Battery Calculator guide
Battery Calculator: How Long Will My Battery Last?
Use our battery calculator to estimate run time from capacity in Ah, mAh or Wh and the load in watts, allowing for depth of discharge and inverter losses.
By the AllCalculator editorial teamUpdated 15 October 20262 min read
Whether it's a leisure battery in a campervan, a power station for camping or a home battery for solar, the question is the same: how long will it last? Our Battery Calculator estimates the run time for any load.
How to use the calculator (Battery Calculator)
- Enter the capacity in Ah, mAh or Wh, and the battery voltage.
- Enter the load in watts or amps.
- Set the usable capacity (depth of discharge) and system efficiency.
- Optionally, enter your charger current to estimate recharge time.
How the run time is worked out
- Energy in Wh = capacity in Ah × voltage.
- Usable energy = total energy × usable capacity × efficiency.
- Run time in hours = usable energy ÷ load in watts.
Usable capacity
You can't use every bit of a battery's capacity without shortening its life:
- Lead-acid and AGM: about 50%.
- Lithium (LiFePO4): 80% to 100%.
A worked example
With the calculator's default settings, a 100Ah, 12V lithium battery running a 60W load:
- Total energy: 100 × 12 = 1,200Wh
- Usable at 80% and 90% efficiency: 864Wh
- Run time: 864 ÷ 60 = 14.4 hours
Tips
- Inverters lose about 10% converting to 230V, more at very light loads.
- Cold weather reduces capacity, especially for lead-acid.
- High loads shorten run time more than the simple maths suggests.
Planning solar? See our Solar Panels Calculator, and for running costs our Energy Bill Calculator.
Frequently asked questions
How long will a 100Ah battery run a 1,000W load?
At 12V with 80% usable capacity and 90% efficiency, about 0.86 hours, or 52 minutes.
How do I convert mAh to Wh?
Divide by 1,000 and multiply by the voltage. A 20,000mAh power bank at 3.7V holds 74Wh.
How long does charging take?
Roughly the capacity to refill divided by the charger current, plus about 15% for losses.
Use the Battery Calculator to size a battery before you buy.
Sources and further reading
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.