For most Portland-area homes, the answer lands between 18kW and 22kW. A gas-heated home usually does well at 18kW to 22kW, an essentials-only backup can run on 10kW to 14kW, and an all-electric home with a heat pump and EV charger pushes into 24kW and up. But the honest answer is that generator size is decided by a load calculation, not by square footage, and the number that matters most is not what your appliances use while running. It is the surge your biggest motor demands the moment it starts.
Here is how sizing actually works, what each size covers, and the Portland-specific wrinkles, from NW Natural gas meters to PGE shutoff zones, that change the answer.
Running watts vs starting watts: the number that actually sizes your generator
Every appliance has a running wattage, the steady power it draws while operating. Resistive loads like water heaters, ovens, and baseboard heat draw the same power from the second they turn on. Motors are different. A stopped motor, like your AC compressor, well pump, or furnace blower, pulls a brief surge of 3 to 7 times its running draw to get spinning. That surge only lasts a second or two, but your generator has to absorb it without the voltage collapsing. If it cannot, the lights sag, the breaker trips, or the engine stalls.
That is why generator sizing comes down to one formula: your home’s continuous running load (with a 25 percent safety buffer, since the code treats a multi-day outage as a continuous load), plus the starting surge of your single largest motor. Only the largest one counts, because your well pump, AC, and sump pump almost never start in the exact same instant.
| Appliance | Typical running watts | Typical starting watts |
|---|---|---|
| Gas furnace blower (1/3 HP) | 500 to 800 | 1,400 to 2,400 |
| Central AC / heat pump (3-ton) | ~3,500 | Up to ~10,500 without a soft starter |
| Deep well pump (1 HP) | ~1,000 | 3,000 to 4,000 |
| Sump pump (1/2 HP) | ~800 | ~2,400 |
| Refrigerator / freezer | 150 to 400 | 600 to 1,200 |
| Electric water heater (50 gal) | ~4,500 | Same (no surge) |
| Electric range / oven | 3,000 to 8,000 | Same (no surge) |
| Level 2 EV charger | 7,600 to 11,500 | Same (continuous) |
Two things jump out of that table. First, a single 3-ton AC can briefly demand more power than an entire small generator produces. Second, an EV charger is a monster continuous load, nearly half of a 24kW unit by itself. Both change your size, and both can be tamed, which we will get to.
What size fits your home: four Portland profiles
| Home profile | Recommended size | Why |
|---|---|---|
| Essentials only. Fridge, gas furnace blower, lights, internet, key outlets. No AC backup. | 10kW to 14kW | Survival loads rarely exceed 3,000 to 4,000 running watts, and the furnace blower is the only real motor. Usually paired with an essentials-circuit transfer panel. |
| Average gas-heated home. Gas furnace and water heater, electric dryer, 3 to 4 ton AC. Most of Beaverton, Tigard, Hillsboro. | 18kW to 22kW | Gas carries the heavy thermal loads, so the AC’s starting surge is the sizing constraint. A 22kW starts it cleanly with the rest of the house running. |
| All-electric home. Heat pump, heat pump water heater, induction range, EV charger. | 24kW to 28kW + load management | Huge continuous loads. The EV charger, dryer, and water heater get wired through management modules so they yield when the heat pump starts. |
| Large rural property. 4,000+ sq ft, well pump, multiple AC units, shop, 400A service. Stafford, West Linn, rural Clackamas. | 28kW air-cooled minimum, often 38kW+ liquid-cooled | A well pump plus multiple compressors is a heavy inductive burden that can crash a smaller alternator if they start together. |
An electronic soft starter installed on your AC or heat pump compressor ramps the motor up gradually instead of slamming it on, cutting the starting surge by roughly 65 to 75 percent. That one device can let a 14kW or 18kW generator comfortably start cooling equipment that would otherwise demand a much bigger unit. It is often the difference between generator tiers, and it costs far less than stepping up a size.
Whole-home vs managed backup: how a 22kW covers a 200-amp house
Here is the part most homeowners have not heard. By code (NEC 702.4), a standby system connected through an automatic transfer switch must either be big enough to carry the panel’s full calculated load, or it must use automatic load management. In practice, nearly every whole-home install in Portland uses management: small smart modules watch the generator’s output, and if it approaches overload they briefly shed the loads you choose, the dryer, the hot tub, the EV charger, so the essentials and the big motors always have room.
That is how a 22kW to 26kW air-cooled unit legally and safely backs up a large modern home without stepping up to an industrial liquid-cooled machine. The skill is in the load calculation and in choosing what sheds first, which is exactly the design work we do on a standby generator installation.
Want the real number for your house?
We run the load calculation and size it to your home, no oversizing and no upsell.
Fuel matters: natural gas derating and your NW Natural meter
Generators are rated on liquid propane. Run the same unit on natural gas, which has far less energy per cubic foot, and its output drops roughly 10 to 15 percent. A “24kW” generator on NW Natural gas really delivers about 21kW to 22kW, and an honest sizing calculation uses that derated number. Homes at elevation, in the West Hills or toward Sandy and Estacada, lose a little more.
A 22kW-class generator near full load draws roughly 235 to 256 cubic feet of natural gas per hour. A standard residential meter flows only about 250 CFH total, and your furnace, water heater, and range need gas at the same time. Most whole-home generator installs in the Portland area require NW Natural to upgrade the meter, and long pipe runs often need an upgraded-pressure line with a step-down regulator at the generator. We coordinate that with NW Natural as part of the install, but it is a real lead-time item to plan for.
Why Portland homes are buying backup: ice storms and PSPS shutoffs
The January 2024 ice storm is the reference case. Over nine days, PGE logged 524,600 cumulative customer outages, peaking at 165,000 customers at once, with 23 transmission lines, hundreds of power poles, and nearly 600 transformers damaged. Parts of the metro sat dark for the better part of a week in sub-freezing temperatures. Storms in the winters since have repeated the pattern on a smaller scale.
Summer brings the opposite problem: Public Safety Power Shutoffs. During high-risk fire weather, PGE proactively de-energizes lines in its designated high fire-risk zones, which include the Portland West Hills, the Tualatin Mountains, the Mt. Hood foothills toward Estacada, and the Columbia River Gorge corridor. And here is the detail that surprises people: when the wind stops, the power does not come right back. Crews must physically patrol every mile of de-energized line before re-energizing, which typically adds hours to a day or more after the weather clears. If you live in one of those zones, your generator needs the fuel supply and endurance for multi-day runs, twice a year, in opposite seasons.
The four sizing mistakes we fix most often
- Oversizing. Engines want to run at half load or more. A big generator loafing at 10 percent load runs cold, fouls plugs and valves, burns more fuel, and wears out early. Bigger is not safer, it is just harder on the machine and the budget.
- Undersizing. Go too small and the load management sheds things you actually care about, like the AC in a summer shutoff or the oven during a winter storm. Fine on paper, miserable in week two of an outage.
- Ignoring starting surge. Adding up running watts and calling it done is how generators stall the first time the thermostat calls for cooling. The surge math is the sizing math.
- Forgetting what is coming. Planning a heat pump conversion, an ADU, or an EV in the next few years? Size for that home, not this one. A proper load calc looks forward, so your generator is not obsolete in three years.
For what a properly sized system costs installed, see our generator installation cost guide.
Frequently asked questions
Will a 22kW generator run my whole house?
For most 2,000 to 3,000 square foot Portland homes with gas heat and gas water heating, yes. A 22kW unit delivers roughly 90 to 100 amps of continuous 240V power, enough for lighting, refrigeration, plugs, and a 3 to 4 ton AC at the same time. All-electric homes usually need more capacity or load management.
What size generator do I need for a 2,000 sq ft house?
Square footage alone does not decide it, your loads do. A typical 2,000 sq ft home with gas heat and central AC lands at 18kW to 22kW. If the same house is all-electric with a heat pump or electric furnace, the requirement climbs to 24kW to 26kW.
Can a standby generator run a central air conditioner?
Yes, if it is sized for the compressor’s starting surge. A 3-ton AC runs on about 3,500 watts but can briefly demand around 10,500 watts at startup. A 22kW unit absorbs that easily, and a soft starter can cut the surge by up to 75 percent so smaller generators can handle it too.
What is the difference between running watts and starting watts?
Running watts are the continuous power an appliance uses while operating. Starting watts are the short surge a motor needs to begin spinning, often 3 to 7 times its running draw. A generator must cover the home’s running load plus the starting surge of its largest motor.
Do I need a new gas meter from NW Natural for a generator?
Usually, yes. A standard residential meter flows about 250 cubic feet per hour, and a 22kW generator near full load can use nearly all of that by itself, before your furnace and water heater draw anything. NW Natural evaluates the total load and typically installs a higher-capacity meter as part of a whole-home generator project.
Is it safe to run a standby generator for a week straight?
Modern standby generators are built for extended emergency runs like the January 2024 ice storm. The catch is maintenance: running around the clock, most units want an oil and filter change after roughly 100 to 200 hours, about 4 to 8 days of continuous operation. Checking oil during a long outage is what keeps the engine alive.
Does a standby generator add value to my home?
A permanently installed, permitted standby system is treated like a major home system upgrade and is a genuine selling point in outage-prone areas like the West Hills and the Gorge corridor. Unlike a portable unit, it conveys with the house, wired, plumbed, and inspected.
Get the size right the first time.
We run a real load calculation, account for your gas supply and future plans, and install Generac-trained with the permit and inspection handled. Licensed in Oregon (CCB# 248553) and Washington (ELECTAP741JB), with 250+ five-star reviews across the Portland metro, the Columbia Gorge, and Southwest Washington.