How many batteries do I need?
Enter your target backup hours and load; we compute required usable Wh and minimum parallel count for one battery SKU (same V & Ah).
Click Calculate backup to estimate how many batteries you need.
Appliance load (multi-device)
Watts × quantity; total feeds the core calculator.
Total load: 0 W
Core battery backup calculator
Round-trip / usable efficiency factor (illustrative).
Click Calculate backup to see backup duration, energy, and inverter hint.
Load vs backup (visual)
Share of a 24 h day your battery can cover at this load (illustrative).
Click Calculate backup to see load vs backup time.
Solar charging integration
Click Calculate backup to estimate solar recharge days.
Cost vs backup comparison
| Item | Value |
|---|---|
| Click Calculate backup for cost breakdown. | |
Daily usage vs backup capacity
If you run the same load hours per day from grid, daily energy is compared to one full usable discharge.
Click Calculate backup to compare daily use vs pack capacity.
Off-grid vs hybrid (summary)
| Mode | Idea |
|---|---|
| Off-grid | Battery is primary storage; solar must cover daily use + recharge. Oversize array & storage for cloudy days. |
| Hybrid / grid-tied + battery | Grid covers shortfalls; battery covers outages & peak shifting. Often smaller Ah than full off-grid. |
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How battery backup works
Batteries store DC energy. Nominal Wh ≈ voltage × amp-hours (per string) × parallel count. You should not discharge 100% of nameplate on most chemistries: apply depth of discharge. The inverter converts DC→AC with losses, so usable AC energy is less than nominal Wh.
How to choose battery capacity
List running watts for devices you need during an outage, add a margin for surge, pick target hours, then solve for Wh. Convert to Ah at your system voltage and account for parallel batteries.
Lead-acid vs lithium
Lead-acid is common and lower cost per kWh upfront but heavier and with lower usable DoD cycles. Lithium often allows higher DoD and efficiency in a smaller footprint—verify BMS, fire rules, and warranty. Gel/AGM sit between for maintenance and spill safety.
Embed
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FAQ
- How long will a battery last?
- Runtime (hours) = usable watt-hours available at the battery bus after DoD and efficiency losses, divided by your load in watts.
- How many batteries do I need?
- Find required usable Wh for load × hours, then divide by usable Wh per battery and round up. Keep series/parallel rules for your inverter voltage.
- What size inverter is required?
- Continuous watts should exceed the sum of running loads; add surge capacity for motors. VA rating is often higher than watts—use manufacturer specs.
- How to calculate backup time?
- Backup time = usable stored energy (Wh) ÷ load (W). This tool applies DoD, chemistry, and inverter efficiency to usable Wh.