Generator Sizing: How to Calculate the Right Generator Size

Getting generator sizing right is one of the most important decisions you’ll make before purchasing a portable or standby unit. An undersized generator causes voltage drops, trips breakers, and can damage motors and sensitive electronics. An oversized one wastes fuel and money. Whether you’re a homeowner preparing for storm season or a contractor powering a job site, this guide walks you through the exact steps to calculate the generator capacity you actually need—with real numbers you can apply today.

Why Generator Sizing Matters More Than You Think

Generators are rated in watts (W) or kilowatts (kW). Every electrical load you connect draws a specific amount of power, and motors draw a surge of extra power the instant they start. If your generator can’t handle that surge, the unit’s built-in breaker trips—or worse, the voltage sags low enough to overheat motor windings. Proper sizing protects your equipment, keeps circuits stable, and ensures the generator runs at 50–75% of its rated capacity, which is the sweet spot for fuel efficiency and engine longevity.

Step 1: List Every Load You Plan to Power

Start by writing down every appliance, tool, or circuit you need the generator to handle simultaneously. For a whole-house standby scenario, pull out your electrical panel schedule. For a portable unit on a job site, list each tool. Here are common loads with typical wattage values:

• Refrigerator: 150 W running / 1,200 W starting
• Sump pump (½ HP): 800 W running / 2,000 W starting
• Window A/C (10,000 BTU): 1,200 W running / 3,600 W starting
• Central A/C (3-ton): 3,500 W running / 7,000 W starting
• Well pump (1 HP): 1,000 W running / 3,000 W starting
• Microwave (1,000 W): 1,000 W running / 1,000 W starting (resistive—no surge)
• LED lighting circuit: 100–300 W running / no significant surge
• Circular saw (15 A): 1,800 W running / 3,600 W starting
• Air compressor (2 HP): 1,500 W running / 4,500 W starting

Check the nameplate on each piece of equipment for exact values. If a motor lists amps instead of watts, multiply amps × voltage (120 V or 240 V) to get VA, which is close enough for sizing purposes on typical residential loads.

Step 2: Calculate Running Watts and Starting Watts

Add up the running watts of every load that will operate at the same time. Then identify the single load with the largest starting watt surge and add that surge on top. The reason you don’t add every surge together is that motors rarely all start at the same instant.

Example: Home Emergency Panel

Suppose you want to run these loads during a power outage:

• Refrigerator: 150 W running
• Sump pump (½ HP): 800 W running
• Well pump (1 HP): 1,000 W running
• LED lighting (two circuits): 400 W running
• Microwave: 1,000 W running
• Phone/laptop chargers: 150 W running

Total running watts: 150 + 800 + 1,000 + 400 + 1,000 + 150 = 3,500 W

Now find the largest starting surge. The well pump surges to 3,000 W, which means an additional 2,000 W above its running draw. The sump pump surges an extra 1,200 W. The well pump surge is largest.

Total required capacity: 3,500 W + 2,000 W = 5,500 W

A 6,500-watt generator covers this load comfortably with headroom. A 5,000-watt unit would be risky, especially as the generator ages and output decreases slightly.

Step 3: Apply a Demand Factor (for Whole-House Standby)

If you’re sizing a permanent standby generator connected through an automatic transfer switch, NEC Article 220 allows demand factors. You don’t assume every circuit in a 200 A panel runs at full capacity simultaneously. A common approach:

• First 10 kW of general loads at 100%
• Remaining general loads at 40%
• A/C or heat at 100% (use whichever is larger—they typically don’t run at the same time)
• Largest motor starting surge added at 25% of its locked-rotor amps (since the generator must handle inrush)

For a typical 2,000-square-foot home with a 3-ton central A/C, gas heat, and an electric range, a calculated demand often lands between 18 kW and 24 kW. This is why the 22 kW standby generator is one of the most popular residential units on the market—it hits the sweet spot for the majority of homes with gas heating.

Job Site Sizing Shortcut

Contractors can use a simpler rule. Add the running watts of all tools that operate simultaneously, then add the starting surge of the largest motor. For a typical framing crew running a circular saw (1,800 W), a miter saw (1,800 W), a compressor (1,500 W running / 4,500 W starting), and chargers (200 W), the math works out to:

1,800 + 1,800 + 1,500 + 200 = 5,300 W running + 3,000 W surge = 8,300 W peak. A 10,000-watt portable generator handles this with room to spare.

Step 4: Choose the Right Fuel Type and Phase

Generator sizing isn’t only about watts. Fuel type affects available run time and convenience:

• Gasoline portable units: 3,000–12,000 W range; affordable but loud, fuel degrades in storage.
• Dual-fuel (gas/propane): Good flexibility; propane stores indefinitely.
• Diesel: More efficient at higher loads; common on job sites above 10 kW.
• Natural gas standby: Unlimited fuel supply from utility line; typical for 16–48 kW whole-house units.

For residential standby generators, also confirm whether your loads require single-phase or three-phase power. Almost all homes use single-phase. Commercial and large industrial sites may need three-phase, which changes the sizing formula (watts = volts × amps × 1.732 × power factor).

Common Generator Sizing Mistakes to Avoid

• Ignoring starting surge: This is the number-one error. A 5,000 W generator cannot reliably start a 3-ton A/C unit even though the running watts may seem to fit.
• Running at 100% rated capacity continuously: Manufacturers rate “maximum” watts for short bursts. Use the “rated” or “continuous” watt figure for sustained loads.
• Forgetting altitude derating: Generators lose roughly 3.5% of output per 1,000 feet above sea level. At 5,000 feet, a 10,000 W generator effectively delivers about 8,250 W.
• Skipping the transfer switch: Backfeeding a panel without a transfer switch is illegal and can electrocute utility workers. Budget for a proper transfer switch as part of the installation.

Generator Sizing Quick-Reference Table

• Essential circuits only (fridge, lights, sump pump): 3,500–5,000 W portable
• Essential circuits + well pump + window A/C: 6,500–8,000 W portable
• Whole-house with central A/C (gas heat): 18–22 kW standby
• Whole-house with electric heat + A/C: 25–48 kW standby
• Job site (typical framing/remodel crew): 7,000–10,000 W portable

Get Your Generator Sizing Right the First Time

Accurate generator sizing saves you from blowing breakers, burning out motors, or wasting hundreds of dollars on a unit that’s too large for your needs. List your loads, add the numbers using the running-plus-largest-surge method,