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What size generator do I need?
Add the appliances you want to run during an outage. This tool adds up their running watts, accounts for the biggest startup surge, then tells you the generator size and class to look for.
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Generator sizing calculator
Start with a preset, then adjust quantities or watts to match your home.
| Appliance | Running W | Surge W | Qty |
|---|---|---|---|
Size class
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A portable fitsHow big a generator do you need
To size a generator, add the running watts of everything you want to power at once, add the single largest startup surge among those items, then add about 25% headroom. A typical home backup load lands around 6,500 to 8,000 watts; an RV or storm-essentials setup often fits in 2,000 to 4,000 watts; and a whole house that runs central AC usually exceeds the portable range and points toward a standby unit.
Reference
Generator sizing chart
Two tables do most of the work: typical appliance wattage, and what generator size different households tend to need. The calculator above is built from the same numbers.
Common appliance wattage
Running watts are the steady draw. Surge watts are the brief spike when a motor or compressor starts. Sizes vary by model; treat these as planning estimates.
| Appliance | Running W | Surge W |
|---|---|---|
| Refrigerator or freezer | 700 | 2,200 |
| Furnace blower (gas, 1/2 hp) | 800 | 2,350 |
| Sump pump (1/3 hp) | 800 | 1,500 |
| Well pump (1 hp) | 2,000 | 4,000 |
| Window AC (10,000 BTU) | 1,200 | 1,800 |
| Central AC (3 ton) | 3,500 | 5,000 |
| Microwave | 1,000 | 1,000 |
| Space heater | 1,500 | 1,500 |
| Lights and outlets (general) | 600 | 600 |
| TV, router and devices | 250 | 250 |
| RV air conditioner (13,500 BTU) | 1,500 | 2,800 |
| Power tools (circular saw) | 1,400 | 2,300 |
Household size to generator size
A starting point by how much you want to run. Your real number depends on your appliances, which is what the calculator works out.
| What you run | Recommended size |
|---|---|
| Fridge, lights, phone, router | 2 to 3 kW |
| Essentials plus furnace and sump pump | 3.5 to 5 kW |
| Most of a small or mid-size home | 6.5 to 8 kW |
| Large home, some climate control | 9 to 12 kW |
| Whole house with central AC | 12 kW and up (standby) |
| RV or camper | 2 to 4 kW |
| Jobsite with power tools | 5 to 8 kW |
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The method
How generator sizing actually works
Buying a generator that is too small is the most common and most frustrating mistake. The unit runs fine until a second motor kicks on, then it stalls or trips. Buying one that is far too big wastes money up front and burns extra fuel every hour you run it. Sizing is the step that gets you the right machine, and it comes down to three numbers.
1. Add up running watts
Running watts, sometimes called rated watts, are the steady power a device draws once it is up and going. A refrigerator settling into its cycle might pull around 700 watts. A furnace blower might pull 800. Lights, a router, a TV, and phone chargers together rarely add up to more than a few hundred. Make a list of everything you genuinely need on at the same time during an outage, then add those running watts together. Be honest about "at the same time." You do not microwave dinner while running the well pump and the space heater in the same instant, and assuming you do inflates the number.
2. Add the single largest surge
Anything with an electric motor or a compressor needs a brief jolt of extra power to start. That spike, called surge or starting watts, can be two to three times the running figure for a second or less. The key insight is that your appliances almost never start at the exact same moment, so you do not add up every surge. You add only the single largest one to your running total. If your well pump has the biggest surge, that is the one number you account for. This is the detail cheap online calculators get wrong by stacking every surge, which pushes people toward generators far larger than they need.
3. Add about 25% headroom
Running a generator at full capacity around the clock is hard on it and leaves nothing in reserve for the day you plug in one more thing. A roughly 25% margin keeps the unit in its comfortable range, extends its life, and gives you room to grow. So the working formula is: total running watts, plus the largest single surge, times about 1.25. That product is the minimum generator capacity to shop for.
Reading the result
Once you have a number, it maps cleanly onto the common size classes. Two to three kilowatts covers an RV or a bare-bones storm kit: fridge, a few lights, and your phone. Three and a half to five kilowatts adds a furnace blower and a sump pump, which is enough to keep a house safe and warm through a winter outage. Six and a half to eight kilowatts is the sweet spot for backing up most of a small or mid-size home. Nine to twelve kilowatts handles a larger home with some climate control, and it is roughly where portable generators top out.
When a portable is not enough
If your must-run list includes central air conditioning or electric heat for the whole house, your total will usually climb past the portable ceiling. That is the honest signal to look at a permanently installed standby generator wired to your panel through a transfer switch, rather than forcing a portable to do a job it cannot. There is no shame in that result. It is the difference between a backup plan that works in a real storm and one that disappoints you at the worst moment. The whole point of sizing first is to spend on the machine that actually fits your house, not the one a salesperson or a generic chart happened to suggest.
A worked example
Say you want to keep a refrigerator (700 running, 2,200 surge), a furnace blower (800 running, 2,350 surge), a sump pump (800 running, 1,500 surge), and general lights and devices (850 running). Your running total is 3,150 watts. The largest single surge belongs to the furnace blower at 2,350, but what matters is the surge above its own running draw, so you add the worst-case startup spike to the running total. Taken together and multiplied by 1.25 for headroom, this household lands in the five to six kilowatt range, comfortably inside portable territory. Change one input, swap in central AC, and the same exercise pushes you toward standby. That sensitivity is exactly why a calculator beats a one-size chart.
Use the tool at the top of this page to run your own list. Adjust the quantities and watts to match the models you actually own, and the recommended size updates with them.
Common questions
What size generator do I need for my house?
For most small to mid-size homes that want fridge, furnace, sump pump, lights, and devices during an outage, a 6,500 to 8,000 watt generator covers it. Homes that need central air conditioning usually exceed the portable range and point toward a standby unit. Run your own appliance list through the calculator above for a number tailored to your home.
How many watts does it take to run a house?
It depends entirely on what you run. Essentials alone (fridge, lights, a furnace blower) often sit around 3,000 to 5,000 watts. Add central air or electric heat and a whole house can climb past 12,000 watts, which is standby territory. There is no single answer, which is why summing your own appliances matters.
What is the difference between running watts and surge watts?
Running watts are the steady power a device uses once it is operating. Surge watts are the brief spike, often two to three times higher, that motors and compressors need for a second when they start. You add up all the running watts, but only the single largest surge, because appliances rarely start at the same instant.
Why do I only add the largest surge instead of all of them?
Because your appliances start at different moments, not simultaneously. The generator only needs to survive the worst single startup spike on top of the steady load. Adding every surge together overstates the demand and pushes you toward a generator that is much larger and more expensive than you need.
What size generator do I need for an RV?
Most RVs run comfortably on a 2,000 to 4,000 watt generator. The biggest single draw is usually the air conditioner, which can surge to around 2,800 watts on startup. If you run a 13,500 BTU AC plus a microwave or coffee maker, aim for the upper end of that range.
How much headroom should I leave?
About 25%. Running a generator at its absolute maximum continuously is hard on the engine and leaves no room for the one extra thing you inevitably plug in. A 25% margin keeps the unit in its efficient range and extends its life.
When should I get a standby generator instead of a portable?
When your must-run load includes central air conditioning or whole-house electric heat, your total typically passes the portable ceiling of roughly 12 kilowatts. At that point a permanently installed standby generator, wired to your panel through a transfer switch, is the honest fit. The calculator flags this for you when your load exceeds portable range.
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