Generator Size Calculator
An undersized generator trips breakers and damages appliances. An oversized one wastes fuel and money. Select the appliances you need to power during an outage, and this calculator determines the exact wattage required — including motor starting surges — with a built-in safety buffer.
Appliance Selector
How Generator Sizing Works
Generator sizing involves two key numbers: running watts (continuous load) and starting watts (momentary surge). The formula:
Required Watts = Total Running Watts + Highest Single Starting Surge
You don't add all starting surges together because appliances don't all start simultaneously. Instead, take the total running load and add the single largest starting surge. This represents the worst-case scenario: everything running while the biggest motor kicks on.
Step-by-Step Sizing Guide
- List every appliance you want to run simultaneously during an outage.
- Find wattage ratings on the appliance nameplate, manual, or use our reference values.
- Sum all running watts. This is your continuous load.
- Identify the largest starting surge — the appliance with the biggest difference between starting and running watts.
- Add running total + largest surge. This is your minimum generator size.
- Apply a 25% safety buffer. This protects the generator and allows for unexpected loads.
Real-World Example
A homeowner wants to run essentials during a winter storm:
- Refrigerator: 200W running, 1,200W starting
- Furnace blower: 800W running, 2,350W starting
- Sump pump: 800W running, 1,300W starting
- Lights + TV + chargers: 400W running (no surge)
- Total running: 2,200W
- Largest surge: furnace at 2,350 - 800 = 1,550W extra
- Peak: 2,200 + 1,550 = 3,750W
- With 25% buffer: 4,688W → 5,000W generator recommended
Common Appliance Wattages
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 200W | 1,200W |
| Freezer | 150W | 900W |
| Sump Pump (1/3 HP) | 800W | 1,300W |
| Furnace Blower (1/2 HP) | 800W | 2,350W |
| Well Pump (1/2 HP) | 1,000W | 2,100W |
| Window AC (10,000 BTU) | 1,200W | 3,600W |
| Central AC (3 ton) | 3,800W | 7,200W |
| Electric Water Heater | 4,500W | 4,500W |
| Microwave (1000W) | 1,000W | 1,500W |
| Coffee Maker | 1,000W | 1,000W |
| Toaster | 1,100W | 1,100W |
| Lights (5 LED bulbs) | 50W | 50W |
Common Generator Sizing Mistakes
- Adding all starting watts together. Appliances don't all start at the same millisecond. Add running watts + the single biggest starting surge.
- Forgetting about starting surges entirely. A 2,200W running load that includes a 2,350W starting surge needs a 3,750W generator, not a 2,500W one.
- Running at 100% capacity. Generators at full load overheat, burn more fuel, and wear out fast. Target 50-75% load.
- Not considering altitude. Generators lose ~3.5% power per 1,000 feet of elevation above sea level. At 5,000 feet, a 7,500W generator produces only ~6,500W.
- Backfeeding into the panel. This is dangerous and illegal. Always use a transfer switch.
Compare your generator fuel costs with our EV vs gas calculator or estimate project electricity costs with the cost per mile calculator. For outdoor projects, check the fence calculator and deck board calculator.
Frequently Asked Questions
How do I determine what size generator I need?
Add up the running watts of everything you want to power simultaneously, then add the highest single starting watt surge from the list. Generators must handle both the continuous running load and the momentary starting surge of motor-driven appliances. Our calculator sums running watts and identifies the peak starting surge automatically.
What is the difference between running watts and starting watts?
Running (rated) watts is the continuous power an appliance needs during operation. Starting (surge) watts is the brief spike of power needed when a motor first kicks on — typically 2-3× the running watts. This surge lasts only a fraction of a second but your generator must handle it or the breaker will trip. Resistive loads (heaters, lights) have no starting surge.
Should I get a generator bigger than my calculated need?
Yes — run generators at 50-75% of rated capacity for optimal efficiency and longevity. A generator running at full load constantly will overheat, consume more fuel, and wear out faster. Also, you may add appliances during an outage you didn't plan for. A 20-25% buffer above your calculated need is the industry recommendation.
Can I run my air conditioner on a portable generator?
Window ACs (5,000-10,000 BTU) need 2,000-4,000 starting watts, manageable for a 5,000-7,500W generator. Central AC (3+ ton) requires 7,000+ starting watts — you need a large portable (10,000W+) or standby generator. Never undersize for AC — the compressor starting surge can damage both the generator and the AC unit.
How many watts does a house use during a power outage?
Essential circuits (fridge, freezer, lights, furnace fan, sump pump, well pump) typically need 3,000-5,000 running watts and 5,000-7,500 watts with starting surges. A full-house setup including AC and electric water heater can exceed 15,000 watts. Most homeowners use a portable 5,000-7,500W generator for essentials or a 20,000W+ standby for whole-house coverage.
What is the difference between portable and standby generators?
Portable generators (3,000-12,000W) run on gasoline, are manually started, and connect via extension cords or a manual transfer switch. Standby generators (10,000-48,000W+) are permanently installed, run on natural gas or propane, start automatically during outages, and connect through an automatic transfer switch. Standby generators cost $5,000-$15,000+ installed but provide seamless whole-house protection.
Can I plug a generator directly into a wall outlet?
Absolutely not — this is called "backfeeding" and is extremely dangerous, illegal in most jurisdictions, and can electrocute utility workers. Always use a transfer switch (manual or automatic) installed by a licensed electrician to safely connect a generator to your home's electrical panel. Extension cords to individual appliances are the only safe alternative without a transfer switch.
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