Quick Answer
Running watts vs starting (surge) watts
Every appliance has a running (rated) number: the continuous draw while it is on. Many also have a starting (surge) number: a short burst when a motor or compressor first spins. Lights, chargers, and most electronics only have running watts. Refrigerators, well pumps, sump pumps, furnace blowers, air conditioners, and shop compressors have both.
If the plate lists volts and amps but not watts, use:
Watts = volts × amps
A 120-volt device drawing 6 amps is about 720 running watts. A 240-volt well pump drawing 8 amps is about 1,920 running watts. That is the number you add to the running column.
Starting watts last a few seconds. Standard wattage worksheets treat surge like this:
- Add all running watts for the items that will be on together.
- Find the one item with the largest starting-watt figure.
- Add the extra start (starting watts minus that item’s running watts) to the running total.
- Compare the running total to the generator’s continuous rating, and the peak to its surge rating.
Do not add every motor’s surge unless they actually start together. Leave unused headroom so a fridge is not fighting the well. If two large motors must start in the same minute (well plus central air), treat that overlap as the peak—or stagger them.
Nameplates win over memory. Locked-rotor amps and hard-start kits change surge. For a medical device, use the manufacturer’s backup notes—not a generic table.
A nameplate worksheet (worked example)
This example is illustrative. The numbers are what you might record from plates, not national averages.
| Load (example plates) | Running W | Starting W | Notes |
|---|---|---|---|
| Refrigerator | 180 | 800 | Cycles on and off |
| LED lights + router + phones | 150 | — | No motor surge |
| Furnace blower | 720 | 1,400 | Heat without electric strips |
| Well pump | 1,920 | 4,800 | Largest motor on the list |
That house does not need whole-home standby just because it has a well. It also cannot live on a tiny inverter if the well and furnace may start while the fridge is running. The honest range is continuous watts for about 3 kW of simultaneous load, plus surge headroom near 6 kW—then you pick a managed portable, an interlocked subpanel, or a small standby.
Change one assumption: add a central air plate of 2,400 W running / 5,000 W starting and keep the well. If both can start in the same minute, the peak is no longer “one motor.” Lock out the AC during well recovery, or move into a standby-oriented range with load management. Square footage never entered the table.
Mini worksheet: apartment / condo Layer-1 only
Many renters and condo owners only need Layer 1—stay safe and informed—plus maybe a fridge for a limited window. Here is a second illustrative nameplate pass with motors off the list:
| Load (example plates) | Running W | Starting W | Notes |
|---|---|---|---|
| LED lamps + phone chargers | 80 | — | Continuous low draw |
| Router / modem / laptop | 70 | — | Keep communications |
| Refrigerator (cycling) | 180 | 800 | Only surge on the list |
| CPAP or similar (if prescribed) | 60 | — | Confirm device maker notes |
Extra surge (fridge only): 800 − 180 = 620 W
That is a compact-inverter or power-station conversation—if you have no well, no furnace blower on the generator, and no inlet. Surge still matters: a weak continuous rating can brown out when the fridge kicks. Record your own plates.
Priority loads vs whole-home: a decision tree
Work top-down. Stop when the next layer is more house than you will actually run.
Layer 1 — Stay safe and informed. Phones, a lamp or two, a radio or router, any prescribed medical device. Low running watts. A compact inverter or power station can cover this if motors are off the list. If a medical device cannot tolerate a transfer blink, ask the manufacturer about a UPS in front of the generator—not instead of a transfer plan.
Layer 2 — Food and water. Refrigerator/freezer (cycling surge) plus a well or sump if you have one. Wells and sumps change the answer more than extra bedrooms. See backup power for well pumps and backup power for medical devices if those apply.
Layer 3 — Habitable heat or cooling. A gas or oil furnace is often just the blower and controls. A heat pump or electric furnace is a large, repeating surge. Space heaters add up room by room. Decide whether you need a conditioned room or the whole thermostat schedule.
Layer 4 — Kitchen convenience. Electric water heaters and electric ranges are high, steady draws. They blow up a portable plan even when the well fits. Many households leave the 240-volt range off the generator and use a kettle or gas cooktop.
Layer 5 — Whole-home. Central HVAC on its normal cycle, kitchen, laundry, well, and “leave the panel alone.” That is a standby conversation, often with load-shedding. It is not what most portable owners should promise themselves.
Common sizing mistakes that waste money
These errors show up in returns, undersized machines, and oversized installs that never get used. Fix the worksheet before you shop.
- Summing every motor’s surge as if they start together. Fridge, well, and furnace rarely lock-rotor in the same second. Adding all three start bumps invents a peak that never happens and pushes you into an unnecessary chassis. Use the single largest extra start unless you know two large motors will overlap.
- Shopping by square footage. “A 2,000 sq ft house needs X kW” ignores heat fuel, municipal vs well water, and whether the electric range stays on the plan. Two identical footprints can need completely different machines.
- Ignoring the well or sump. A quiet lighting plan collapses the first time the pump recovers the tank. If water or basement protection is on the list, put the nameplate in the running column before you fall in love with a compact inverter.
- Treating the surge rating as continuous capacity. Cartons advertise a peak that lasts seconds. Your simultaneous running total must fit under the continuous rating with headroom; the surge rating only covers the brief start bump.
- Buying watts with no transfer path. A machine that cannot legally feed the furnace or well is an extension-cord appliance, not a house plan. Budget the inlet, interlock or transfer switch, and electrician time as part of “size,” not as an afterthought.
- Trusting the yellow energy sticker or memory instead of the nameplate. Stickers are often annual energy, not running watts. Locked-rotor amps and aftermarket hard-start kits change surge. Walk the equipment with a phone flashlight and write volts × amps.
- Leaving the electric range and water heater “maybe” on the list. Those loads are high and steady. Keeping them as soft maybes forces a standby-sized quote when a kettle and a gas cooktop would have kept you in a portable band.
Why square footage is a weak proxy
Two houses can share a 2,200-square-foot footprint and need different machines.
- House A: gas heat, municipal water, LED lighting, one fridge. Critical-load running watts stay modest.
- House B: heat pump, private well, electric water heater, electric range, a workshop compressor. Same footprint; completely different surge and fuel story.
Square footage does not know your heat fuel, whether water is pumped, or whether you will run a dryer. The sizing tool uses sq ft as a weak prior, then overrides it with backup goal, HVAC count, and load checkboxes. Skip the well and the range will be wrong.
Portable vs standby size bands (qualitative)
Do not shop a kilowatt number until you know the chassis. Bands below are planning language, not model picks.
Compact inverter / small portable. Layer 1, plus a fridge if you manage other loads. Cords to specific appliances. Not a house plan.
Mid-size portable. Layer 2 if you stagger motors: fridge + lights + one pump or a window AC. Dual-fuel is a fuel choice, not a size choice (dual-fuel explained).
Large portable. Can feed a prioritized subpanel through an inlet and interlock if the watts fit. Still owner-started and refueled by hand. This is the crossover where people compare portable vs standby on convenience rather than watts.
Small or mid standby. Automatic transfer for hardwired essential circuits or a managed panel. Makes sense when nobody is home to start a machine, or a sump or well cannot wait.
Larger standby. Broader HVAC, multiple compressors, or “don’t think about the panel.” Install complexity (pad, fuel, permit, transfer gear) dominates. Watts still come from an electrician’s load calculation.
The tool’s result labels follow the same idea: portable or inverter-focused, large portable or small standby, or standby-oriented. “See generators in this range” means compare categories and real ratings—not a brand podium.
You still need a transfer path
Sizing answers how many watts. It does not answer how those watts enter the house.
- Extension cords only reach what you can plug in. Outdoor-rated, adequate gauge. They do not run a hardwired well, furnace, or sump unless those already have a legal inlet.
- Never backfeed a dryer or range outlet. That energizes the service drop. Ready.gov and fire-service guidance treat indoor running and backfeeding as the two unforgivable setup errors.
- Interlock or transfer switch is how a portable feeds selected circuits without backfeed. An interlock keeps the main and generator breakers from being on together. A manual transfer switch moves a listed set of circuits. An ATS is what most standby systems use.
- Permits and a licensed electrician are the normal path onto the service panel. Neutral bonding is a local-code question, not a wattage-worksheet finish.
Read transfer switch vs interlock before you treat a portable as the house generator. Placement is part of the path: outdoors, away from doors and windows, never in a garage. Pair this with the homeowner outage checklist for fuel and CO alarms.
How to use the generator sizing tool
Open the generator sizing tool. The form is a planning estimator, not a dealer load study.
- Approximate square footage. A ballpark. It nudges the estimate; it does not replace the checkboxes.
- Backup goal. Critical loads only, Most important household loads (default), or Broader whole-home coverage—Layer 1–2 vs 3–4 vs 5. “Broader” still scales the estimate up even if you leave the range unchecked.
- Preferred fuel path. No strong preference, Portable fuel storage, Natural gas service, or Propane tank. Fuel does not change physics; it changes which chassis is realistic. Street gas favors standby. Propane can feed dual-fuel portables or a tank-fed standby. Cans in a shed point toward portable planning.
- Central HVAC units. Count compressors or air handlers you might start (0–4). Zero is valid if heat is a gas furnace blower only.
- Critical loads. Refrigerator/freezer, well, sump, electric water heater, electric range/oven, medical or critical electronics. Well, water heater, and range move the estimate more than an extra bedroom.
- Workshop / large garage. Yes only if welders, compressors, or a loaded shop panel are part of the plan.
- Calculate Estimate. Read the category first, then the estimated running watts and startup planning load. Those figures are a worksheet, not a lab measurement.
If the category surprised you, toggle the well, the range, or HVAC count and see which input did the work. Then walk the house and confirm the two or three largest nameplates. If your recorded peak is far above the tool, trust the plates and talk to an electrician.
See generators in this range—then match continuous and surge ratings to your worksheet, not to a model number. Local trees and winter ice still take feeders down even when the Atlantic season is quiet.
Next step
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