What Size Generator Do You Actually Need? Sizing It Without Overbuying

Most people buy too much generator. They add up the nameplate wattage of everything in the house, arrive at a frightening number, and buy a unit that is heavier than they can move, thirstier than they can fuel, and louder than their neighbours will tolerate.

The correct method starts from the opposite end. Decide what has to run, find the real running watts, account for motor startup surge, and buy the smallest unit that covers it. That number is usually far lower than the first estimate, and the savings compound across purchase price, fuel consumption, and how long you can actually operate.

Running watts and starting watts are different numbers

Every generator is advertised with two figures. The larger one is starting or surge watts, which the unit can supply for a few seconds. The smaller one is running or rated watts, which it can supply continuously.

The running figure is the one that matters for almost everything. The starting figure only matters because of motors.

Anything with an electric motor draws a large multiple of its running current for the first second or two while the rotor comes up to speed. A refrigerator compressor running at 150 watts may pull 600 to 1,000 watts at startup. A well pump running at 800 watts may surge past 2,000. That transient is brief, but if the generator cannot supply it the motor stalls, the breaker trips, or in bad cases the motor overheats.

Resistive loads have no surge at all. Lights, heating elements, and electronics draw the same on startup as they do continuously.

So the sizing rule is: total running watts of everything on simultaneously, plus the surge of the single largest motor that might start while everything else is already running.

Not the sum of every surge. Motors do not all start at the same instant, and designing for that produces a generator twice the size you need.

Typical draws for the things people actually run

Two entries in that table decide most sizing questions. Central air conditioning and electric resistance heat or cooking are what push a household into large generator territory. Almost nothing else does.

Working the numbers

Take a common case. Gas furnace, refrigerator, a chest freezer, lights, and a phone charging.

Running watts: furnace blower at 600, refrigerator at 150, freezer at 80, lights at 200, electronics at 100. That totals 1,130 watts.

Largest single motor surge: the furnace blower, at roughly 1,800.

The requirement is 1,130 running watts and enough headroom for the 1,800 watt surge on top of the other loads. A 3,000 watt inverter generator covers this comfortably, and it will spend most of its life loaded at around a third of capacity, which is where fuel efficiency and engine life are best.

The instinct is to buy 7,500 watts because it is only slightly more expensive. Resist it. A larger generator burns more fuel at any given load, weighs substantially more, and is louder, and the fuel consumption is the part that determines how long you can actually run during a multi day outage.

Add a well pump to that same scenario and the picture changes. The pump surges past 2,500 watts, and it may start while the furnace is already running. That case wants 5,000 to 6,500 running watts, or a soft start device on the pump.

Add central air and you are in whole house territory, typically 10,000 watts and up, which for most people means a standby unit on a permanent gas connection rather than anything portable.

The off grid power sizing calculator works the same arithmetic for battery systems, and the load inventory step is identical for both.

Inverter, conventional, or standby

Conventional portable generators run a fixed engine speed to produce sixty hertz and are the cheapest per watt. They are loud, typically in the seventy to eighty decibel range at operating distance, and produce electricity with more waveform distortion than sensitive electronics prefer. Fine for pumps, heaters, and tools. Less good for a laptop or a furnace control board.

Inverter generators convert to DC and back, which lets the engine throttle down under light load. That makes them dramatically quieter, often in the fifties, considerably more fuel efficient at partial load, and produces clean power suitable for anything. They cost more per watt and top out lower in size. For household backup where the load is intermittent, they are usually the right choice.

Standby generators are permanently installed, wired to the panel, run on natural gas or propane, and start automatically when the utility fails. They are the most expensive by a wide margin once installation is included, and they solve the two problems portables do not: fuel logistics and being present to start it.

The fuel question decides more than people expect

A generator is a fuel storage problem wearing an engine.

Gasoline is the default and the worst for storage. It degrades in months, faster with ethanol content, and stabiliser extends that to perhaps a year rather than indefinitely. Storing enough for a multi day outage means storing and rotating a substantial quantity of a flammable liquid, in approved containers, away from the house.

Propane stores essentially indefinitely, which is its great advantage. It is also easy to buy in quantity, and a large tank removes the refuelling problem entirely. It delivers slightly less power than gasoline for the same engine, typically around ten percent, and regulators can struggle in extreme cold.

Natural gas removes fuel storage entirely because it comes from the utility. That is either the best option or a false comfort depending on whether your gas supply survives the event. Texas in 2021 demonstrated that gas delivery and electrical supply can fail together, and the breakdown of that event is worth reading before assuming gas will be there.

Dual fuel and tri fuel units run on more than one, which is genuinely useful and worth the modest premium.

The arithmetic that matters: a mid size portable at half load consumes very roughly half a gallon of gasoline per hour. That is twelve gallons a day if run continuously. Almost nobody stores twelve gallons a day for a week. Which leads to the most important operational point on this page.

Run it intermittently, not continuously

A refrigerator holds temperature for hours with the door closed. A freezer holds considerably longer. A house holds heat for a while depending on insulation and outdoor temperature.

Running a generator for two to three hours, twice a day, keeps food frozen, brings the house back up to temperature, recharges devices and batteries, and pumps water, on a fraction of the fuel that continuous operation demands. It also gives the engine rest, reduces noise complaints, and makes your fuel supply last three or four times as long.

This single habit does more for outage endurance than buying a larger unit.

Safety, briefly and seriously

Carbon monoxide from generators kills people every winter, and the mechanism is always the same: the unit was too close to the house, or in a garage, or in a partially enclosed space. Twenty feet from any structure, exhaust pointed away, and never in a garage or breezeway even with the door open. A battery powered carbon monoxide alarm inside the house is not optional if you own a generator.

Backfeeding, meaning powering the house by plugging the generator into an outlet, energises the utility line outside your home and can kill line workers restoring power. It also bypasses your panel's protection. The legitimate solutions are an interlock kit on the main panel, which is inexpensive, or a manual transfer switch. Both are electrician work in most jurisdictions and both are worth doing properly.

Never refuel a hot engine. Let it cool. Gasoline vapour and hot exhaust manifolds are a predictable combination.

And run it under load for twenty minutes every few months. The most common failure mode for a backup generator is that it has not been started since it was purchased, and carburettors gum up with ethanol blended fuel sitting in them. Stabilised fuel and a quarterly test start solve it.

What to buy, in one paragraph

Inventory the loads that genuinely have to run, add the running watts, add the surge of the largest single motor, and buy an inverter generator at that figure with modest headroom rather than double. Choose dual fuel if the option exists, because propane storage solves the problem gasoline creates. Install an interlock kit rather than improvising. Buy a carbon monoxide alarm at the same time. Then run it quarterly so it works when you need it.

For most households with gas heat, that lands somewhere between 3,000 and 5,000 watts, which is a unit one person can move and fuel for several days. The 10,000 watt purchase is for houses with central air or electric heat, and those households should look seriously at whether a battery based system covering the furnace alone would serve them better for less.

Related: How to heat a house when the power is out covers the furnace electrical problem in detail. How to read a grid reliability report for your region covers whether extended outages are likely where you live.

This page is general information. Generator installation and panel work are regulated in most jurisdictions and are worth doing with a licensed electrician.

Last reviewed: July 2026