Add the running watts of everything you will run at once, add only the largest single starting surge, then add 20% headroom. The example outage list — fridge, freezer, sump pump, furnace blower, microwave, lights, TV, computer and router, coffee maker — totals 5,545 running watts, peaks at 7,895 watts, and lands on a 10,000 W class generator.
Running watts, starting watts, and the 2–3 second surge
Every appliance has two numbers. Running watts are what it draws continuously once it is going. Starting watts are what a motor-driven appliance needs for the 2–3 seconds it takes to start — in Generac's words, “starting watts are extra watts needed for 2–3 seconds to start motor-driven products.” Resistive and electronic loads — a water heater, a space heater, lights, a TV — have no surge at all. Motor loads — refrigerator compressors, pumps, furnace blowers, washing machines — can need two to three times their running draw for those few seconds, and a generator that cannot cover the surge stalls or trips exactly when the fridge kicks on.
This page's table stores additional starting watts: the extra watts above running watts, following Generac's worksheet convention. Total starting watts for one appliance are running + additional. Keeping the convention straight is the whole game, because the two ways incumbents mix it up both produce a wrong size: summing every appliance's starting watts oversizes the unit (motors do not all start in the same second), while adding an appliance's total starting watts on top of a running total that already includes its running watts double-counts them. The formula here is the worksheet formula: total running watts, plus the single largest additional surge, once.
The appliance table — with its sources
| Appliance | Running W | Additional starting W |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Freezer (standalone) | 700 | 2,200 |
| Sump pump (½ HP) | 1,050 | 2,200 |
| Well pump (⅓ HP) | 1,000 | 2,200 |
| Furnace blower (½ HP) | 800 | 2,350 |
| Window AC (10,000 BTU) | 1,200 | 1,800 |
| Central AC (24,000 BTU, ~2-ton) | 3,800 | 4,950 |
| Microwave (1,000 W) | 1,000 | 0 |
| Electric water heater | 4,000 | 0 |
| Washing machine | 1,150 | 2,250 |
| Electric clothes dryer | 5,400 | 1,350 |
| LED lights (10 bulbs) | 100 | 0 |
| TV (55" LED) | 90 | 0 |
| Computer (laptop) + router | 105 | 0 |
| Space heater | 1,800 | 0 |
| Electric oven / range (8" element) | 2,100 | 0 |
| Coffee maker | 1,000 | 0 |
| Garage door opener (½ HP) | 720 | 700 |
| CPAP (medical device) | 60 | 0 |
Sources: (1) Generac, Home Standby Generator Ownership Guide — “Typical Power Needs by Appliance” / Wattage Worksheet (running watts + additional starting watts per appliance), corroborated by Generac Support (support.generac.com, “What does surge/starting wattage mean?”). Generac lists “Refrigerator/Freezer” as one row at 700 / +2,200; this table applies those values to a standalone freezer as the conservative typical, and labels the row accordingly. The computer row is a visible composite: Generac laptop 85 W + Wi-Fi modem/router 20 W. (2) Honda Power Equipment, “What Wattage Generator Do I Need?” wattage estimation guide (powerequipment.honda.com) — the garage-door-opener row: Honda states 720 W running and 1,420 W total starting, converted here to +700 W additional (1,420 − 720), and used as the cross-check in the variance notes below.
Every figure is a typical value that yields to the appliance nameplate. Sources disagree on several rows, and the table takes the Generac worksheet value as the defensible typical: a 10,000 BTU window AC is 1,200 running / +1,800 additional at Generac versus 1,500 running / 2,200 total starting at Honda — real units vary by efficiency and age. The electric water heater is 4,000 W at Generac versus 4,500 W at Honda, because element sizes differ by tank. LED lighting is a per-bulb typical (10 W), editable per row — the default row is 10 bulbs at quantity 1. Two rows carry explicit labels rather than source claims: the LED row is a labeled typical, and the CPAP row (60 W) appears in neither the Generac worksheet nor the Honda home table — it is the top of the 30–60 W typical range used across sizing tables for a CPAP without heated humidification. Check your device label, especially with a heated humidifier, which can raise the draw well above 60 W. It is presented as a typical, never as a manufacturer-sourced figure.
The central AC warning: no row varies more. Generac's own table runs from 1,500 / +3,000 at 10,000 BTU to 6,000 / +6,700 at 40,000 BTU, so the 24,000 BTU preset here is a midpoint, not a measurement. If your list includes central AC, read the outdoor unit's nameplate (LRA/RLA figures) or ask about a soft-start kit rather than trusting any preset — including this one.
Why 20% headroom
The default margin is 20% for three stated reasons. First, generators should not run at 100% of rating continuously — sustained loading is conventionally kept to roughly 80% of rated capacity, and sizing the unit at peak × 1.20 is the worksheet-level expression of that practice. Second, the margin absorbs nameplate variance between table values and your actual appliances, plus voltage sag under load. Third, it covers the one forgotten small load every outage list has. Generac's own worksheet example rounds a 6,600 W calculated need up to a 7.5 kW unit — the same buy-the-next-size-up behavior the size-class output encodes. If you are running only resistive and electronic loads with verified nameplates, you can lower the margin; at altitude or on propane — both of which derate a generator's output — raise it instead.
Portable classes, and where standby begins
The recommendation maps onto the common portable classes — 2,000 / 3,500 / 5,000 / 7,500 / 10,000 W — always rounding up to the smallest class that covers the computed minimum. A list needing 3,588 W lands in the 5,000 W class, not the 3,500 W class it just misses. Past 10,000 W the output reads “14,000 W+ — whole-house standby territory”: at that point the conversation changes from which portable to buy to transfer switches, fuel supply, and a licensed electrician's load calculation. The running-load percentage in the results keeps the continuous picture visible alongside the surge picture — a unit whose peak fits but whose running load sits at 95% of its class will live a hard life.
Worked example
An outage list of nine appliances, each quantity 1, at the default 20% headroom: refrigerator (700 / +2,200), freezer (700 / +2,200), sump pump ½ HP (1,050 / +2,200), furnace blower ½ HP (800 / +2,350), microwave (1,000 / 0), LED lights (100 / 0), TV (90 / 0), computer + router (105 / 0), and coffee maker (1,000 / 0). Tick exactly those rows in the calculator to reproduce it:
- Total running watts: 700 + 700 + 1,050 + 800 + 1,000 + 100 + 90 + 105 + 1,000 = 5,545 W
- Largest single additional surge: 2,350 W — the furnace blower, named in the output
- Peak starting watts: 5,545 + 2,350 = 7,895 W
- Recommended minimum: 7,895 × 1.20 = 9,474 W
- Size class: 10,000 W — running load 5,545 ÷ 10,000 = 55.5% of the class
Two checks worth running yourself. Essentials only — refrigerator plus TV — give 790 W running, a 2,200 W surge, a 2,990 W peak, and a 3,588 W recommendation, which lands in the 5,000 W class because 3,588 exceeds the 3,500 W class. And two refrigerators (quantity 2 on that row alone) give 1,400 W running with the surge still 2,200 W — quantity never multiplies the surge, because one unit starts at a time — for a 3,600 W peak and a 5,000 W class recommendation.
Carbon monoxide: the non-negotiable block
Never run a generator indoors or in a garage — carbon monoxide kills. Operate it outdoors only, away from windows, doors, and vents, so exhaust cannot drift inside, and follow the manufacturer's instructions for your unit. No sizing result on this page matters if this rule is broken. Fuel and altitude also derate real output: a generator on propane, or working at elevation, delivers less than its sea-level gasoline rating, which is one more reason the headroom margin exists.