Generator Size Calculator

Generator Sizing for Real Outages

Running vs. starting watts

Resistive loads (bulbs, TVs, chargers, space heaters) draw their rated watts, full stop. Motor loads (compressors, pumps, fans) draw 2–3× their running rating for a moment at startup — the locked-rotor surge. Generators publish both numbers for the same reason: a '4,000/3,200 W' unit sustains 3,200 continuously with 4,000 available briefly. Undersizing shows up as breaker trips or stalls exactly when the fridge and well pump happen to start together — which load-sequencing (start things one at a time) prevents.

The classes and what they honestly cover

2,000–2,200 W inverters (quiet, ~50 lbs) run a fridge, lights, WiFi, and phone charging — genuine essentials-only outage coverage. 4,000–6,500 W portables add a furnace fan, well pump, or window AC with load management. 7,500–12,000 W covers most homes minus central AC luxury. Whole-home standby units (14–26 kW, permanently installed, auto-start) are the everything-including-AC tier at $10–20K installed. The honest question isn't 'what runs my house' but 'what runs my 72-hour outage' — most families discover 4,500 W covers it.

The safety rules that are actually non-negotiable

Carbon monoxide kills roughly 70–100 people per year around US generators — always outdoors, 20+ feet from the house, exhaust pointed away, never in garages even with the door open. Backfeeding through a dryer outlet energizes the utility line and can electrocute lineworkers; the legal connections are a transfer switch ($500–1,500 installed) or interlock kit, or simply extension cords to individual appliances. And refuel only when off and cool — gasoline on a hot muffler is the second-most-common generator emergency.

Generator size by coverage goal: a reference grid

Generator sizing works from the loads you'll run, accounting for motor starting surges. This grid shows typical wattage needs and the matching generator class for common outage-coverage goals.

Motors are the sizing game: a fridge running at 700 W demands ~2,200 W for the two seconds its compressor starts. The saving grace is that starts are sequential — size for running load plus the single largest motor's surge, not everyone's surge at once. Managing which motor starts when is how a smaller generator runs a house with a larger total surge.

Common mistakes in generator sizing

• Ignoring starting watts. Motors draw 2–3× their running rating at startup. Sizing only for running watts means breaker trips when the fridge and well pump start together.
• Sizing for "everything at once." Loads start sequentially. Size for running load plus the largest single surge, then stagger startups — that's how a 5,000 W unit runs a 7,000 W-surge house.
• Backfeeding through an outlet. Plugging a generator into a dryer outlet energizes the utility line and can electrocute lineworkers. Use a transfer switch or interlock, or extension cords to appliances.
• Running it in or near the house. Carbon monoxide kills. Always 20+ feet outdoors, exhaust pointed away — never in a garage, even with the door open.

Running vs. starting watts, and the safety non-negotiables

Resistive loads — bulbs, TVs, chargers, space heaters — draw their rated watts flat. Motor loads — compressors, pumps, fans — draw 2–3× their running rating for a moment at startup, the locked-rotor surge, which is why generators publish both numbers: a "4,000/3,200 W" unit sustains 3,200 continuously with 4,000 available briefly. The classes cover honest coverage tiers: a 2,000–2,200 W inverter runs a fridge, lights, WiFi, and charging (genuine essentials); 4,000–6,500 W adds a furnace fan or well pump with load management; 7,500–12,000 W covers most homes minus central AC; and whole-home standby units (14–26 kW, auto-start) are the everything tier. The safety rules are non-negotiable: carbon monoxide kills dozens of people a year around generators, so always run outdoors 20+ feet from the house with exhaust pointed away, never in a garage even with the door open. Backfeeding through a dryer outlet can electrocute lineworkers — the legal connections are a transfer switch or interlock kit, or extension cords to individual appliances. And refuel only when the unit is off and cool.

Inverter versus conventional, and why it matters

Beyond raw wattage, the choice between an inverter and a conventional generator affects what you can safely power and how much fuel you burn. Inverter generators produce clean sine-wave electricity, safe for laptops, phones, and the sensitive electronics in modern furnace control boards — conventional generators produce dirtier power that can damage them. Inverters also throttle their engine to match the load, burning roughly half the fuel at partial load and running 15–20 decibels quieter, which matters enormously during a multi-day outage in a neighborhood. Conventional generators run their engine at full speed regardless of load, making them louder, thirstier, but cheaper per watt for large capacities. For anything electronics-adjacent or where noise matters — which describes most home backup situations — the inverter premium repays itself in fuel savings alone over a long outage, before counting the quiet and the protected electronics. The practical read: essentials-and-electronics coverage strongly favors an inverter, while a large conventional unit makes sense for pure high-wattage brute-force needs like running well pumps and power tools where sine-wave purity is irrelevant.

Frequently asked questions

What size generator runs a refrigerator and freezer?

Both together need ~1,200–1,500 running watts with surges to ~3,000 if they start simultaneously — a 3,000–3,500 W generator handles it comfortably; a 2,200 W inverter manages if the starts don't overlap (they rarely do).

Can a generator run central air conditioning?

A 3-ton central AC needs ~3,500 running / 8,000+ starting watts — realistic only on 10,000+ W portables or standby units, ideally with a soft-start kit ($300) that cuts the surge by half and lets 6,500 W generators manage smaller AC systems.

Inverter generator vs. conventional — worth the premium?

Inverters produce clean sine-wave power (safe for laptops and modern furnace boards), throttle to match load (half the fuel burn at partial load), and run 15–20 dB quieter. For anything electronics-adjacent or neighborhood-adjacent, yes — the premium repays in fuel alone over a long outage.

What size generator runs a refrigerator and freezer?

Both together need about 1,200–1,500 running watts, with surges to ~3,000 if they start simultaneously — a 3,000–3,500 W generator handles it comfortably. A 2,200 W inverter manages if the starts don't overlap, which they rarely do. Add lights and WiFi and you're still under a 4,000 W unit.

Can a portable generator run central air conditioning?

A 3-ton central AC needs ~3,500 running and 8,000+ starting watts — realistic only on 10,000+ W portables or standby units. A soft-start kit (~$300) cuts the surge by about half, letting some 6,500 W generators manage smaller AC systems. For most homes, covering essentials without AC is the practical portable-generator goal.