What Size Generator Do You Need? Size It Once, Size It Right
By the VoltConvert team · June 12, 2026
Every hurricane season, ice storm and grid scare sends a wave of buyers at the generator aisle armed with a guess. Half buy too small and trip it the first night; the other half overpay for capacity they'll never load. The right size comes from forty minutes of arithmetic. Here is the professional method, homeowner edition.
Step 1 — List what genuinely must run
Outage power is triage, not normal life. The usual essentials: refrigerator/freezer (~700 W running), furnace blower or fan (~800 W), sump pump if you have one (~800 W), lights and internet (~300–400 W), phone charging and a TV (~200 W). A window AC adds 1,000–1,500 W; a well pump 1,000–2,000 W. Whole-house central AC and electric water heating are usually what push you from a portable into standby-generator territory — decide deliberately whether they make the list.
Step 2 — Total the running watts
Use nameplate or measured running watts, not maximums. A typical essentials list lands between 2,500 and 5,000 W. (To sanity-check any single appliance against a circuit or extension cord while you're at it, the watts to amps calculator is the tool.)
Step 3 — The surge question, where most sizing fails
Motors don't start politely. Refrigerator compressors, sump pumps, well pumps and AC units briefly draw 3–6× their running watts at start. The practical rule: take your running total and add the starting surplus of the single largest motor (they rarely all start in the same second). Example: 3,000 W running, largest motor is a sump pump that runs at 800 W but starts at ~2,400 W → plan for 3,000 + 1,600 = 4,600 W of capability. Generator spec sheets list both "running/rated watts" and "starting/peak watts" — your running total must fit under the rated figure with the surge under the peak figure.
Step 4 — Convert to the unit on the price tag
Larger units and rental equipment are quoted in kVA, and the exchange rate is power factor: kVA = kW ÷ 0.8 for typical mixed household loads. The 4.6 kW requirement above is 5.75 kVA — meaning the sensible purchase is the 6.5–7 kVA class, because the last rule of sizing is to run a generator at 70–80% of its rating, not 100%. Continuous full-load running shortens engine life and leaves zero margin. The kW to kVA calculator and the single-phase version do this step with the formula shown.
Step 5 — The decisions the math doesn't make
Fuel: gasoline is everywhere but stores poorly; propane stores indefinitely and burns cleaner; dual-fuel units hedge. Connection: extension cords for a couple of appliances, or a transfer switch/interlock (installed by an electrician, required by US code for backfeeding a panel) for whole-circuit convenience — never the dryer-plug "suicide cord," which endangers line workers and is illegal everywhere that has electricity. Noise and placement: inverter generators run dramatically quieter, and every generator runs outdoors only, far from windows — carbon monoxide kills more people after storms than the storms do.
The one-paragraph version
Total your essential running watts. Add the starting surplus of your biggest motor. Divide by 0.8 to get kVA, then buy the next size up so you cruise at 70–80% load. For most homes that lands between 4 and 8 kVA portable — or pushes you to a standby unit if central AC made the essentials list. Forty minutes with a notepad and two calculators, and you'll buy exactly once.
Related calculators
kW To kVA · Single Phase kW To kVA · Watts To Amps · kVA To Amps