What Size Generator Do I Need?

How to Size a Generator

Every device you plan to power has two wattage figures, and you need both: running watts and starting watts.

Running watts (also called rated watts) is the continuous power a device draws while operating normally. Starting watts (surge watts) is the brief spike of power that electric motors need for the fraction of a second it takes to get spinning. Motors in saws, compressors, pumps, and air conditioners typically draw two to three times their running wattage at startup. Resistive loads — space heaters, lights, microwaves, and electric water heaters — produce no starting surge at all.

To size a generator correctly:

• Add up the running watts of every device you plan to run simultaneously. This is your total running load.
• Identify the single device with the largest starting surge. Add only that surge on top of the total running load. This gives you your peak starting load.
• Apply a 20–25% headroom buffer above the peak load. This is the industry-standard safety margin that protects against nuisance trips and extends generator life.
• Select the next standard generator size above your buffered number (common sizes: 3,000W, 5,000W, 7,500W, 10,000W, 12,000W).

For a deeper look at generator rental options by region, see the generator rental guide.

Worked Example: Sizing a Generator for a Construction Site

Suppose a small framing crew needs to run three tools simultaneously: a 7⅜" circular saw, a 1 HP air compressor, and a hammer drill. Here is how the math works out.

Step 1 — Total running watts:

• Circular saw: 1,400–1,500W running
• Air compressor (1 HP): 1,400–1,600W running
• Hammer drill: 1,000W running
• Total running load: approximately 3,900–4,100W

Step 2 — Starting surge: The air compressor has the largest starting surge at 3,600–4,800W. Add the highest figure (4,800W) to the total running load: 4,100 + 4,800 = 8,900W peak.

Step 3 — Apply 25% headroom: 8,900 × 1.25 = 11,125W.

Step 4 — Select standard size: Round up to the next standard size — a 12,000-watt generator. A conventional portable in the 10,000W–12,000W range, or a small towable diesel unit, handles this crew comfortably with room to start additional small tools.

Construction Tool Wattage Reference

The figures below are typical industry averages. Check the nameplate on your specific tool for exact ratings, which can vary by manufacturer and model.

Tool	Running Watts	Starting Watts
Circular Saw (7⅜")	1,400–1,500	2,300–3,000
Miter Saw (10")	1,800	3,300
Table Saw (10")	1,800–2,000	2,000–4,500
Air Compressor (½ HP)	1,000	2,000
Air Compressor (1 HP)	1,400–1,600	3,600–4,800
Angle Grinder	1,000	2,000
Hammer Drill	1,000	3,000
Concrete Mixer	1,600	3,000
Pressure Washer (gas engine)	2,400	8,400
Reciprocating Saw	960	~1,920
Planer	800–950	~1,600–1,900

Household Appliance Wattage Reference

Home backup loads are generally more predictable than job site loads, but motor-driven appliances like refrigerators and AC units still carry a large starting surge. Electric water heaters and space heaters are purely resistive — their starting watts equal their running watts.

Appliance	Running Watts	Starting Watts
Refrigerator	700–800	1,200–2,200
Central AC (2 ton)	3,800	4,950–11,400
Window AC (10,000 BTU)	1,200	3,600
Sump Pump (⅓ HP)	800–1,140	1,300–2,850
Furnace Blower (½ HP)	800	1,300–2,350
Well Pump (⅓ HP)	575–1,000	1,440–2,200
Microwave	1,000	1,000
Electric Water Heater	4,000	4,000
Space Heater	1,500–1,800	1,500–1,800
Washing Machine	1,150–1,200	2,250–2,300
LED Lighting Circuit	~50	~50

Generator Types and Fuel Consumption

Choosing the right generator category matters as much as getting the wattage right. Fuel efficiency and noise level vary significantly across types.

• Portable inverter generators (1–3 kW): Run on gasoline. At 50% load, a 2 kW inverter generator typically consumes around 0.18 gallons per hour — highly efficient. Produce clean sine-wave power safe for sensitive electronics. Quiet enough for residential neighborhoods and outdoor events.
• Conventional portable generators (3–8 kW): Run on gasoline. Fuel consumption at 50% load ranges from 0.35 to 0.70 gallons per hour depending on output. The workhorses of small job sites and home backup. Heavier and louder than inverter units but significantly less expensive.
• Large portable generators (8–12 kW): Run on gasoline, sometimes dual-fuel (gasoline/propane). Fuel consumption at 50% load is typically 0.60 to 1.20 gallons per hour. These units often weigh 200+ lbs and require wheel kits or a hand truck to reposition.
• Standby and whole-house generators (10–26 kW): Permanently installed, typically running on propane or natural gas. Propane consumption at 50% load runs from roughly 0.97 to 3.0 gallons per hour depending on size. Activate automatically when grid power fails.
• Towable and industrial generators (20–1,000+ kW): Diesel-powered units on trailers, available from most equipment rental companies. A 20 kW diesel unit consumes approximately 1.1 gallons per hour at 50% load; a 100 kW unit consumes roughly 4.2 gallons per hour. These are the standard choice for large construction sites, events, and emergency response.

Altitude Derating and Safety

Generator output drops measurably at elevation. Naturally aspirated (non-turbocharged) generators lose 3.5% to 4% of rated power for every 1,000 feet above sea level. That adds up fast on mountain job sites:

• At 5,000 ft: approximately 15–20% power loss
• At 8,000 ft: approximately 24–32% power loss
• At 10,000 ft: approximately 30–40% power loss

A generator rated at 10,000 watts at sea level effectively delivers only 8,000–8,500 watts at 5,000 feet. When renting for high-altitude applications, ask the rental company specifically about altitude-derated output or turbocharged models, which are substantially less affected.

The 25% Headroom Rule

Size 20–25% above your calculated peak load. The buffer covers motor efficiency losses under load, measurement uncertainty in nameplate ratings, and the tool or appliance you didn't think of at planning time. Running at 70–80% of rated capacity is the efficiency sweet spot — lower fuel consumption per kWh and longer service life than pushing the unit near its limit continuously.

Electrical Safety Requirements

On construction sites, OSHA requires GFCI protection on all 120V, 15- and 20-amp receptacles supplied by a generator. A limited exemption applies to portable generators under 5,000 watts that are two-wire, single-phase, and equipped with fully insulated conductors — but most job site units do not meet all three criteria simultaneously. Generators over 5,000 watts, or any unit providing 120/240V, must have GFCI protection. The National Electrical Code (NEC) additionally requires a transfer switch for any generator connected to a building's permanent electrical system, preventing dangerous backfeed onto utility lines.

For carbon monoxide safety, never operate a generator indoors, in a garage, or under a partially open awning. Maintain at least 3 to 4 feet of clearance on all sides, and keep exhaust directed away from windows and doors. CO poisoning from portable generators causes dozens of fatalities in the United States every year, typically during or after major storms when units are moved indoors to avoid weather.