Select your appliances
Each appliance shows its running watts (what it draws continuously) and starting watts (the brief spike when it turns on). Use the + button to add more than one of the same appliance.
Add an appliance not in the list. Running watts are what it draws continuously; surge watts are the startup spike (leave blank if it has no motor).
Your results
Select appliances to get started
Check the boxes above for everything you want to power during an outage.
How does the calculator size a generator?
Running watts are what an appliance draws continuously. Surge watts are the brief spike when a motor starts. Generac's wattage worksheet puts it at "typically 2X running watts"; the appliance table below spans about 1.2x to 4x. Your generator must handle the total running load plus the surge of the single largest motor, assuming one motor starts at a time, which is the same assumption Generac's own sizing worksheet makes. We add a 20% safety buffer on top.
Rule of thumb: a 2,000 sq ft home running essentials (fridge, lights, furnace fan, well pump, a window AC) typically needs a 5,000–7,500 watt portable. Whole-home coverage with central AC needs a standby generator.
More tools and guides
Explore the rest of the generator toolkit:
- Solar generator & power station size calculator (battery Wh you need for camping, RV & blackouts)
- Fuel & runtime estimator (how long will your generator run on a tank?)
- Generator wattage chart (running & starting watts for home appliances)
- What will a 7,500 watt generator run? (real appliance list, and what won't fit)
- What size generator for a well pump? (1/2 HP and 1 HP, plus the 240 volt catch)
- What size generator for a sump pump? (1/2 HP and 1 HP, and why storms are the worst case)
- What size generator for a fridge and freezer? (plus how long food stays safe)
- Portable vs standby generator (which should you buy?)
Frequently asked questions
What size generator do I need for a 2,000 sq ft home?
For essentials only (fridge, lights, furnace fan, well pump, a window AC), a 5,000 to 7,500 watt portable generator is typically enough. For whole-home coverage that also runs central air conditioning, you generally need a standby generator in the 12,000 to 25,000 watt range. Use the calculator above with your actual appliances for a precise number.
What is the difference between running watts and starting watts?
Running watts are what an appliance draws continuously while it operates. Starting watts (also called surge watts) are the brief spike, usually 2 to 3 times running watts, that a motor needs for a second or two when it first turns on. Your generator must handle the total running load plus the surge of the single largest motor.
How do I calculate what size generator I need?
Add up the running watts of every appliance you want to power, then add the starting surge of the single largest motor. That gives your peak load. Add a 20% safety buffer on top, and round up to the nearest available generator size. This calculator does all of that for you automatically.
Can a portable generator run a whole house?
It depends on the load. A portable generator can run a whole house if you keep the total load under its rated running watts, which usually means running essentials rather than everything at once. For automatic whole-home coverage that handles central AC and large appliances, a professionally installed standby generator is the practical choice.
What size generator do I need to run a refrigerator and a sump pump?
A refrigerator draws about 300 running watts and up to 1,200 starting watts. A 1/2 HP sump pump draws about 800 running watts and up to 2,150 starting watts. Together, with a 20% buffer, you are looking at roughly a 3,500 to 4,000 watt generator to run both safely.
Is a 5,500 watt generator enough for a house?
For essentials, yes. A 5,500 watt generator can run a refrigerator, lights, a furnace fan, a well pump, and a window AC, but not all at once with a central AC or an electric water heater. If you need whole-home coverage with central air, you will need a larger standby unit.
How much does a whole home standby generator cost?
Standby generators typically range from about $3,000 to $6,000 for the unit, plus installation, which can add another $1,500 to $3,000 depending on your setup. The exact cost depends on the size you need and your local electrician's rates.
Disclaimer: This is an estimate for planning purposes only. Actual wattage varies by make, model, and age of your appliances. Always check the nameplate (label) on each appliance for its true running and starting watts, and consult a licensed electrician before installing or connecting a generator. Never run a generator indoors.