Generator sizing is where most buyers make their most expensive mistake — in either direction. Too small and the generator bogs, stalls, or damages the very appliances it was bought to protect when a motor starts. Too large and you've overspent on capacity, fuel, and maintenance for watts you'll never draw. The correct method is a load list: sum the running watts of everything you expect to operate simultaneously, add the largest starting surge, apply headroom, and match to a standard generator size. The calculator below builds that list for you with realistic preset values you can edit to match your actual appliances.
Our whole-house generator sizing guide and the 2026 sizing chart cover the scenario-based approach; this page is the deeper interactive version with a live load list and the engineering math to verify any dealer's recommendation.
Generator Sizing Calculator
Build your load list · running watts + largest starting surge + 25% headroom → recommended kW
Method: Required = Σ running watts + max(starting − running). Recommended = next standard size above required × 1.25 headroom, derated for altitude. Preset watts are typical values — check your appliance nameplates. Don't forget the transfer switch sizing chart.
The Sizing Method: Running Watts, Starting Watts, Headroom
Every motorized appliance — refrigerator compressors, pumps, fans, air conditioners — draws two very different currents. Running watts is the steady draw once the motor is spinning. Starting watts (locked-rotor or surge) is the 2–5× spike for the first second or two while the motor fights up to speed. Resistive loads like heaters, dryers, and incandescent lights have no surge: starting equals running.
The field-proven sizing method:
Required watts = Σ running watts + largest (starting − running) surge
Recommended kW = next standard size ≥ Required × 1.25
Why only the largest surge rather than all of them? Because motors almost never start simultaneously, and a modern automatic transfer switch with load management guarantees they won't. The 25% headroom covers measurement error, altitude, heat, aging, and the load you forgot to list. Running a generator at 50–80% of rated capacity is also where fuel efficiency and engine life are best — chronic light loading below 30% wet-stacks diesel engines, and chronic full-throttle operation wears everything.
Worked Example 1: The Critical-Loads House (7.5 kW)
The classic outage kit: refrigerator (700 W run / 2,200 W start), furnace blower (800/2,300), lights (400/400), and a ½ hp sump pump (1,050/3,200):
Running total = 700 + 800 + 400 + 1,050 = 2,950 W
Largest surge = 3,200 − 1,050 = +2,150 W (the sump pump)
Required = 5,100 W → ×1.25 = 6,375 W → 7.5 kW portable
This is the most common portable-generator answer in America, and it fits our 6–10 kW range comfortably. Add a window AC (1,200/3,600) and the required jumps to 7,600 W → 9,500 with headroom → a 10 kW unit.
Worked Example 2: Whole Home with Well Pump (10 kW)
Rural home: refrigerator (700/2,200), window AC (1,200/3,600), 1 hp well pump (2,000/6,000):
Running = 3,900 W · largest surge = 6,000 − 2,000 = 4,000 W · required = 7,900 W → ×1.25 = 9,875 → 10 kW
Well pumps are the silent sizing killer in rural installs — their surge dominates everything else on the list. Our rural generator sizing guide digs into pump-heavy load lists specifically.
Worked Example 3: True Whole-Home Standby (22–26 kW)
A 2,500 sq ft all-electric home running central AC (3,500/10,500), water heater (4,500), refrigerator, freezer, lights, and electronics: running ≈ 10,450 W, largest surge +7,000 W, required ≈ 17,450 W, ×1.25 ≈ 21,800 W → 22 kW standby. That's why the 22 kW class is the best-selling standby size in the country — see what you need to know about 22 kW generators, the 26 kW guide for larger homes, and the 12/16/20/24 kW head-to-head for the full decision ladder. Browse the actual units in 20–28 kW standby or 22 kW generators.
Typical Running & Starting Watts Reference
| Appliance | Running W | Starting W | Surge multiple |
|---|---|---|---|
| Refrigerator / freezer | 700 | 2,200 | 3.1× |
| Sump pump (½ hp) | 1,050 | 3,200 | 3.0× |
| Well pump (1 hp) | 2,000 | 6,000 | 3.0× |
| Furnace blower (½ hp) | 800 | 2,300 | 2.9× |
| Window AC (10k BTU) | 1,200 | 3,600 | 3.0× |
| Central AC (3 ton) | 3,500 | 10,500 | 3.0× (soft-start cuts to ~1.5×) |
| Electric water heater | 4,500 | 4,500 | 1× (resistive) |
| Electric dryer | 5,400 | 5,400 | 1× (resistive) |
| EV charger (30 A) | 7,200 | 7,200 | 1× (continuous!) |
Standard Generator Sizes by Use Case
| Size class | What it covers | Type |
|---|---|---|
| 2–3.5 kW | Camping, tailgating, one fridge + lights | Inverter portable |
| 5–7.5 kW | Critical loads: fridge, furnace fan, sump, lights | Portable |
| 9–12 kW | Critical loads + window AC or well pump | Large portable / small standby |
| 14–20 kW | Partial whole-home, one central AC with management | Standby |
| 22–26 kW | True whole-home for most 2,000–3,500 sq ft houses | Standby |
| 30–48 kW | Large estates, small commercial | Liquid-cooled standby |
| 60 kW+ | Commercial and industrial | Engineered systems |
Fuel Consumption by Size (Planning Figures)
Runtime per tank or per propane cylinder scales with size and load. Approximate consumption at half load:
| Generator size | Gasoline (gal/h @50%) | Propane (lb/h @50%) | Natural gas (ft³/h @50%) |
|---|---|---|---|
| 5 kW portable | 0.4 | 1.2 | — |
| 7.5 kW portable | 0.6 | 1.7 | — |
| 10 kW | 0.8 | 2.1 | 110 |
| 14 kW standby | — | 2.6 | 140 |
| 22 kW standby | — | 3.6 | 200 |
| 26 kW standby | — | 4.2 | 230 |
These numbers decide fuel logistics: a 22 kW standby on propane burns through a 500-gallon tank in about five days of continuous half-load operation, while the same unit on natural gas runs as long as the utility gas flows. The fuel-type comparison breaks down the full trade-offs, and liquid-cooled units become the right answer above ~30 kW or for extended runtimes.
Altitude & Temperature Derate
| Elevation | Engine output | What to do |
|---|---|---|
| 0–3,000 ft | 100% | No adjustment |
| 3,000–6,000 ft | ~90% | Divide required watts by 0.9 (one size up usually covers it) |
| Above 6,000 ft | ~80% | Divide by 0.8; confirm with manufacturer derate tables |
Thin air starves naturally aspirated engines of oxygen. Most manufacturers publish derates of roughly 3–4% per 1,000 ft above a baseline; the 10%/20% bands above are safe planning figures. High ambient heat (100 °F+) takes another few percent. Diesel and turbocharged engines suffer less.
Common Generator Sizing Mistakes
- Summing every starting watt. This assumes every motor in the house starts in the same second — it doesn't happen, and it oversizes the generator by 30–50%.
- Ignoring starting watts entirely. The mirror-image error: a 4,000 W generator "covers" 3,500 W of running load on paper, then stalls the moment the well pump kicks.
- Forgetting EV chargers are continuous loads. A 30 A EVSE pulls 7,200 W for hours. Under NEC, continuous loads need 125% headroom on the supply — treat EV charging as a scheduling decision, not a background load, or size up deliberately.
- Skipping the transfer switch. The generator is only half the system. An undersized or manual switch negates an automatic standby unit. Match it with the ATS types guide and transfer switch inventory.
- No headroom. A generator running at 98% capacity in a July outage is a failure waiting for a heat wave. Twenty-five percent headroom is the industry norm for a reason.
- Buying on surge rating. Portable generators advertise peak watts prominently; the rated/running figure is what sustains your house. Compare running watts only.
Portable vs. Standby: Where the Line Sits
The load list usually decides the format before budget enters the conversation. Under about 10 kW of required capacity, portable gasoline and dual-fuel units compete on price and flexibility — you store the unit, wheel it out, and back-feed through a manual interlock or transfer panel. Above 10–12 kW, or whenever automatic unattended operation matters (vacation homes, medical equipment, sump pumps in storm season, businesses), standby units on natural gas or propane win outright: they self-test weekly, start within seconds of an outage, and run for days without refueling trips. The crossover zone — homes needing 9–14 kW — is where the honest comparison happens: a large portable plus a manual transfer switch costs a third of a standby install but demands that someone be home, awake, and willing to work in the rain. Price both paths in our 10–14 kW standby and 10–12 kW portable collections before deciding.
One note on inverters vs. conventional portables at the small end: inverter generators cost more per watt but deliver clean power for electronics, run dramatically quieter, and throttle down at light loads to save fuel — the right call for camping and electronics-heavy critical loads. Conventional open-frame units deliver more surge watts per dollar, which matters when the load list is dominated by pumps and compressors.
Finally, remember that the calculator's "required" number assumes loads you actually listed. Before ordering, walk the property with the list: garage freezers, basement dehumidifiers, well-house heat tape, and the second refrigerator in the garage are the classic forgotten loads that turn a well-sized 10 kW into an overloaded one.
Generator or Battery — or Both?
Generators win on multi-day outages, heavy motor loads, and cold climates. Batteries win on silence, instant switchover, daily cycling against time-of-use rates, and indoor air quality. The honest answer for many homes is a battery for the first hours and a generator for the long tail — size the battery side with the Battery Runtime Calculator and compare the economics in Battery Backup vs. Generator. Whatever you land on, the circuits between matter: verify feeders with the Voltage Drop Calculator and conductors with the Wire Ampacity Calculator, then get the whole package quoted.
Frequently Asked Questions
What size generator do I need to run a refrigerator and a furnace?
A refrigerator (700 W running, 2,200 W starting) plus a ½ hp furnace blower (800 W running, 2,300 W starting) needs 1,500 W running + 1,500 W surge = 3,000 W required, or about 4 kW with headroom. A quality 4–5 kW portable handles it; 7.5 kW leaves room for lights and a sump pump.
What size generator do I need for a whole house?
Most 2,000–3,500 sq ft homes with central air land between 20 and 26 kW for true whole-home coverage, which is why 22 kW is the best-selling standby class. Homes with gas heat and water heating can drop to 14–16 kW. Build your actual load list in the calculator above rather than guessing from square footage.
What's the difference between starting watts and running watts?
Running watts are the steady draw of an appliance in normal operation. Starting watts are the 2–5× spike lasting one to two seconds while a motor accelerates. Generators publish both; size to running watts plus the largest single starting surge, not the sum of all surges.
Can a generator be too big?
Yes. Oversized generators cost more to buy, fuel, and maintain, and chronic light loading (under ~30%) causes wet-stacking in diesels and carbon buildup in all engines. Aim for 50–80% loading at typical demand — the 25% headroom rule naturally lands you there.
Does altitude really reduce generator power?
Yes. Naturally aspirated engines lose roughly 3–4% of output per 1,000 feet of elevation. Plan on −10% between 3,000 and 6,000 feet and −20% above 6,000 feet, and confirm against the manufacturer's derate table for your exact model.
Do I need an automatic transfer switch with a standby generator?
Yes — an ATS is what makes a standby generator "standby": it detects the outage, starts the unit, and transfers load in seconds, then reverses the process when utility power returns. Size the switch to your service amperage, not the generator output.
