HOW LONG WILL IT LAST
Battery Backup Time Calculator
Enter your battery bank and load to see how many hours of backup you’ll actually get.
Your battery bank & load
Sets a recommended depth of discharge below — you can still override it.
Enter one battery’s specs below, plus how many are wired in series.
Auto-filled from battery type. Edit it if your manufacturer specifies a different figure.
Extra buffer left unused, on top of your depth of discharge limit.
Estimated backup time
How to use this calculator
- Pick your battery type. This sets a sensible depth-of-discharge default — lithium tolerates much deeper cycling than lead-acid, so the number will jump when you switch types.
- Describe one battery. Enter the voltage and Ah capacity printed on a single unit, unless you’re using “Preconfigured bank,” in which case enter the whole bank’s rated voltage and capacity directly.
- Choose how they’re wired. Series wiring adds voltages together (two 12V batteries in series make 24V); parallel wiring adds capacities together (two 100Ah batteries in parallel make 200Ah at the same voltage).
- Enter your load. Add up the running wattage of everything you want powered during an outage — this is the same “connected load” figure a solar sizing calculator would use.
- Read the result. Backup time updates live as you type. The battery graphic shows how much of your total capacity is actually usable versus reserved as a safety margin or off-limits under your DoD setting.
How the numbers are calculated
- Total bank energy (Wh) = bank voltage × bank capacity (Ah).
- Usable energy (Wh) = total energy × depth of discharge × (1 − safety reserve).
- Effective load (W) = appliance load ÷ inverter efficiency, since the inverter itself consumes some power converting DC to AC.
- Backup time (hours) = usable energy ÷ effective load.
- Battery current (A) = effective load ÷ bank voltage — useful for checking your battery’s max discharge rating.
Worked example
Two 12V, 100Ah tubular batteries wired in series make a 24V, 100Ah bank — 2,400 Wh total. At a 50% depth of discharge and a 5% safety reserve, that’s about 1,140 Wh usable. Running a 300W load through a 90% efficient inverter draws roughly 333 W from the battery, giving a backup time of about 3.4 hours — right at the edge of “Good” on the scale below.
Battery type comparison
Approximate figures — always check your specific battery’s datasheet, since quality and brand vary widely. For deeper technical background on depth of discharge and cycle life, Battery University is a solid independent reference.
| Type | Typical DoD | Cycle life | Notes |
|---|---|---|---|
| Lithium (LiFePO₄) | ~90% | 3,000–5,000 | Lightest, longest life, highest upfront cost. |
| Tubular Lead Acid | ~50% | 1,000–1,500 | Common for solar/UPS use in Pakistan, tolerates frequent cycling. |
| AGM | ~50% | 400–600 | Maintenance-free and spill-proof, moderate cost. |
| Gel | ~50% | 500–700 | Handles slow, deep discharge well; sensitive to fast charging. |
| Flooded Lead Acid | ~40% | 300–500 | Lowest upfront cost, needs regular water top-ups and ventilation. |
Frequently asked questions
Related tools
Use these alongside your backup time result to plan the rest of your system.
Related articles
A few guides worth reading once you know your backup time:
Actual backup time varies with battery age, temperature, discharge rate, and real-world inverter performance — treat these numbers as a planning estimate, not a guarantee.