“How many hours will it run?” is the first question every buyer asks. You can answer it yourself in three steps with nothing more than your load list.
Step 1 — Add Up Your Essential Loads
List only what must run during an outage: fridge (~150W), lights (~100W), router and TV (~100W), fans (~150W). A typical essential-load total is 400–600W. Non-essentials like water heaters and ovens are usually excluded.
Step 2 — Apply the Runtime Formula
Backup hours = (battery kWh × usable fraction × inverter efficiency) ÷ load kW. For a 5.12kWh LiFePO4 battery: 5.12 × 0.9 × 0.95 ≈ 4.4kWh delivered. At a 500W load that is about 8.8 hours; a 10.24kWh battery doubles it.
- 5.12kWh battery, 500W load → ~8–9 hours
- 10.24kWh battery, 500W load → ~17–18 hours
- 10.24kWh battery, 1.5kW load (with one inverter AC) → ~5–6 hours
Step 3 — Add Margin for Reality
Startup surges, high ambient temperature and aging all reduce runtime. Size with 20–25% headroom, and remember solar panels recharge the battery during daylight — in practice a well-matched system runs essential loads indefinitely through daily outages.
