Quick answer: Add the running watts of everything you want to power at once, add the single largest starting surge, then allow about 20% headroom. Essentials such as a fridge, sump pump, furnace fan, lights and microwave usually need a 5,000–7,500 W generator.
Buying a generator that is too small means tripped breakers and stalled motors during an outage. Buying one far too large wastes money and fuel. The right size is easy to find with a short list and a little arithmetic.
Running Watts and Starting Watts
Appliances with motors, such as refrigerators, pumps and fans, need a short burst of extra power to start. This starting (surge) wattage can be two to three times the running wattage. Your generator must handle all running loads plus the biggest surge.
Appliance Wattage Chart
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator (ENERGY STAR) | 132–192 | 1,200 |
| Sump pump, ½ hp | 1,050 | 2,150 |
| Furnace fan, ½ hp | 875 | 2,350 |
| Window AC, 10,000 BTU | 1,500 | 2,200 |
| Microwave, 1,000 W | 1,500 | 1,500 |
| Electric water heater | 4,500 | 4,500 |
| Washing machine | 1,200 | 1,200 |
| Electric dryer | 5,400 | 6,750 |
| Garage door opener | 720 | 1,420 |
| Coffee maker | 600 | 600 |
| Flat-screen TV, 46 in | 190 | 190 |
Figures from Honda’s generator wattage guide. Check your own appliances’ labels when you can.
Step-by-Step Example
- List running watts: fridge 192 + sump pump 1,050 + furnace fan 875 + microwave 1,500 + coffee maker 600 + TV 190 + lights 300 = 4,707 W.
- Find the biggest extra surge: the furnace fan needs 2,350 − 875 = 1,475 W extra to start.
- Peak = 4,707 + 1,475 = 6,182 W.
- Add headroom: 4,707 ÷ 0.8 = 5,884 W running capacity. A 6,500 W generator covers both.
Skip the math: tick the appliances you want to run and it does this math for you. Open the free Generator Size Calculator →
Typical Generator Sizes
| Generator | What it can usually run |
|---|---|
| 2,000–3,500 W inverter | Fridge, lights, TV, phone charging |
| 5,000–7,500 W portable | Essentials plus sump pump, furnace fan and microwave |
| 8,000–12,000 W large portable | Most circuits, staggering big loads |
| 14,000–26,000 W standby | Whole house including central AC |
Tips to Get More From a Smaller Generator
- Start the largest motor first, then add smaller loads.
- Do not run the dryer, water heater and range at the same time.
- Use LED bulbs to cut lighting load.
- Install a transfer switch or interlock so you can power hard-wired loads safely.
Safety
Never run a generator indoors, in a garage or near windows. Carbon monoxide is deadly and odorless. Never backfeed power through an outlet; it can kill utility workers and damage your home’s wiring.
Portable vs Standby Generators
| Portable | Standby | |
|---|---|---|
| Typical size | 2,000–12,000 W | 10,000–26,000 W+ |
| Fuel | Gasoline, propane or dual fuel | Natural gas or propane |
| Start | Manual (some electric start) | Automatic within seconds |
| Connection | Cords or transfer switch / interlock | Automatic transfer switch |
| Best for | Essentials, occasional outages | Whole-home backup, frequent outages |
Inverter Generators
Inverter generators produce very clean power that is safe for computers, TVs and sensitive electronics, and they throttle down at light loads to save fuel and noise. They are usually 2,000–4,000 W, so several homeowners pair two units or choose a larger open-frame model for heavy loads.
How Long Will a Generator Run?
Run time depends on load and tank size. Many portable generators run 8–12 hours on a full tank at half load. Running at a lighter load, about 50–75% of capacity, gives the best mix of fuel economy and engine life. Store fuel safely and rotate it, or choose a propane or natural gas model for long outages.
Sizing Mistakes to Avoid
- Adding up starting watts for every appliance, which oversizes the generator. Only the largest surge matters if motors start one at a time.
- Ignoring 240 V loads such as well pumps, which need a generator with a 240 V outlet.
- Forgetting that the furnace blower is needed for gas heat to work during winter outages.
Sources
Frequently Asked Questions
What size generator do I need to run a refrigerator?
A refrigerator needs about 130–200 running watts and 1,200 starting watts, so even a 2,000 W inverter generator can run one.
Will a 7,500 watt generator run a house?
It can run most essentials, including a sump pump, furnace fan, fridge, lights and microwave, but not central AC and large electric appliances together.
What size generator for central air?
Usually a 14,000 W or larger standby generator, sized by an installer for your AC’s locked-rotor current.
Do I need a transfer switch?
Yes, to connect a generator to household circuits safely and legally.
What size generator do I need for a sump pump?
A ½ hp sump pump needs about 1,050 running and 2,150 starting watts, so a 3,500 W generator can run it with a few small loads.
Related Guides
- What Size Wire Do I Need? Amps, Distance and Voltage Drop
- What Size Water Heater Do I Need? Tank and Tankless Sizing
- What Size Air Conditioner Do I Need? BTU by Room Size
- How Many Solar Panels Do I Need to Power a House?
Try the free Generator Size Calculator or browse all home improvement calculators.