example
These are the values the calculator opens with, so you can see exactly what it does with them. Change any value on the page and the result updates; the figures below come from the same calculation.
| Input | Value |
|---|---|
| Battery voltage | 12 V |
| Battery capacity | 100 Ah |
| Depth of discharge | 50% |
| Inverter efficiency | 90% |
| Load power | 200 W |
With those inputs, the calculator returns:
- Total energy: 1,200 Wh
- Usable energy: 540 Wh
- Estimated runtime: 2.7 hours
Assumptions and limitations
- Assumes a constant load; real loads vary and some appliances surge at startup.
- Battery capacity is reduced at low temperatures and high discharge rates.
- Does not account for battery ageing and reduced capacity over time.
- An estimate, not a guarantee - test your actual setup before relying on it.
Frequently asked questions
How do I calculate how long a battery will last?
Convert the battery's amp-hours to watt-hours by multiplying by the voltage, then multiply by the depth of discharge and inverter efficiency to find usable energy. Divide by the load power in watts to get runtime in hours. For example, a 12 V 100 Ah battery at 50% depth of discharge powering a 200 W load through a 90% efficient inverter lasts about 2.7 hours.
What depth of discharge should I use?
For lead-acid batteries, limit depth of discharge to 50% to avoid shortening the battery's life. For lithium iron phosphate (LiFePO4) batteries, 80% to 90% is safe. Using more than the recommended depth of discharge regularly will reduce the number of cycles the battery can deliver.
Why does my battery last less than the label says?
The label capacity is measured under ideal conditions at a low discharge rate over 20 hours. Real-world factors - higher discharge rates, low temperatures, inverter losses and depth of discharge limits - all reduce the usable energy. A 100 Ah lead-acid battery realistically provides about 50 Ah of usable energy, not 100.
How does inverter efficiency affect runtime?
An inverter converts the battery's DC power to AC power for household appliances, losing 5% to 15% of the energy as heat. A 90% efficient inverter means only 90% of the battery's usable energy reaches the load, reducing runtime proportionally.
What is the difference between Ah and Wh?
Amp-hours (Ah) measure charge capacity - how much current a battery can deliver over time. Watt-hours (Wh) measure energy capacity - how much total energy the battery stores. Watt-hours = amp-hours × voltage. Two batteries with the same Ah rating but different voltages store different amounts of energy.
Does temperature affect battery life?
Yes - cold temperatures reduce a battery's available capacity, sometimes by 20% to 30% at freezing point for lead-acid. High temperatures increase capacity slightly but accelerate ageing and shorten overall cycle life. Lithium batteries handle temperature variation better but still have an optimal operating range.