Plan realistic backup power in minutes with smart load inputs. Tune surge, efficiency, and autonomy for accuracy. Make confident choices for outages and daily resilience.
| Appliance | Qty | Running (W) | Surge (W) | Hours/day | Essential |
|---|---|---|---|---|---|
| Refrigerator | 1 | 150 | 900 | 8 | Yes |
| LED lighting (whole home) | 1 | 300 | 300 | 5 | Yes |
| Wi‑Fi + router | 1 | 20 | 20 | 24 | Yes |
| Well pump | 1 | 800 | 2400 | 1 | Yes |
| Microwave | 1 | 1100 | 1100 | 0.3 | No |
| Space heater | 1 | 1500 | 1500 | 2 | No |
Typical essential circuits in a modern home often total 600–2,000 running watts, while whole‑home comfort loads can push 4–10 kW. Starting events dominate sizing: a 1 hp pump may surge near 2–3 kW for seconds. This calculator applies a surge diversity factor so you can model realistic overlap instead of assuming every motor starts together. Updated appliances, tighter insulation, and efficient lighting reduce daily energy, but large intermittent loads still control peak sizing. Treat results as planning targets, then confirm continuous ratings, breaker limits, and manufacturer surge curves before purchase.
Energy drives battery size. Lighting, networking, refrigeration, and small electronics frequently land around 3–8 kWh per day for essential use. Add cooking, laundry, or space heating and the profile can exceed 15–30 kWh daily. The load table converts watts and hours into kWh/day and then scales that total to your selected backup window to estimate required stored energy.
Delivered backup is always lower than nameplate capacity. Inverter efficiency reduces available AC energy, and round‑trip losses reduce what returns after charging. Depth of discharge limits usable storage to protect battery life. For example, an 80% DoD and 90% round‑trip efficiency means only about 72% of rated capacity is typically usable before adding temperature derating.
The inverter target is based on surge‑adjusted peak watts plus headroom for continuous operation and future circuits. Transfer current is estimated from inverter kW and your split‑phase voltage, supporting quick checks against common 100 A, 200 A, or critical‑loads panels. If you plan to run high‑draw HVAC or resistance heat, consider a higher continuous rating than the peak alone suggests.
A generator can shrink battery capacity by covering long outages and recharging storage. Many homeowners size a generator to meet peak load with a margin and then use batteries for silent overnight operation. If solar contribution is predictable, net daily energy drops, but peak watts remain unchanged. Use the chart to spot the biggest contributors and decide what to shed first.
It estimates how much extra motor starting power overlaps in time. Lower values assume starts are staggered; higher values assume more simultaneous starts.
The model adjusts for inverter losses, round‑trip losses, and depth of discharge limits. Those factors reduce usable energy compared with the rated battery capacity.
Include them only if you plan to power them during outages. Excluding non‑essentials is the fastest way to reduce inverter and battery targets while keeping critical circuits covered.
Multiply volts by amps for an approximate watt value. Use 120 V for standard outlets and 240 V for large appliances, then refine using nameplate ratings.
Solar mainly reduces energy required over time. Peak inverter sizing is still driven by instantaneous load and surge behavior, even when solar production is strong.
It is indicative. Runtime varies with load cycling, temperature, battery aging, and inverter behavior. Use it to compare scenarios, then validate with real usage data.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.