When the power goes out, having a portable generator can mean the difference between comfort and crisis. But buying one that is too small leaves you stranded, while overspending on a unit that is too large wastes money and fuel. Learning how to calculate what size portable generator you need for your house is the single most important step before making a purchase.
Our team has spent years testing generators in real outage scenarios, from hurricane prep to winter storm recovery. We have seen firsthand what happens when someone guesses their power needs instead of calculating them. This guide walks you through the exact method we use, step by step, so you can size your generator with confidence.
By the end of this article, you will know how to tally your appliance wattage, account for starting surges, apply safety margins, and pick the right generator capacity for your specific home. If you want to jump straight to product options after calculating your needs, check our guide to the best portable generators for home use.
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Every electrical appliance draws power in two different ways, and understanding the difference is the foundation of generator sizing. Running watts (also called continuous or rated watts) represent the steady power an appliance needs to keep operating. Starting watts (also called surge watts) represent the extra burst of power needed to get motor-driven appliances going.
Here is why this matters so much: a refrigerator might only need 700 running watts to stay cold, but it requires around 2,200 starting watts to fire up its compressor. If your generator cannot handle that initial surge, the appliance will not start at all. Worse, it could trip your generator's breaker or damage the appliance motor.
Motor-driven appliances like air conditioners, well pumps, refrigerators, freezers, and washing machines all have significant starting watt requirements. Resistive loads like lights, televisions, and phone chargers do not need extra surge power. They draw the same wattage at startup as they do during continuous operation.
You will typically find both running and starting wattage listed on the appliance itself or in its user manual. If you only see amps, you can convert: watts = amps x volts. Most household appliances run on 120-volt circuits, while larger items like electric dryers or ranges use 240 volts.
Many homeowners ask about the 20/20/20 rule, and it gets confused across different sources online. The rule refers to three key safety practices related to generator placement and operation. First, place your generator at least 20 feet from your home. Second, keep it at least 20 feet from any windows or doors. Third, never run it for more than 20 hours without letting it cool down and checking the oil.
This rule exists primarily to prevent carbon monoxide poisoning, which kills dozens of people every year during power outages. It also protects your generator from overheating and premature engine wear. We cover more safety details later in this guide, but always remember the 20/20/20 principle when setting up your portable generator.
Now we get to the core question. The calculation method involves seven clear steps that anyone can follow, no electrical engineering degree required. Grab a pen and paper or open a spreadsheet, because you will need to do some simple addition.
Start by writing down every appliance, device, and light fixture you want to run during an outage. Be realistic about what is essential versus what would be nice to have. Most homeowners focus on refrigeration, lighting, heating or cooling, water pumps, and communication devices.
Think about your daily routine during a power outage. You need to keep food cold, have working lights, maintain some climate control, and charge phones. Medical equipment like CPAP machines or oxygen concentrators should be top priority if anyone in the household depends on them.
Next to each appliance on your list, write down its running wattage. Check the appliance label, the user manual, or the manufacturer's website. If the label shows amps instead of watts, multiply amps by 120 (for standard outlets) to get watts. We have included a comprehensive reference table below to help with common items.
For any appliance with a motor, you need to find its starting wattage. This includes refrigerators, freezers, air conditioners, well pumps, sump pumps, furnaces, washers, and microwaves. Starting watts are typically 2 to 3 times the running wattage. A refrigerator rated at 700 running watts might need 2,100 starting watts.
If you cannot find the starting wattage listed anywhere, a safe rule of thumb is to multiply the running watts by 3 for items with motors. For well pumps and air conditioners, the surge can be even higher. Users on forums like r/Generator frequently report underestimating well pump requirements, which trips their generator instantly.
Sum up the running watts for every appliance on your list. This gives you your baseline continuous load. For example, if your refrigerator runs at 700 watts, your freezer at 500 watts, your lights at 300 watts, and your TV at 200 watts, your total running watts come to 1,700 watts.
This is where many people make their first mistake. You do not add all starting watts together, because appliances rarely all start at the exact same moment. Instead, find the single highest starting wattage on your list and add that to your total running watts.
For example, if your air conditioner needs 4,500 starting watts and that is the largest surge on your list, add 4,500 to your total running watts. This accounts for the worst-case scenario where everything is running and then your biggest motor kicks on.
Take your total from Step 5 and multiply it by 1.2, or simply add 20%. This gives you your minimum generator capacity requirement. The formula looks like this: (Total Running Watts + Highest Starting Watts) x 1.2 = Minimum Generator Size.
This 20% buffer is not optional in our experience. It accounts for voltage drops, aging appliances that draw more power over time, and any devices you forgot to include. Running a generator at absolute maximum capacity stresses the engine and shortens its lifespan considerably.
The 80% rule means you should never run a generator at more than 80% of its rated capacity for extended periods. If you need 8,000 watts of power, you should buy a generator rated for at least 10,000 watts. This rule keeps your generator running efficiently, prevents overheating, and extends engine life.
Forum users and electricians consistently recommend this rule. Running at full capacity causes voltage fluctuations that can damage sensitive electronics. It also burns fuel faster and increases wear on components. Think of it like a car engine: cruising at 80% of redline is sustainable, but constantly redlining will destroy it.
Use these tables as a starting point, but always verify with the actual labels on your specific appliances. Wattage varies significantly between models, ages, and efficiency ratings.
Refrigerator (standard): 700 running watts, 2,200 starting watts. The compressor cycles on and off, so it does not draw surge power constantly. Energy-efficient models may use less.
Freezer (chest): 500 running watts, 1,500 starting watts. Like refrigerators, the surge happens when the compressor kicks in.
Microwave oven: 1,000 to 1,500 running watts. Microwaves draw full power whenever they are running but have minimal surge.
Coffee maker: 800 to 1,200 running watts. Resistive load with no significant starting surge.
Electric stove or oven: 2,000 to 5,000 running watts depending on how many burners are active. These are power-hungry items that may not be practical on a portable generator.
Dishwasher: 1,200 running watts, 1,800 starting watts. The pump motor creates the surge requirement.
Toaster: 800 to 1,500 running watts. Quick bursts of high power but no starting surge.
Central air conditioner (10,000 BTU): 1,200 running watts, 3,500 starting watts. Window units are similar but slightly lower.
Central air conditioner (24,000 BTU): 3,800 running watts, 11,000 starting watts. This is one of the most demanding appliances in any home.
Furnace blower (gas furnace): 800 running watts, 2,400 starting watts. The blower motor creates the surge.
Space heater: 1,500 running watts. No starting surge, but it draws maximum power continuously.
Electric water heater: 4,000 running watts. This is extremely demanding and usually not practical for portable generator backup.
Washing machine: 1,200 running watts, 3,500 starting watts. The motor surge is significant.
Gas clothes dryer: 700 running watts, 1,800 starting watts. The drum motor creates the surge. Electric dryers need much more power.
Well pump (1/2 hp): 1,000 running watts, 3,000 starting watts. Well pumps are notorious for high starting surges.
Well pump (1 hp): 2,000 running watts, 6,000 starting watts. If you rely on a well for water, plan your generator size around this appliance.
Sump pump (1/3 hp): 800 running watts, 1,300 starting watts. Essential during storms when power outages coincide with flooding.
LED light bulb: 10 running watts each. Ten bulbs add up to just 100 watts total.
Television (LED, 55-inch): 120 running watts. Minimal power draw.
Laptop charger: 60 to 90 running watts. Very low demand.
Phone charger: 5 to 20 running watts. Negligible load.
Internet router and modem: 30 to 50 running watts combined. Keeping connectivity alive is important during emergencies.
CPAP machine: 200 to 400 running watts depending on whether it has a heated humidifier.
Many readers want a quick reference based on their home's square footage. While the detailed calculation above is always more accurate, this chart gives you a reasonable starting point. These recommendations assume you are running essential circuits only, not your entire home at full capacity.
For a small home, apartment, or cabin, a 3,000 to 5,000 watt portable generator typically handles your essential needs. This covers a refrigerator, several lights, a small window AC unit, and basic electronics. You will not be running central air or an electric water heater on this size.
A 5,000 to 7,500 watt generator is usually sufficient for homes in this range. This capacity can handle a refrigerator, freezer, furnace blower, well pump, lighting, and some electronics. It gives you enough headroom to run most essentials simultaneously during an extended outage.
For a typical 3-bedroom home, plan for a 7,500 to 10,000 watt generator. This accommodates the essentials plus a window AC unit or small central air system. If you have a well pump or want to run a washing machine, lean toward the higher end of this range.
A 10,000 to 12,000 watt generator is recommended for larger homes. At this size, you can run central air conditioning along with all essential appliances. This is also where standby generators start becoming a more practical option than portable units.
For homes over 2,500 square feet, you are looking at 12,000 watts or more. At this point, portable generators stretch their practical limits. A standby generator rated at 15,000 to 22,000 watts is usually the better choice for whole-house backup power.
Remember that square footage is only a rough proxy for power needs. A 1,200 square foot home with a well pump, electric water heater, and central air will need more generator capacity than a 2,000 square foot home with gas heating, municipal water, and window units. Always do the full calculation for the most accurate result.
Let us walk through a complete calculation so you can see exactly how this works. Our example is a typical 3-bedroom, 1,800 square foot home with gas heat and municipal water.
The appliances this family wants to power during an outage are: refrigerator, chest freezer, furnace blower (gas furnace), 6 LED lights, TV and internet router, two phone chargers, a microwave, and a window AC unit (10,000 BTU).
Here are the running watts: Refrigerator at 700W, freezer at 500W, furnace blower at 800W, 6 LED lights at 60W total, TV at 120W, router at 40W, phone chargers at 20W, microwave at 1,200W, and window AC at 1,200W. Total running watts: 4,640W.
Now for starting watts, we look at the motorized items. Refrigerator needs 2,200W, freezer needs 1,500W, furnace blower needs 2,400W, and window AC needs 3,500W. The highest single starting surge is the window AC at 3,500W.
Calculation: Total running watts (4,640) + highest starting surge (3,500) = 8,140 watts. Add 20% safety margin: 8,140 x 1.2 = 9,768 watts. For the 80% rule, divide by 0.8: 9,768 / 0.8 = 12,210 watts.
This family should look for a generator rated around 10,000 to 12,000 running watts. If they skip the window AC and only run essentials, a 7,500 watt generator would handle the rest comfortably. This is exactly how to calculate what size portable generator you need for your house.
Sizing your generator correctly is important, but using it safely is even more critical. Portable generators cause dozens of deaths and injuries every year, almost entirely from preventable mistakes.
Never operate a portable generator indoors, in a garage, or in any enclosed space. Carbon monoxide is colorless, odorless, and deadly. Place your generator at least 20 feet from your home, away from windows, doors, and vents. The wind should carry exhaust away from the house, not toward it.
Modern generators often include built-in CO sensors that automatically shut the engine down if dangerous levels are detected. This feature has saved lives, and we strongly recommend choosing a model with this safety technology. Never override or disable a CO sensor.
Backfeeding means plugging a generator directly into a wall outlet to power your home's wiring. This is extremely dangerous and illegal. It can electrocute utility workers repairing power lines and start fires in your electrical system. Always use a proper transfer switch installed by a licensed electrician.
A transfer switch safely isolates your home's electrical system from the grid while allowing your generator to power selected circuits. Manual transfer switches are affordable and straightforward. Automatic transfer switches cost more but switch over instantly when the power goes out.
Store generator fuel in approved containers, away from living spaces and heat sources. Never refuel a running or hot generator. Let it cool down first to prevent fires. Gasoline degrades over time, so use fuel stabilizer if you are storing it for more than a few months.
Once you know your wattage requirements, the next decision is portable versus standby. Both have their place, and the right choice depends on your budget, power needs, and how often you experience outages.
Portable generators cost less and offer flexibility. You can move them between locations, use them for camping or job sites, and store them when not needed. Their wattage output ranges from about 2,000 to 12,000 watts. They require manual setup during an outage: wheeling them out, connecting fuel, starting the engine, and flipping the transfer switch.
Standby generators are permanently installed and connect directly to your home's electrical system. They turn on automatically within seconds of a power outage. Their capacity ranges from 7,000 to 22,000+ watts for residential models. They run on natural gas or propane, so you never need to worry about fuel storage.
For homes needing more than 12,000 watts of backup power, standby generators are typically the better investment. They also require professional installation, which adds to the cost. If you are weighing options, our guides to home backup generators and the best standby generators offer detailed comparisons.
We see the same errors repeated across forums, reviews, and real-life outage stories. Avoiding these common pitfalls will save you money, frustration, and potential equipment damage.
Mistake 1: Only counting running watts. This is the number one error. If you add up running watts and buy a generator based on that number, your generator will stall the moment a motor-driven appliance tries to start. Always account for starting watts.
Mistake 2: Forgetting the well pump. Well pumps have enormous starting surges, often 5,000 to 6,000 watts for a 1 hp model. Many homeowners buy a generator that handles everything else, only to discover their water does not work because the pump cannot start.
Mistake 3: Ignoring the 80% rule. Running your generator at maximum capacity wears out the engine faster and risks voltage drops that damage electronics. Size up by at least 20% to keep your generator running efficiently.
Mistake 4: Not planning for future needs. If you might add a chest freezer, medical equipment, or a larger AC unit, factor that in now. It is cheaper to buy a slightly larger generator than to replace one that is too small a year later.
Mistake 5: Using cheap extension cords. Undersized or damaged cords cause voltage drops, overheating, and fire risks. Use heavy-duty, outdoor-rated cords sized for your generator's output. If you use inverter generators for camping or home use, the clean power output protects sensitive electronics, but you still need quality cables.
A 2,000 square foot home typically needs a 7,500 to 10,000 watt portable generator for essential circuits. This covers a refrigerator, freezer, furnace blower, lighting, electronics, and a window AC unit. If you want to run central air conditioning, you may need 10,000 to 12,000 watts. Always calculate your specific appliance wattage for the most accurate sizing.
The 20/20/20 rule means placing your generator at least 20 feet from your home, keeping it at least 20 feet from windows and doors, and not running it for more than 20 hours without checking oil and letting it cool. This rule prevents carbon monoxide poisoning and protects the generator engine from overheating.
A 7,500 watt generator can power essential circuits in a medium-sized home (1,000 to 1,800 square feet). It handles a refrigerator, freezer, furnace blower, lights, electronics, and either a small window AC or a well pump. It will not run central air conditioning or an electric water heater. For full whole-house backup, you need significantly more capacity.
A 5,000 watt generator runs essentials in a small home: a refrigerator, several LED lights, a furnace blower (gas heat), a TV, phone chargers, and a microwave. You can also run a small window AC unit (5,000 BTU). It cannot handle central air, electric water heaters, well pumps over 1/2 hp, or multiple large appliances simultaneously.
The 80% rule states that a generator should never be loaded beyond 80% of its rated capacity for continuous operation. If you need 8,000 watts of power, buy a generator rated for at least 10,000 watts. Running at full capacity causes overheating, voltage fluctuations that damage electronics, increased fuel consumption, and shortened engine life.
To determine the right generator size, list every appliance you need during an outage, find their running watts, add the highest single starting watt surge, then multiply by 1.2 for a safety margin. Most homes need 5,000 to 12,000 watts for essential backup. Small homes may need only 3,000 watts, while large homes with central air and well pumps may need 12,000 watts or more.
Figuring out how to calculate what size portable generator you need for your house comes down to a simple process: list your appliances, find their running and starting watts, add them up with the largest surge, then apply a 20% safety margin and the 80% load rule. The math is straightforward, and doing it right means you will have reliable power when you need it most.
Once you know your wattage requirement, the next step is finding a generator that meets those specs. Check out portable generators for job sites or our home generator guides for specific recommendations. Do the calculation, pick the right size, and stay powered through the next outage.