Quick answer
The inverter must handle the total power of everything running at once (plus start-up surges); the battery must store enough energy for the runtime you need. Energy needed = load (W) × hours. Because batteries shouldn't be fully drained, divide by the usable fraction: about 80–90% for lithium (LiFePO4), often around 50% for lead-acid.
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Shop all inverter products →Key takeaways
- Inverter size = peak load (watts), with headroom for surges.
- Battery energy = load × hours ÷ usable depth of discharge.
- Lithium gives more usable capacity and longer life than lead-acid.
- Measure real loads; don't guess.
- Security loads are small and constant, ideal for backup.
01Worked example
- 01List the loadsNVR + 8 cameras (PoE switch) ~150 W, router 15 W, fence energizer ~20 W, gate standby ~10 W: about 200 W
- 02Decide runtime8 hours overnight
- 03Energy needed200 W × 8 h = 1,600 Wh
- 04Allow for depth of dischargeLithium at 80%: 1,600 ÷ 0.8 = 2,000 Wh (2 kWh)
- 05Add marginLosses and battery ageing: choose around 2.5 kWh
ImportantFigures above are illustrative. Real loads are measured on site, and IR, heaters and gate openings change consumption.
02Lithium vs lead-acid
| Lithium (LiFePO4) | Lead-acid | |
|---|---|---|
| Usable capacity | About 80–90% | Often around 50% for good life |
| Cycle life | Much longer | Shorter |
| Weight | Lighter | Heavier |
| Upfront cost | Higher | Lower |
| Cost over its life | Often lower | Often higher |
03Inverter sizing
- Add up everything that runs at once
- Motors (pumps, gate motors, fridges) draw a surge at start-up
- Choose an inverter with continuous rating above the load and surge rating above the peaks
- Put security loads on a dedicated backed-up circuit
See on-grid vs hybrid and changeover switches.
04Frequently asked questions
How long will my batteries last in an outage?
Usable battery energy (Wh) divided by the load (W) gives the hours, e.g. 2,000 Wh ÷ 200 W = 10 hours.
Can I run everything in the house on backup?
Possible but expensive. Most people back up essential circuits: security, lights, router, fridge.
Why did my inverter trip when the pump started?
Motor start-up surges can exceed the inverter's surge rating.
