People ask me this question every fall before storm season. Sometimes the answer is “a lot smaller than the salesman is telling you.” Sometimes it is “twice what you were thinking.” The math is not hard, but most people get it wrong in one of two predictable ways — buying a 2000W inverter to power a fridge plus an air conditioner (way undersized, will overload constantly) or buying a 12,000W open-frame portable to power three lights and a phone charger (massively oversized, runs out of fuel in 8 hours doing nothing). I have helped neighbors and family members size generators for years, and here is the ladder I climb to get the right number every time.
The 30-second answer: The question “what size generator do I need” comes down to one calculation: add the running watts of everything I want on at the same time, then add the highest single starting wattage on top. That total is the surge capacity my generator needs. For most homes wanting fridge + freezer + furnace blower + a few lights + the occasional microwave, the answer is 4,000-5,500 running watts. For homes adding well pump or central AC, 7,000-10,000 watts. For whole-house standby coverage, 14,000-22,000 watts standby unit. The 80% rule: never plan to run sustained loads above 80% of the generator’s rated continuous output.
The math — running watts versus starting watts
Two numbers matter for every appliance: running watts (what it draws while operating normally) and starting watts (the brief surge when a motor kicks on). Light bulbs, electric heaters, and resistive loads have starting watts equal to running watts — no surge. Motors are different. A refrigerator compressor draws 150-200W running and 600-800W for the first 1-3 seconds of startup. That 3-4x surge is what determines my generator’s required surge capacity.
The math I use: add up all the running watts of devices that will be on simultaneously. Then find the single appliance with the highest starting watts, and add that on top. I do not stack multiple starting watts because — if I plan it right — I never start two big motors at the same time. I stagger them.
IMAGE_NEEDED: Photo of a Kill-A-Watt style meter plugged into a refrigerator with the watt reading visible, captioned “$15 watt meter pays for itself the first storm. The labels on appliances are often optimistic — measured numbers are real.”
Appliance reference table (typical running and starting watts)
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator (standard) | 150-200 | 600-800 |
| Chest freezer | 100-200 | 500-700 |
| Furnace blower (1/2 HP) | 800-1200 | 1800-2500 |
| Well pump (1/2 HP) | 1000 | 2000-2500 |
| Well pump (1 HP) | 1800-2200 | 4000-5000 |
| Sump pump | 600-800 | 1300-1700 |
| Window AC, 10,000 BTU | 1200 | 1800-2400 |
| Window AC, 15,000 BTU | 1800 | 2500-3000 |
| Central AC, 3-ton | 3500-4000 | 7000-8000 |
| Microwave (1000W) | 1000-1500 | — |
| Coffee maker | 800-1200 | — |
| Toaster | 800-1500 | — |
| LED lights (per bulb) | 8-15 | — |
| Incandescent lights (per bulb) | 60-100 | — |
| TV (LED, 55-inch) | 100-200 | — |
| Phone / laptop chargers | 10-50 each | — |
| CPAP machine | 30-100 | — |
| Electric water heater (40 gal) | 3000-4500 | — |
| Garage door opener | 500-800 | 1200-1600 |
These are typical numbers. The label on my actual appliance is the source of truth — and a Kill-A-Watt style meter ($15-25) is more accurate still because it measures what is actually drawn rather than the conservative label maximum.
The ladder — four tiers of home coverage
I climb the ladder by what someone actually wants to power, not by what the generator manufacturer wants to sell me.
Tier 1 — The minimum essentials (2,000-3,000 surge watts)
What it covers: fridge, freezer, a few lights, phone charging, maybe a small TV. No furnace blower, no well pump, no AC.
The math: Fridge running 200W + freezer running 150W + four LED lights 60W + TV 150W + phone chargers 50W = 610 running watts. Add 800W starting surge for the fridge = 1,410 surge requirement. Round up to a 2,000-2,500W generator with a 1,500-1,800W running capacity.
What I would buy: An inverter generator in this range — Honda EU2200i, Champion 2500W inverter, Westinghouse iGen2500. Inverters give cleaner power that will not fry sensitive electronics, and they sip fuel at low load. $500-1,200 depending on brand.
Tier 2 — The modest setup (4,000-5,500 surge watts)
What it covers: Everything in Tier 1 plus a furnace blower, microwave occasionally, coffee maker, sump pump if needed. Still no AC, still no well pump.
The math: Tier 1 base 610W + furnace blower 1000W = 1,610W running. Add 2,500W starting surge for the furnace = 4,110 surge requirement. Round to 4,500-5,500W surge / 3,500-4,500W running generator.
What I would buy: A conventional portable in this range — Champion 4,000-5,500W series, Predator 5,000W, Generac GP5500. Conventional (non-inverter) portables in this size run $400-700. Inverter versions of the same size are quieter and cleaner but $1,500-2,500.
Tier 3 — Comfortable backup (7,000-10,000 surge watts)
What it covers: Tier 2 plus well pump and/or window AC and a few more conveniences. Most rural homes with a well land here.
The math: Tier 2 running 1,610W + well pump 1,000W + window AC 1,200W + microwave 1,200W (occasionally) = 5,010W running. Well pump starting 2,500W as the largest motor surge = 7,510 surge required. Round up to 8,000-9,000W surge / 6,500-7,500W running generator.
What I would buy: Champion 7,500-9,000W or Generac GP8000, Predator 8750. $700-1,200. At this size I usually skip the inverter version unless I have a lot of sensitive electronics — the inverter premium gets steep above 5,000W.
Tier 4 — Whole-house coverage (10,000+ surge watts)
What it covers: Central AC, electric water heater, well pump, full kitchen — essentially everything in a normal house.
The math: Central AC running 3,500W + well pump 1,000W + fridge 200W + freezer 150W + furnace blower 1,000W + lights and small loads 600W = 6,450W running. Central AC starting surge 7,000W = 13,450 surge required. Round to 14,000W+ surge.
What I would buy: At this point I am usually looking at standby home generators rather than portables. Generac Guardian 14kW, Kohler 14RES, Cummins QuietConnect — all in the $4,000-7,000 range installed. They run on natural gas or propane, start automatically, and handle the load management to stagger startups. The peace of mind on a 14kW standby that runs itself is hard to beat.
If I want a portable in this size range, I am looking at Champion 12,000W or DuroMax XP13000EH dual-fuel. Big, heavy, loud, fuel-thirsty, but functional. $1,500-2,500.
The 80% rule — why I do not buy to the edge
Generator manufacturers publish two numbers: surge (peak, short duration) and running (continuous). I never plan to run sustained loads above 80% of the running watt rating. Here is why: a generator at 100% rated load for hours is generating more heat than its cooling system can shed indefinitely, the engine wears faster, and any motor inrush from a fridge cycling on will exceed surge capacity and trip the breaker.
So if my math says I need 5,000 watts running, I buy a 6,500-watt generator. If I need 7,500 running, I buy a 9,000. The headroom is not waste — it is reliability under load.
IMAGE_NEEDED: Photo of a generator’s data plate showing both surge and running wattage ratings, captioned “Two numbers, two meanings. Surge is for the brief motor inrush. Running is what I can sustain.”
The inverter question — when to pay the premium
Inverter generators cost 2-4x more than conventional units of the same wattage. Worth it for:
- Sensitive electronics (laptops, smart TVs, modern furnace ECM blowers, anything with a sensitive circuit board)
- Camping or RV use where noise matters (inverters run 25% quieter than open-frame)
- Light-load efficiency (inverters throttle the engine to load and sip fuel at 25-50% capacity)
Not worth it for:
- Pure resistive loads — lights, heaters, well pumps
- Higher-output requirements (5,000W+ inverters get prohibitively expensive)
- Tools and construction use
For home backup with mixed loads including sensitive electronics, a Tier 2 inverter is usually the sweet spot — Honda EU3000iS or Champion 4250 inverter. About $1,500-2,400.
Dual-fuel and tri-fuel — the propane question
Dual-fuel generators run on either gasoline or propane (LP). Some tri-fuel units add natural gas. Costs about $100-200 more than the gasoline-only version of the same wattage. Tradeoffs:
Propane advantages: Indefinite storage life (the carb problems in generator won’t start mostly do not happen with propane), clean burning, no fuel stabilizer needed. Great for storms and infrequent use.
Propane disadvantages: About 10% lower power output on propane vs gasoline (a 5,000W gas / 4,500W propane unit), requires a 20-100 lb propane tank, propane gels in cold weather below about 20°F.
For storm prep on a unit that may sit 11 months between uses, dual-fuel is worth the extra $150 for the propane option alone. I would rather store a 20-lb propane tank than a 5-gallon can of treated gasoline.
The 240V question — do I need it?
Most portable generators above 4,000W have at least one 240V outlet (usually a NEMA L14-30 twist-lock). I need 240V if I want to power: an electric water heater, an electric dryer, a deep well pump (some are 240V), or to feed a transfer switch / interlock for the main panel of the house. Pure 120V coverage is fine for everything else.
Safety — sizing affects more than convenience.
An undersized generator that constantly runs near its surge capacity trips breakers, overheats, and ages itself fast. An oversized generator running at 10% load is inefficient and tends to wet-stack (incomplete combustion deposits in the exhaust). But the safety-critical sizing issue is the transfer switch / interlock connection. Never backfeed a house through a wall outlet — that can kill a utility lineman on the grid and is illegal in every state. Always install a proper transfer switch or interlock rated for the generator’s output. See generator safety for the full set.
Spend versus capability.
The price-to-watt curve is not linear. $400 buys 4,500W portable. $700 buys 7,500W. $1,200 buys 9,000W. But $4,000+ is the entry point for a 14kW standby unit installed. The big jump is the standby tier — the portable tiers are all in roughly proportional pricing. For sustained outages and convenience, standby. For occasional use and value, portable. For occasional use with sensitive electronics, inverter portable. I would not buy beyond Tier 3 portable — once I need that much capacity reliably, the standby starts to make sense for the auto-start alone.
Common mistakes I see
1. Adding starting watts of multiple motors. If my furnace, fridge, and well pump all surge at the same time, that is 12,000+ watts. But they will not — and I do not stack the starting watts on paper. I pick the single biggest motor and add that. Plan to start big motors one at a time during the outage.
2. Buying based on continuous (running) watts instead of surge. Some marketing leads with the smaller number. The surge watts is what determines whether the breaker trips on a fridge kick-on. Buy by the surge rating.
3. Forgetting the inverter premium for sensitive electronics. A conventional generator can produce “dirty” power with voltage spikes that fries laptops and smart TV electronics over time. If I have ECM-blower furnaces, smart-home gear, or any other sensitive circuit board running on the generator, inverter is worth the cost.
4. Forgetting fuel storage and runtime. A 7,500W generator at 50% load runs about 8-10 hours on a 6-gallon tank. A 36-hour outage means refueling 3-4 times — and I need that fuel on hand. If I cannot store 20+ gallons of treated gasoline safely, dual-fuel propane is the better answer.
5. Picking by brand reputation instead of math. Honda is reliable but expensive. Champion is the value choice in most tiers. Predator is the budget play. Generac dominates standby. Match the brand to the budget after I know the wattage I need — not the other way around.
IMAGE_NEEDED: Photo of a portable generator on a pad with a power cord running to a transfer switch box, captioned “Right-sized to the load, plumbed in through a proper transfer switch. That is the setup that works.”
Video — wattage and sizing walkthrough
Frequently asked questions
What size generator do I need to run my whole house?
If “whole house” means central AC, electric water heater, electric range, and everything else running at once, that is 14,000-22,000W standby territory. If “whole house” means everything except those big resistive heat loads (AC, water heater, range), 10,000-12,000W portable can do it with load management. Most people land in the 7,500-10,000W range because they accept living without electric heat or cooking during an outage.
Can I run my 5-ton central AC on a portable generator?
Not realistically. A 5-ton (60,000 BTU) central AC pulls 5,500-7,000W running and surges to 10,000-15,000W on startup. By the time I have a portable big enough to surge through that, I am at 15kW+ portable territory which is heavy, loud, and fuel-hungry. Standby unit makes more sense at that point.
How do I figure starting watts if the label only shows running watts?
For motors (anything that hums or spins): multiply running watts by 3 and use that as starting watts. For resistive loads (heaters, light bulbs, toasters): starting watts equals running watts, no surge. Conservative estimate, but better than under-sizing.
Is it better to buy too big or too small?
Too big is the safer mistake. An oversized generator runs at low load, sips fuel, lasts longer, and handles unexpected loads. An undersized generator runs near surge capacity, trips breakers, overheats, and ages fast. If I am unsure between two sizes, I round up.
Do I need an inverter generator for my computer or phone?
For brief charging from a quality conventional generator, no — phone chargers and laptop power bricks have their own conditioning. For running a desktop computer or sensitive medical equipment continuously, inverter is worth it. The line is roughly “anything plugged in for hours” gets inverter; “anything plugged in for minutes” can run on conventional.
How much fuel will I use during an outage?
Rough rule: a portable at 50% load consumes about 0.5-0.75 gallons of gasoline per hour. A 24-hour outage at moderate load is 12-18 gallons. Standby units on natural gas use 1-3 cubic meters per hour, on propane 1.5-3.5 gallons per hour. Stock 25-30 gallons of treated gasoline for a typical multi-day outage if running a portable.
Watts you really need
- Answering what size generator do I need starts with: running watts of everything I want on at once, plus the single highest motor starting surge.
- Four tiers: essentials only (2,000-3,000W surge), modest (4,000-5,500W), comfortable with well or AC (7,000-10,000W), whole-house (14,000W+ standby).
- The 80% rule: never plan to run sustained loads above 80% of the running watt rating. Always buy with headroom.
- Inverter premium ($1,500-2,400 vs $500-700 for same watts conventional) is worth it for sensitive electronics or RV use; skip it for tools and resistive loads.
- Dual-fuel (gasoline + propane) is worth the $100-200 premium for storm prep because propane stores indefinitely without the carb problems gasoline causes.
- If unsure between two sizes, round up. Oversized is the safer mistake.
Once I have my generator, the most common problems I will encounter are covered across the troubleshooting guides — generator won’t start (almost always the carb on a stored unit), starts then dies (also the carb), runs but no power (the electrical end), keeps tripping breaker (usually overload), and general maintenance. For a comprehensive sizing calculator that runs through every appliance, Champion Power Equipment has a good one, and the manufacturer pages at Generac have model-specific spec sheets. For replacement parts and accessories like watt meters and transfer switches, Amazon has the standard inventory.