# What Size Generator Do I Need for My House? · Larder & Well > Source: https://larderandwell.cc/power/what-size-generator-do-i-need-for-my-house Power ## Sizing a Generator for a House How to work out the generator a household needs in an outage, starting from the loads that cannot wait: the well pump, the refrigerator and freezer, and the furnace fan. By Ruth Calloway, November 12, 2026 To size a generator for your house, list the things that must run at the same time, add up what each draws while running, then add the starting surge of the single biggest motor on the list. A house on city water that only needs its refrigerator, freezer, furnace fan and lights usually lands between 3,500 and 5,000 watts, and a house that also needs a 240-volt well pump usually needs a 7,000 to 8,000 watt machine with a 240-volt outlet. The rest of this guide shows how to get your own number instead of trusting a range, because the pump in your well and the age of your fridge decide it more than the size of the house does. **Your next step tonight:** walk the house with a notepad and a flashlight and write down every appliance you would miss after a day without power. Ready.gov opens its [power outage guidance](https://www.ready.gov/power-outages) with that same step, an inventory of what in your home relies on electricity. ### Running watts and starting watts Every appliance has a running figure, the power it draws once it is up and working. Anything with a motor or a compressor also has a starting figure, a short surge it pulls in the first second or two while the motor gets turning. Refrigerators, freezers, well pumps, sump pumps, furnace blowers and air conditioners all have one. Light bulbs, chargers, a microwave and a space heater do not. The surge is the number that catches people out. Champion Power Equipment's [sizing guide](https://www.championpowerequipment.com/news-updates/what-size-generator-do-i-need/) puts the start of a typical refrigerator at 800 to 1,200 watts, several times what it uses once running. Norwall's guide to [portable generator sizing](https://norwall.com/power-expert/Sizing-Guide-for-Portable-Generators) says capacitor-start motors, the kind found in pumps and compressors, need three to six times their running power to start. A generator that cannot deliver that burst sags, its breaker trips, or the motor hums without turning, which is hard on the motor. Generators carry two ratings to match. The larger one, often printed biggest on the box, is the surge or peak rating it can hold for a moment. The smaller one is what it can hold for hours. Size the everyday load to the smaller figure and the motor start to the larger one. ### Essential loads: what a house must run To size a generator for essential loads, be strict about what counts as essential. In a winter outage the list for most rural and suburban houses is short: water, cold food, heat and a little light. Everything else can wait for the line crew or be run one at a time. Typical figures from the two sizing guides above give a starting point. Your own appliances may differ, so treat these as placeholders until you have read the nameplates. | Load | Running (W) | Starting (W) | | ------------- | ------------ | -------------- | | Refrigerator | 150 to 400 | 800 to 1,200 | | Chest freezer | 100 to 500 | 500 to 1,000 | | Furnace fan | 300 to 800 | 800 to 1,600 | | Sump pump | 800 to 1,050 | 1,300 to 2,150 | | Well pump | about 1,050 | about 3,200 | | LED bulb | 8 to 15 | none | | Microwave | 600 to 1,200 | none | | Space heater | 750 to 1,500 | none | The well pump line is Norwall's worked example for a modest residential pump. Pumps vary more than any other item on the list, which is why the next section is about them alone. Two items deserve a place on the list even though they draw little. A furnace blower brings gas or oil heat back to the whole house, which our guide to [heating a house without power](/power/how-to-heat-a-house-without-power) covers in detail. And phone and radio chargers keep you in touch with outage updates and warming-center notices for the cost of a few dozen watts. ### A generator for the well pump and refrigerator The question most rural readers bring is exactly this pairing, and the well pump is the harder half. A refrigerator is a small motor on a 120-volt plug. A submersible well pump is a bigger motor, often hundreds of feet down, and in most American homes it is wired for 240 volts on its own double breaker. That voltage matters before the wattage does. A small generator with only 120-volt household outlets cannot run a 240-volt pump, however many watts it makes. Look for a unit with a 120/240-volt twist-lock outlet, the kind an electrician wires to a generator inlet, and confirm the pump's voltage on its control box or on the breaker in the panel. Then the surge. A forum thread on [sizing a generator for a submersible well pump](https://www.tractorbynet.com/forums/threads/sizing-generator-for-submersible-well-pump.309058/) at TractorByNet shows what it looks like in practice. One rural owner reports that his 3/4-horsepower, 240-volt pump pulls a 5,500 running, 8,500 starting watt generator down far enough to dim the lights and knock some electronics offline each time it kicks on. In the same thread, a pump maker's reply to another owner put the locked-rotor draw of a 5-horsepower irrigation pump at 121 amps for a fraction of a second, against 27.5 amps once running. For the common fractional-horsepower house pump, a generator rated around 7,000 to 8,000 running watts handles the pump plus the fridge, a freezer, the furnace fan and lights, which matches the worked example below. If your pump is 1 horsepower or more, or deep enough that its motor is larger than usual, ask the well contractor or the pump maker for its generator requirement before you buy. Pump makers publish generator sizing tables for their motors, and a well contractor can read your motor's nameplate on a service call. A pressure tank buys you room to manage. The thread's original poster planned to run the pump only to fill the tank and then switch its breaker off, so the pump and the other motors never start together. A household that fills the tub, the stock tanks and a few water jugs each morning can get by on fewer pump starts. Our page on [how much water to store per person](/water/how-much-water-to-store-per-person) covers keeping a stored reserve for the hours the generator is off. ### The sizing math, worked through Norwall's guide sets out the method in four steps, and Champion's matches it. Total the running watts of everything that will run together. Find the single largest starting surge. Add that surge to the running total of everything else to get the peak the generator must survive. Then add a margin so the machine is not living at its limit. Running loads stack up and the biggest motor start sits on top of them. The generator's surge rating has to reach at least the tip of that spike. Norwall's own example house runs a sump pump, a well pump, a furnace fan, a refrigerator, a toaster and a hair dryer. Everything except the well pump adds up to 4,800 running watts. Adding the pump's 3,200-watt start gives 8,000 watts, the minimum surge the generator must supply. With the pump running instead of starting, the total is 5,850 watts, and a 25 percent margin lifts that to about 7,300\. A generator rated near 7,500 running and 9,400 starting covers it. Trim that list to a winter outage and the number drops. Take out the toaster and hair dryer, add a chest freezer and a dozen LED bulbs, and the running total falls to around 3,500 watts before the pump starts. That is the household that gets away with a smaller machine if it is careful about what starts when. The margin is not padding. Norwall describes a real case where a 5,500-watt generator carrying about 4,000 watts of load kept tripping its breaker. The air conditioner and the refrigerator compressors were starting at the same moment, and their combined surge passed 7,000 watts. Any two motors can line up like that, and a freezer and a well pump will eventually do it at 3 a.m. ### Three sizes that cover most houses Once the list is written, most households fall into one of three groups. * **City water and gas heat, about 3,500 to 5,000 watts.** Refrigerator, freezer, furnace blower, lights, chargers and a microwave used on its own. A 120-volt generator can do it, though a 120/240-volt one leaves room to add a circuit later. * **A well, about 7,000 to 8,000 watts with a 240-volt outlet.** The same list plus a fractional-horsepower submersible pump. This is the size most rural houses need, and the one where a proper panel connection becomes close to essential because the pump is hard-wired. * **Most of the house, 10,000 watts and up.** Central air, an electric water heater, an electric range or a large pump. Champion lists central air and a tank water heater among the loads that push a house past 7,000 watts. At this size many owners move to a permanently installed standby unit fed by natural gas or a propane tank. Square footage rules of thumb skip the point. A small farmhouse with a deep well needs more generator than a large suburban house served by a municipal main and gas heat. ### Fuel changes the rating A dual-fuel or tri-fuel generator does not make the same power on every fuel. Champion's guide says propane typically delivers about 10 percent less than gasoline and natural gas 10 to 15 percent less. Read the propane rating on the spec sheet, which is printed separately, and size to it if propane is what you will be burning in February. Many preppers pick propane anyway, because it does not go stale in storage and a large tank can carry a generator through a long outage without a run to a gas station that may have no power either. Gasoline needs stabilizer and rotation, and diesel is mostly for larger machines. Whichever you choose, cold weather is hard on starting, so start the generator once a month and run it under load. ### Staggering loads to run a smaller generator A smaller generator run with discipline can cover what a bigger one would carry on autopilot. The trick is to keep motors from starting together, because the surge is what sizes the machine. * Run the well pump on its own to fill the pressure tank and the water jugs, then turn its breaker off. * Let the freezer and refrigerator run for an hour or two, then switch to the furnace fan. A full freezer held shut keeps food frozen for about two days, Ready.gov says, so it does not need power around the clock. * Cook, make coffee or run the microwave while the motors are idle. * Leave the water heater and range off the generator unless it is sized for them. A switch box with circuit selectors, or a main-panel interlock, makes this a matter of flipping breakers. Our page on [how long freezer food lasts in an outage](/food-storage/freezer-power-outage-how-long-food-lasts) gives the hold times to plan the rotation around. ### Connecting it to the house The pump, the furnace and many sump pumps are hard-wired, so they cannot be plugged into the generator with a cord. They are fed through a manual transfer switch or a panel interlock with a generator inlet on the outside wall, and Champion's guide says that work belongs to a licensed electrician. Plugging a generator into a wall outlet to reach those circuits is backfeeding, which can kill a lineworker on the street. For plug-in loads, Ready.gov calls for heavy-duty extension cords run from the generator. Keep the generator outdoors, at least 20 feet from doors, windows and an attached garage, with a battery carbon monoxide alarm on every floor. Our [generator safety guide](/power/generator-carbon-monoxide-safety) covers placement, CO and backfeeding in full, and is worth reading before the first cold night. ### My take: what I would buy I'd size from the well pump outward. If the house has a 240-volt pump, I would not consider a generator without a 120/240-volt outlet and enough surge to start that pump with the fridge already running, which for most homes means the 7,000 to 8,000 watt class. I would pay for the transfer switch or interlock at the same time, since a pump you cannot connect safely is a pump that stays off. On city water, I'd buy the smaller machine and spend the difference on fuel storage and a furnace hookup. A 5,000-watt generator that is maintained, fueled and connected does more for a family in an ice storm than a big one that will not start. Either way, write the list of loads, and the order in which you will run them, on a card taped inside the panel door. The rest of our guides to keeping a house running without the grid are in the [power section](/power). ### Sources * [Ready.gov: Power Outages](https://www.ready.gov/power-outages) * [Champion Power Equipment: What Size Generator Do I Need?](https://www.championpowerequipment.com/news-updates/what-size-generator-do-i-need/) * [Norwall PowerSystems: Portable Generator Sizing Guide](https://norwall.com/power-expert/Sizing-Guide-for-Portable-Generators) * [TractorByNet: Sizing Generator for Submersible Well Pump](https://www.tractorbynet.com/forums/threads/sizing-generator-for-submersible-well-pump.309058/) How we write: we work from government guidance, manufacturer documentation and long-running owner discussions, and we say what we have not tested. See our [editorial policy](/editorial-policy). ### Generator sizing: questions Will a 3,500 watt generator run a refrigerator? Yes, with room left for lights and a phone charger. A modern fridge starts at roughly a kilowatt and then settles to a few hundred watts. What a unit that small cannot do is start a 240-volt well pump. Can I run a well pump on a 120-volt generator? Not if the pump is wired for 240 volts, which most submersible pumps in American homes are. You need a generator with a 120/240-volt outlet and a transfer switch or interlock to feed the pump circuit. Does square footage tell me what size generator I need? Only roughly. Two houses of the same size can need very different machines: one with gas heat and city water, the other with a deep well and an electric range. Size from the list of loads. Is it bad to run a generator at full load? Running at its limit for hours wears the engine harder and leaves nothing for a motor that kicks on. Sellers of generators advise keeping a quarter of the capacity spare. How do I find a pump or appliance wattage? Read the nameplate. If it shows only volts and amps, multiply them. For a well pump, the horsepower and voltage are on the control box in the basement or well house. Written by Ruth Calloway Pantry, water and homestead skills Ruth Calloway writes Larder & Well's guides to food storage, water, off-grid power and the old household skills. She works from extension-service bulletins, USDA and Ready.gov guidance, manufacturer manuals and years of homesteader discussion threads, and she says plainly where the sources disagree. She does not claim to have tested what she describes; when a method is untested or risky, her articles say so. [rcalloway@larderandwell.cc](mailto:rcalloway@larderandwell.cc) Last checked 2026-11-12 Every guide on generators, fuel and keeping warm without the grid is in the [power section](/power). --- [Larder & Well](/) Keep the household fed, watered and warm when the stores close. Food storage, water, off-grid power, homestead skills and the old ways, written for American households that want to stay fed, watered and warm when the grid, the economy or the supply chain fails. [Send a correction or a question](/contact) © 2026 Larder & Well. Independent, written from sources you can open. A page that ever carries a paid link says so above it. Nobody pays for a place in our guides.