Home Generator Sizing Calculator: Watts, Surge, and Runtime Estimate

Running watts (also called rated watts) is the steady power an appliance draws while it is operating normally. Starting watts (also called surge watts or peak watts) is the much higher power a motor-driven appliance requires for 1 to 2 seconds when it first starts. The compressor in a refrigerator, the motor in an AC unit, and the pump in a well pump all need a startup surge because the motor must overcome its own inertia to go from stationary to full operating speed. Resistive loads (electric heaters, incandescent lights, toasters, coffee makers) have no startup surge and draw the same amount from start to running. The practical implication: a generator sized only on running watts will trip its breaker or stall when a motor appliance starts while other loads are running, because the combined running watts plus the motor’s starting surge exceeds what the generator can supply. Always use starting watts for motor loads when sizing a generator.

Follow this systematic process: Step 1: List every appliance you want to power during an outage. Step 2: Find the running watts and starting watts for each one (from the nameplate, owner’s manual, or the presets in this calculator). Step 3: Add up all running watts to get your total continuous load. Step 4: Identify the motor with the biggest starting surge (starting watts minus running watts). Step 5: Add that extra surge to your total running watts to find peak demand. Step 6: Multiply peak demand by 1.20 to apply the 20% safety margin. Step 7: Round up to the nearest available generator size. This calculator performs steps 3 through 7 automatically once you enter your load list. For a whole-house standby generator, have a licensed electrician perform a load calculation; they will also size the automatic transfer switch and ensure the installation meets your local electrical code.

To run a 3-ton central AC unit (3,500 running / 10,500 starting watts) alongside basic essential loads (refrigerator, lights, devices, furnace fan): peak demand of approximately 12,000 to 14,000 watts. With the 20% safety margin: minimum rated generator capacity of 14,400 to 16,800 watts. The practical recommendation for running a 3-ton central AC plus essential home loads is a generator rated at 15,000 to 18,000 watts. A 20-SEER high-efficiency unit running the same size compressor draws approximately half the watts (1,750 running / 5,250 starting), which reduces the required generator to approximately 8,000 to 10,000 watts. Note that many US installers do not recommend trying to run a central AC on a portable generator at all, as the constant cycling of the AC compressor creates repetitive surge demand on the generator and the AC unit may not cool effectively when the generator runs near its limits. Window AC units (500 to 900 watts running) are a much more generator-friendly cooling solution for outage scenarios.

A 10,000-watt generator can cover most essential home loads but is typically not large enough to run a 3-ton central AC plus a full home load simultaneously. A 10kW generator handles: refrigerator (150W), furnace fan (800W), lighting (200W), electronics (300W), and a window AC (900W) or small central AC (2-ton at 2,500W running) comfortably. Adding an electric dryer (5,000W) or electric water heater (4,500W) to the load list pushes total running load past 8,000W and the startup surge of the AC could push peak demand near or above the 10kW limit. For truly whole-house backup in a home with a 3-ton or larger AC, electric dryer, and electric water heater, 15,000 to 22,000 watts is a more realistic minimum. For households with natural gas dryer, gas water heater, and gas furnace (needing only the fan motor, not the heating element), a 7,500 to 10,000W generator covers most scenarios reasonably well.

A transfer switch is an electrical device that safely disconnects your home from the utility grid before connecting it to a generator. It prevents the dangerous condition called backfeed, where generator power travels back through your meter and onto the utility lines, creating a potentially lethal voltage hazard for utility workers trying to restore power. In the US, connecting a generator to home wiring without a proper transfer switch (by plugging extension cords into outlets or by using a suicide cord to connect to an outlet) is illegal under the National Electrical Code and creates serious safety risks. There are two main types: a manual transfer switch (a panel mounted next to the main breaker panel that a homeowner manually switches during an outage) and an automatic transfer switch (ATS, used with standby generators, which senses utility power loss and automatically starts the generator and switches the home to generator power within 10 to 30 seconds). Standby generators always include or require an ATS. Portable generators used to power specific loads via extension cords do not technically require a transfer switch, but connecting a portable generator to any home wiring requires a properly installed transfer switch. Transfer switch installation costs $500 to $2,000 for a manual switch covering selected circuits, or $1,500 to $4,000 for a whole-house automatic transfer switch.

The choice between portable and standby generators depends on outage frequency and duration in your area, your budget, and what loads you need to power. Portable generators: lower upfront cost ($500 to $3,000), no installation required for basic use with extension cords, require manual setup and refueling, limited to gasoline or dual-fuel (gas and propane), and must be operated outdoors away from the home. Best for: areas with infrequent outages (one or two per year), partial home backup, and budget-conscious buyers. Standby generators: higher cost ($3,000 to $10,000 equipment plus $2,000 to $5,000 installation), automatic start with no user intervention needed, run on natural gas or propane from your existing utility line (no refueling required during extended outages), whole-house coverage, and include an automatic transfer switch. Best for: areas with frequent outages (Northeast ice storms, Southeast hurricanes, Midwest severe weather), homes with medical equipment requiring uninterrupted power, or homeowners who travel and cannot be present to manually start a portable generator. Most US generator installers recommend Generac, Kohler, or Briggs and Stratton for residential standby units.

An inverter generator produces electricity through a two-stage process: the engine generates AC power, which is then converted to DC and back to clean AC power through electronic inverter circuits. This produces power with very low total harmonic distortion (THD, typically under 3 percent), which is safe for sensitive electronics including laptops, phones, CPAP machines, and medical equipment. A conventional generator produces power with higher THD (typically 10 to 25 percent), which can shorten the life of sensitive electronics and may cause equipment errors. Inverter generators are also significantly quieter (50 to 60 decibels at 25 percent load versus 70 to 80 decibels for conventional units) and more fuel-efficient because the engine speed automatically adjusts to match the load rather than running at a constant 3,600 RPM. The tradeoff: inverter generators are more expensive per watt and typically max out at 3,500 to 4,000 watts for a single unit (though two compatible units can be paralleled for double the output). For powering laptops, phones, CPAP machines, or any sensitive electronics, an inverter generator is the appropriate choice. For powering large motor loads like well pumps and AC units, a conventional portable or standby generator is more practical.

Yes, but well pumps require careful generator sizing because they have significant starting surge requirements. A 0.5 HP well pump draws 500 running watts but surges to 1,250 starting watts (2.5x ratio). A 1 HP well pump draws 750 running watts and surges to 1,875 starting watts. A 1.5 HP pump: 1,100W running, 2,750W starting. To run a well pump plus essential home loads (refrigerator, lights, furnace fan), size the generator using the peak demand formula: total running watts + extra surge of the well pump. Example with a 1 HP pump plus 1,500W other loads: peak demand = 2,250W + (1,875 – 750) = 2,250 + 1,125 = 3,375W. With 20% margin: 4,050W, so a 5,000W generator handles this load. Most well pumps in US homes are on 240V circuits, which requires a generator with a 240V outlet. Verify your portable generator has a 240V output (most 5kW and larger portable units do). For a 240V deep well pump, use a 4-prong 14-30 outlet with a properly rated cord.

The US Consumer Product Safety Commission and the National Fire Protection Association both recommend placing portable generators at least 20 feet from any door, window, or vent that could allow carbon monoxide to enter the home. This includes garage doors, even when open. CO is colorless and odorless and accumulates silently; people often fall asleep before they realize they are affected. In 2020 and 2021, generators were involved in approximately 85 CO fatalities per year in the US, with the majority caused by operating generators in garages, basements, or too close to the home. CPSC guidelines also recommend: never run a generator in a basement, crawl space, or attached garage; exhaust must point away from the home; install battery-operated CO detectors on every floor of the home within 10 feet of sleeping areas; and if CO alarms sound, immediately move to fresh air and call 911 before going back indoors. Standby generators are installed outdoors on a permanent pad and vent away from the structure as part of the installation; they do not have the same proximity concerns as portable units.

The 20% safety margin means the recommended generator rated capacity is 1.20 times the calculated peak demand. If peak demand is 8,000 watts, the recommended rated capacity is 9,600 watts, rounded to the next available size of 10,000 watts. The margin serves three purposes: it accounts for measurement uncertainty (real appliance draws often vary from published figures by 5 to 15 percent depending on equipment age and operating conditions); it prevents the generator from running at or above its rated capacity for extended periods (which causes accelerated wear and overheating); and it provides capacity for loads you may have overlooked or added later. Most US generator manufacturers recommend operating their units at a maximum of 80 percent of rated capacity for continuous operation, which is exactly the operating point created by the 20 percent sizing margin. Running at 100 percent of rated capacity for hours at a time significantly reduces engine and alternator life and increases the risk of generator failure during the outage you bought it to protect against.

A conventional portable generator can damage sensitive electronics if it produces power with high total harmonic distortion (THD). Conventional generators running at variable loads produce THD levels of 10 to 25 percent, which can cause laptop power supplies, phone chargers, CPAP machines, and medical devices to run hotter, fail sooner, or produce errors. Inverter generators produce power with under 3 percent THD, which is comparable to or cleaner than typical US grid power, and are safe for all electronics. To protect sensitive electronics connected to a conventional generator: use a UPS (uninterruptible power supply) or surge protector with line conditioning between the generator and the electronics; these devices smooth the generator’s output to safe THD levels. Most modern power supplies in laptops and phones have some tolerance for power quality variation, but medical equipment (CPAP, home dialysis, infusion pumps) should only be connected to inverter generators or through a line-conditioning UPS. Check the owner’s manual for any medical equipment you plan to power during an outage.

Each fuel type has advantages and disadvantages for US home backup: Gasoline: most portable generators run on gasoline, which is widely available but degrades in as little as 30 days without fuel stabilizer, requires rotation and fresh storage, and can be difficult to obtain during major regional outages when gas stations lose power. Propane: dual-fuel and propane-only generators run on stored propane tanks, which have an indefinite shelf life and can be stockpiled. A large 500-gallon propane tank provides days to weeks of fuel depending on load. Propane generators produce slightly less power than gasoline at the same engine size. Natural gas: standby generators connected to the utility gas line never run out of fuel (as long as gas service is not disrupted), making them ideal for extended outages. Natural gas outages are rare but do occur in severe disasters. Diesel: diesel generators are more fuel-efficient than gasoline and diesel has a longer shelf life (1 to 2 years with stabilizer), but diesel portable generators are heavier, louder, and less common for residential use in the US. Dual-fuel (gas and propane): the most practical choice for most US homeowners using a portable generator, offering flexibility to use whichever fuel is available and the ability to store propane for long-term readiness.

A modern ENERGY STAR-certified refrigerator uses approximately 150 watts running, with a starting surge of approximately 450 watts when the compressor starts. An older refrigerator (pre-2001) may draw 300 to 400 watts running, with a 900 to 1,200-watt starting surge. The refrigerator’s compressor starts and stops automatically to maintain temperature, which means it will trigger its starting surge multiple times per hour during generator operation. For generator sizing, always use the starting watt figure (450 watts for a new model) rather than the running watt figure, because the generator will need to supply the startup surge repeatedly throughout the outage. A small 2,000W inverter generator can safely run a modern refrigerator (150W running / 450W starting) plus basic lighting and device charging with capacity to spare. The key concern during outages: keep the refrigerator door closed as much as possible. A refrigerator maintains safe temperatures for approximately 4 hours without power if unopened, a full freezer for 48 hours. Running the generator intermittently (a few hours on, a few hours off) can extend your runtime significantly while keeping food safe.

A dual-fuel generator is a portable generator that can run on either gasoline or propane (LP gas), with a switch or knob to select the fuel source. Most dual-fuel generators produced by US brands like Champion, DuroMax, and Westinghouse run primarily on gasoline for maximum power and efficiency, and can switch to propane at the same or slightly reduced wattage. The advantage of dual-fuel capability: propane stores indefinitely without degrading (unlike gasoline, which degrades in 30 to 60 days without stabilizer), so you can maintain a propane supply ready for emergencies without rotation. During a major regional outage (hurricane, ice storm) when gasoline is sold out at every station, your stored propane supply continues to work. The tradeoff: propane has about 91,502 BTU per gallon versus approximately 114,000 BTU per gallon for gasoline, meaning propane produces less power per gallon and generators produce 10 to 15 percent less rated output on propane than on gasoline at the same engine settings. For backup preparedness, a dual-fuel generator with a 100-pound propane cylinder (approximately 23 gallons) provides 40 to 60 hours of mid-load runtime without any fuel resupply needed.

Signs a generator is overloaded: the generator’s circuit breaker trips (the most common and safest indicator); the engine slows to a lower-pitched sound (called bogging down) as it struggles to maintain speed under excessive load; the output voltage drops below 110V or 220V (a digital multimeter at the outlet can measure this); the generator produces less output frequency than 60Hz (some modern units have digital displays showing frequency and voltage); or the generator produces visible exhaust smoke from laboring at excess load. If your generator trips its breaker during operation: immediately turn off or disconnect the highest-wattage load, reset the breaker, and restart. If it trips again, you have a load that exceeds the generator’s capacity. To find the culprit: add loads one at a time starting with the lowest-wattage items and test after each addition. When the generator begins to bog or trips, the most recently added load (or its startup surge) is the overloading factor. Refer to this sizing calculator and add that load’s starting watts to your existing running total to see the correct generator size needed.

Generator fuel consumption depends on the generator size, your actual load, and the fuel efficiency of the specific unit. At approximately 50 percent of rated load: a 5,000W generator drawing 2,500W of actual load consumes approximately 0.35 to 0.45 gallons per hour. A 7-gallon tank provides 15 to 20 hours of runtime. At 75 percent of rated load (more typical for an essential-loads scenario): the same 5,000W generator draws 3,750W of actual load and consumes 0.50 to 0.65 gallons per hour, giving 11 to 14 hours on a 7-gallon tank. At full load (100 percent, not recommended for extended operation): 0.75 to 1.0 gallon per hour, or 7 to 9 hours on a 7-gallon tank. Inverter generators are significantly more fuel-efficient at partial loads because they reduce engine speed to match demand; the Honda EU2200i, for example, runs 8.1 hours on a 1.1-gallon tank at 25 percent load. This calculator estimates fuel consumption using an average factor of 0.16 gallons per kWh of output at 75 to 80 percent load efficiency, which is consistent with most portable generator specifications from major US brands including Champion, Honda, Generac, and Westinghouse.