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Calculate generator backup power needs
Generator sizing is where most people overspend—or end up disappointed. The goal is to match the generator to your critical loads and include starting watts for motors and compressors.
If you are considering a battery-based solution, note that solar generators are typically limited by battery capacity and inverter surge capability, so starting watts still matter.
For battery-based systems, see also: Solar Generator Sizing Guide →
Step 1: choose your backup strategy
- Full-home / uninterrupted operation: larger system, higher cost, more planning.
- Emergency mode: power only critical items and avoid running big loads at the same time (more economical).
If you’re comparing fuel vs battery options, review: Solar vs Gas Generator →
Step 2: list appliances and note “running” vs “starting” watts
Many appliances have a higher starting load (surge) than their running load. That surge often determines the generator size.
Example: minimal emergency requirements (basic home)
| Appliance | Run watts | Start watts |
|---|---|---|
| Refrigerator | 700 | 2800 |
| Chest freezer | 500 | 2500 |
| Lights (10 × 100W) | 1000 | 1000 |
| Microwave (1500W) | 1500 | 1500 |
| Radio | 100 | 100 |
| Television | 800 | 800 |
| Gas furnace fan (½ hp) | 500 | 1250 |
| Total | 4850 | 9700 |
Common appliance wattage reference
| Appliance / Load | Run watts | Start watts |
|---|---|---|
| VCR | 150 | 150 |
| Vacuum cleaner | 250 | 250 |
| Electric water heater | 4500 | 4500 |
| Sump pump (½ hp) | 500 | 1250 |
| Well pump (1 hp) | 1000 | 2500 |
| Well pump (2 hp) | 2000 | 5000 |
| Well pump (3 hp) | 3000 | 7500 |
| Air conditioner (12,000 BTU) | 2800 | 7000 |
| Air conditioner (24,000 BTU) | 4200 | 10500 |
| Air conditioner (32,000 BTU) | 5000 | 12500 |
| Heat pump / electric heat | 10000 | 10000 |
| Electric space heater | 1500 | 1500 |
| Window fan | 250 | 250 |
| Small drill motor | 500 | 500 |
| Electric motor (½ hp) | 500 | 1250 |
| Electric motor (1 hp) | 1000 | 2500 |
| Electric motor (2 hp) | 2000 | 5000 |
Step 3: add headroom (don’t run at 100%)
If you’re planning a permanent setup, allow additional capacity. Power needs often grow over time, and operating a generator below its maximum output is typically easier on the system and can be more efficient.
Compare practical system types: