Home Backup & Portable Sizing
Last reviewed: April 2026
What size generator do you need? Calculate total wattage from your appliances and get the right generator size for home backup or job site use. This calculator runs entirely in your browser — your data stays private, and no account is required.
Choosing the right generator means adding up the running watts of everything you want to power simultaneously, then accounting for starting watts (surge power) of motor-driven appliances. A generator that's too small will overload and shut down. One that's too large wastes fuel and money. The sweet spot is a generator rated at 20–25% above your calculated peak demand.
Every appliance has two wattage ratings. Running watts is the continuous power needed to operate. Starting watts (surge watts) is the extra power needed for the first few seconds when a motor starts — typically 2–3× the running watts. Refrigerators, AC units, sump pumps, and well pumps all have significant starting surges. Your generator must handle the highest starting surge on top of the running load of everything else.
Refrigerator: 150–400 running, 1,200 starting. Sump pump (1/3 HP): 800 running, 1,300 starting. Window AC (10,000 BTU): 1,200 running, 3,600 starting. Central AC (3-ton): 3,500 running, 7,000 starting. Well pump (1/2 HP): 1,000 running, 2,100 starting. Furnace fan: 700 running, 2,100 starting. Microwave: 1,000 running. Lights (LED): 10–15 per bulb. Phone/laptop charger: 25–100 watts.
3,000–5,000 watts (portable): Essentials only — fridge, some lights, phone charging, sump pump. 5,000–7,500 watts: Essentials plus a window AC or well pump. 7,500–10,000 watts: Most of the house minus central AC. 10,000–15,000 watts: Nearly full house including central AC. 15,000–25,000+ watts (standby): Whole-house backup including central HVAC, electric range, and dryer.
Portable generators ($500–$2,500) run on gasoline, are manually started, and connect via extension cords or a transfer switch. Good for occasional outages. Standby generators ($3,000–$15,000+ installed) run on natural gas or propane, start automatically when power fails, and connect permanently to your electrical panel. Essential for areas with frequent or extended outages, medical equipment needs, or home offices.
A 5,000-watt portable generator consumes roughly 0.75 gallons of gasoline per hour at 50% load. A 7,500-watt unit uses about 1 gallon/hour. For extended outages, fuel storage becomes critical. Standby generators on natural gas avoid this issue but consume 150–300 cubic feet per hour depending on load.
| Use Case | Watts Needed | Generator Size | Fuel Type |
|---|---|---|---|
| Phone/laptop charging | 200–500W | Portable inverter | Gas/battery |
| Essential circuits (fridge, lights) | 3,000–5,000W | Mid-size portable | Gas/dual fuel |
| Most home circuits | 7,500–10,000W | Large portable | Gas/propane |
| Whole house | 15,000–25,000W | Standby | Natural gas/propane |
Choosing the right generator size requires adding up the running wattage of everything you want to power simultaneously, then accounting for the starting (surge) wattage of motor-driven appliances that draw 2–3 times their running watts during startup. Undersizing means tripped breakers and potential equipment damage; oversizing wastes fuel and money.
Start by listing every appliance, system, and device you need to run during an outage. For each, note both its running watts and starting watts. Add all running watts together, then add the single highest starting wattage surge on top — because not everything starts simultaneously. Key household loads: refrigerator: 100–400 running, 1,200–2,200 starting. Sump pump: 800–1,050 running, 1,300–2,500 starting. Well pump: 750–1,500 running, 1,500–3,000 starting. Central AC (3 ton): 3,000–3,500 running, 4,500–7,000 starting. Electric furnace blower: 500–800 running, 1,000–1,500 starting. Window AC unit: 500–1,500 running, 900–2,500 starting. Microwave: 600–1,200 running, same starting (resistive load). LED lighting (10 fixtures): 100–200 running, same starting.
Portable (2,000–4,000 watts): Powers essentials — lights, refrigerator, phone chargers, small appliances. Cannot run central AC or well pumps. $300–$1,200. Ideal for camping, tailgating, and limited emergency backup. Mid-range portable (5,000–8,000 watts): Powers most essential circuits including a sump pump, well pump (1/2 HP), refrigerator, lighting, and small appliances simultaneously. Cannot run central AC. $500–$2,000. Large portable (8,000–12,000 watts): Can power a small central AC (2 ton) or window units plus all essentials. Requires a transfer switch for safe whole-house connection. $800–$3,000. Standby (12,000–48,000 watts): Permanently installed, automatically activates within seconds of power loss. Powers entire home including central AC and electric range. $3,000–$15,000 plus $1,000–$3,000 installation. Whole-house standby (22,000+ watts): Runs everything in a typical home simultaneously with no load management required. $5,000–$20,000+ installed.
Gasoline: Most common for portables. Widely available but degrades within 30 days without stabilizer, making long-term storage challenging. Consumption: 0.5–1.5 gallons per hour depending on load. Propane (LP): Stores indefinitely, cleaner burning, and produces less carbon monoxide per BTU than gasoline. Slightly lower energy density means 10–15% more fuel consumption per kWh. Available in portable tanks (20 lb) or plumbed from a whole-house tank. Natural gas: Unlimited supply via utility line — no refueling concerns. Available only for standby generators. Produces the least power per unit of fuel but the convenience is unmatched. Dual-fuel: Many modern portables run on gasoline or propane, providing flexibility. Diesel generators (commercial and large standby) offer superior fuel efficiency and longevity but higher upfront cost.
Never connect a generator directly to your home's electrical panel by plugging into a wall outlet — this creates backfeed that sends electricity back through the transformer and onto utility lines at lethal voltages, endangering utility workers. A transfer switch ($200–$1,000 plus installation) isolates your home from the grid when the generator is operating. Manual transfer switches require you to physically flip circuits; automatic transfer switches (standard with standby generators) detect outages and switch seamlessly. A load management panel (interlock kit: $50–$200, plus electrician installation) provides a budget-friendly alternative for portable generators, preventing backfeed while allowing you to power selected circuits through your existing breaker panel.
Carbon monoxide from generators kills dozens of Americans annually. Never operate a generator indoors — including garages, basements, and enclosed porches. Place it at least 20 feet from any window, door, or vent, with the exhaust directed away from the building. Install battery-powered CO detectors on every floor of your home. Many modern generators include built-in CO sensors that automatically shut down the engine if dangerous levels are detected — a feature worth prioritizing when purchasing.
A generator that won't start during an actual emergency is worse than no generator at all. Monthly maintenance includes running the generator under load for 15–30 minutes to circulate oil, burn off moisture, and verify operation. Add fuel stabilizer (like Sta-Bil) to stored gasoline — untreated gasoline degrades in 30 days, gumming carburetors and fuel lines. Change oil after every 100 hours of operation or annually, whichever comes first. Replace spark plugs and air filters annually. Keep spare oil, filters, spark plugs, and a full fuel supply on hand before storm season. For standby generators, schedule annual professional maintenance ($150–$300) that includes load testing, transfer switch verification, battery testing, coolant level check, and comprehensive inspection. A well-maintained generator provides reliable power for 20–30 years of service.
Consider your local outage frequency and duration when choosing between portable and standby options — if you experience multiple outages per year lasting 8+ hours, a standby generator's convenience and automatic operation justify the higher investment cost.
See also: Electricity Cost · AC BTU Calculator · Solar Payback · Energy Savings · Appliance Energy Cost
→ Starting watts are the critical spec, not just running watts. A refrigerator runs at 150W but surges to 1,200W on startup. A sump pump runs at 800W but surges to 2,400W. Your generator must handle the highest simultaneous starting surge plus all running loads.
→ You don't need to power everything simultaneously. A 5,000W generator can run a house if you manage loads — don't start the well pump while the AC is cycling on. Stagger startups and you can use a smaller (cheaper, more fuel-efficient) generator.
→ Inverter generators are best for electronics. Standard generators produce "dirty" power with voltage fluctuations that can damage sensitive electronics. Inverter generators (Honda EU series, etc.) produce clean sine-wave power safe for computers and phones, but cost 2–3× more.
→ A 7,500W generator handles most home essentials. That covers: refrigerator, freezer, sump pump, furnace blower, well pump, lights, and phone charging. Central AC requires 15,000–25,000W — only whole-house standby generators handle this. See our Electricity Bill Calculator and Appliance Energy Calculator.
See also: Electricity Bill · Appliance Energy · Energy Savings · Solar Payback