Generator Size Calculator

How to Use This Generator Size Calculator

Work through the checklist the way you would during a real outage. Check the box next to each appliance that has to stay on: the refrigerator, the furnace blower if you heat with gas, a well or sump pump if you have one, lights and a way to keep phones and the internet alive. Leave off anything you can live without for a few hours, because every box you add pushes the recommended size up a step.

The calculator totals the running watts of everything you selected, then finds the single appliance with the largest starting surge and adds that surge on top of the rest of the load. That second number is the moment your generator has to survive: the well pump kicks on while the fridge and furnace are already running. The recommended size covers both numbers, and the last card tells you whether that load fits a portable unit or pushes you into a whole-home standby.

These are typical wattage values for standard residential appliances. Your exact draw is printed on each unit's nameplate, and the guide below explains how to read one. If your house has an odd load, a heat pump with resistance heat, a workshop, or a big pool pump, swap in the real nameplate numbers before you buy. Also run the electrical load calculator on this site to confirm your panel and transfer switch have room for the generator feed.

Running Watts vs Starting Watts: The Two Numbers That Matter

Every generator is rated with two numbers, and undersizing happens when people look at only one. Running watts is the continuous power the unit can produce hour after hour. Starting watts, also called surge watts, is the extra power it can deliver for a few seconds while a motor spins up from a dead stop.

Almost anything with a motor draws far more at the instant it starts than it does once it is running. A refrigerator compressor needs about 700 running watts but surges near 2,200 watts. A 1/2 HP well pump runs on 1,000 watts and surges toward 3,000. A 3 ton central air conditioner is the extreme case, roughly 3,500 running watts and up to 8,000 watts of surge. Purely resistive loads, like light bulbs, a microwave and an electric water heater, have no meaningful surge, which is why they are listed with matching numbers above.

The sizing rule that handles this is simple: your total running load must stay under the generator's running rating, and your total starting load must stay under the generator's surge rating. The surge rarely happens for everything at once, so the practical calculation takes the single biggest surge and adds it to the running watts of all the other appliances, which is exactly what the calculator does.

Essential Circuits vs Whole-Home Backup: Decide Before You Size

The single biggest decision in generator sizing is scope, and it comes before any math. An essential-circuits backup keeps a short list running, the fridge, furnace, a pump, lights and electronics. A whole-home backup feeds the entire panel, so the AC, electric water heater, dryer and every other circuit work exactly as they do on grid power.

Most homes genuinely need only the essential list during an outage, and a 5,000 to 7,500 watt portable generator covers it at a fraction of the cost. The whole-home approach is attractive until you see what it demands: adding a 3 ton AC unit and an electric water heater to the load can push the requirement past 15,000 watts, which means a professionally installed standby generator and usually a larger gas line. Size for what you truly need, not for the fantasy of running every appliance at once.

There is a middle path used by many homeowners: cover the essentials on a portable generator, and leave the big 240-volt loads off unless the outage drags on. A generator sized for essentials still gives you heat, cold food, water and light, which is what outages actually threaten. You can always start the generator and run one large appliance briefly by switching a transfer switch, as long as you stay under the surge rating.

How to Read an Appliance Nameplate

Every appliance carries a metal or sticker nameplate that tells you its real electrical draw, and for sizing your own generator it beats any typical-value table. The plate is usually on the back, the bottom, the door frame, or near the motor. You are looking for one of three things: a wattage number, an amperage number, or a locked-rotor number on larger motors.

• Watts (W): If the plate lists watts directly, that is the running draw. Multiply by a surge factor of 2 to 4 for motor loads, or use the locked-rotor value below.
• Amps and volts: Most plates list amps. Multiply amps by volts to get watts. A fridge drawing 6 amps at 120 volts uses 720 watts. A dryer drawing 30 amps at 240 volts uses 7,200 watts, which is why electric dryers dominate a generator load.
• LRA (locked rotor amps): On an air conditioner or heat pump nameplate, LRA is the starting surge. Multiply LRA by 240 volts to get the surge watts. A condenser with an LRA of 30 needs about 7,200 watts at the moment the compressor starts, which is exactly the surge this calculator estimates.

If a nameplate is gone, the typical values above are a safe starting point. When in doubt, round up to the next size. A generator that is slightly oversized for your load just runs with margin, while one that is undersized by a few hundred watts stalls exactly when the well pump kicks on in the dark.

Fuel Types Compared: Gasoline, Propane, Natural Gas and Diesel

Portable generators mostly burn gasoline, and they have one weakness: in a regional outage, gas stations lose power too. If you plan around a portable unit, store treated fuel in approved cans, rotate it every few months, and remember that a generator at full load drinks roughly 0.5 to 0.75 gallons of gasoline per hour per 5,000 watts. A 5,000 watt unit under load burns about a gallon every two hours.

Propane and dual-fuel generators let you switch between gasoline and propane from a BBQ-style tank. Propane stores indefinitely, burns cleaner, and gives the engine a slightly lower output, roughly 10% less than gasoline, but it removes the stale-fuel problem entirely. A 20-pound tank runs a 5,000 watt generator for only a few hours under load, so propane users should plan on a 100-pound tank or several small ones.

Natural gas is what makes a whole-home standby generator feel automatic. It taps the same gas line that feeds the furnace and water heater, so there is no refueling and no storage. The trade-off is that a standby unit must be installed with a gas line sized for its draw and an automatic transfer switch, which is why installed prices run $4,000 to $12,000. Diesel generators are common on farms and in workshops for their fuel efficiency and long engine life, but less common in homes because of noise, smell and the cold-start behavior of diesel in winter.

Match the fuel to your reality. If outages are short and rare, a gasoline portable with a transfer switch is the honest value. If you lose power every winter for days, the natural gas standby unit stops being a luxury and starts being the cheapest reliable option you can install.

Generator Size Chart by Wattage Class

Surge ratings in the table assume the common 25% headroom above running watts. Match your calculator's running total to the first row whose running watts clear it, then confirm the row's surge rating clears your worst-case starting number. If the surge forces you up a row, that is the row to buy.

Transfer Switches and Interlock Kits: The Only Safe Ways to Connect

The most dangerous generator mistake is also the most tempting one in an outage: plugging the generator into a wall outlet with a double-ended extension cord. That cord backfeeds the panel and energizes the utility transformer outside, which can electrocute a lineman working to restore your power. It is illegal under the NEC, and it can also start a fire in your own walls.

The two approved solutions both physically separate your house from the grid while the generator runs. A manual transfer switch is a small sub-panel installed next to your main panel. A licensed electrician moves the essential circuits you chose onto it, and when the power fails you flip the generator breaker and start the unit, which feeds only those circuits. An interlock kit is a sliding plate on the main panel that makes it impossible to have the main breaker and the generator breaker closed at the same time. It is cheaper than a transfer switch and lets the generator feed any circuit you switch on, as long as you respect the generator's rating.

A whole-home standby generator skips this step entirely because it comes with an automatic transfer switch that disconnects the house from the grid and starts the engine within about ten seconds of an outage. Whatever route you choose, the installation should be done by an electrician who will pull the permit, because both a transfer switch and a generator feed change how your electrical load connects to the panel.

Portable vs Whole-Home Standby: Pick the Tool for Your Outages

A portable generator, 3,000 to 11,000 watts, is the budget answer. It costs $500 to $3,000, stores in the garage, and rolls out when needed. The work is on you: wheel it outside away from windows, connect it to the transfer switch, fuel it, and repeat every 8 to 12 hours while it runs. It also must stay dry and never run in the garage or basement, where engine exhaust can kill within minutes. For occasional short outages, this is the sensible choice for most homes.

A whole-home standby generator, 13,000 to 24,000 watts, is bolted to a concrete pad outside, wired to your gas line and panel, and tested weekly by its own controller. When the grid drops, it starts itself, transfers the house, and runs on natural gas for days with nobody home. The price reflects that convenience: $4,000 to $12,000 installed, plus a gas line upgrade if your existing line is too small. It earns its keep in climates with long winter outages, for homes with medical equipment, or for anyone who travels and cannot babysit a portable.

There is a legitimate middle tier, the large portable with a wheel kit and a manual transfer switch, that runs the essentials plus a well pump and costs far less than a standby. Match the category to how often the power actually fails where you live, and you will not overpay for automation you rarely use.

5 Generator Sizing Mistakes That Leave You in the Dark

1. Adding running watts and ignoring surge. A fridge and a well pump together may total only 1,700 running watts, well inside a 3,000 watt generator, but when both motors try to start near the same moment the surge can exceed 4,000 watts and stall the unit. Always size against the worst-case starting number.
2. Sizing for the whole panel when you only need essentials. Every circuit you include doubles the generator you buy. A 5,000 watt portable covers a normal essential list, while a whole-home load with central AC and an electric dryer demands a 17,000 to 22,000 watt standby. Decide the scope before you do the math.
3. Assuming the nameplate watts are the whole story. A nameplate lists running draw, and on a motor it may list locked-rotor amps that translate to the surge. Forgetting that the fridge compressor and the AC condenser surge at startup is how people undersize by a full generator class.
4. Forgetting that output drops with altitude and age. Gasoline engines lose roughly 3% of their output per 1,000 feet of elevation, so a generator sized exactly for sea level can stall at 5,000 feet. Buy one class above your calculated need and you absorb altitude, cold fuel and a tired engine.
5. Planning to backfeed through an outlet. A double-ended cord is dangerous, illegal and a common cause of death for line workers. The same dollars spent on a transfer switch or interlock kit make the generator safe to use and legal to install.

Worked Example: Sizing a Backup for a 2,400 Square Foot Home

Take a typical suburban home with a gas furnace, a fridge, a deep freezer, a sump pump and normal lights and electronics. During an outage the family wants the essentials plus a microwave and the internet. The calculator adds the running watts: 400 for lights and electronics, 700 for the fridge, 700 for the freezer, 800 for the furnace blower, 800 for the sump pump and 1,000 for the microwave, for a total of 4,400 running watts.

The largest surge belongs to the refrigerator compressor at 2,200 watts. The worst-case starting moment is that surge plus everything else already running, 2,200 plus the other 3,700 watts, for a starting total of 5,900 watts. A 6,500 watt portable generator clears both numbers with a little margin, so that is the recommendation: a 6,500 watt running unit rated near 8,100 watts of surge, connected through a manual transfer switch.

Now add central air conditioning to the list, the moment most people discover their generator plan changed. The 3 ton condenser adds 3,500 running watts and an 8,000 watt surge, so the total climbs to about 7,900 running watts and the worst-case surge to more than 10,000. That load no longer fits a wheel-out portable, and the honest answer becomes a 13,000 to 15,000 watt whole-home standby, or a soft-start kit on the AC that cuts the condenser surge enough to keep the 6,500 watt portable in the picture. Run your own list through this calculator, and let the surge number, not the running total, make that call for you.