Generator Sizing Calculator
Find what size generator you need. Pick your appliances to get the running and starting watts, a recommended generator size, and an estimated runtime.
Quick Scenarios
Pick a scenario to load typical appliances, then adjust to match your needs.
Select Your Appliances
Kitchen
Heating & Cooling
Water & Sump
Laundry
Power Tools
Lights & Electronics
Wattages are typical estimates — check each appliance's nameplate. Size for about 50–75% continuous load, and consult an electrician for any standby or transfer-switch installation.
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What Size Generator Do I Need?
Picking the right size generator is the difference between keeping your essentials running through an outage and overloading a unit that's too small — or wasting money on one that's too big. The answer depends on what you want to power. This calculator lets you pick your appliances and instantly adds up the running watts, finds the largest starting surge, and recommends a generator size with a safety margin, plus whether a portable or standby unit fits. Select a scenario like Storm backup or Whole house to start, adjust the quantities, and you'll get a clear wattage target and a per-appliance breakdown — no signup, no sales pitch.
How to Size a Generator: Running vs Starting Watts
Every appliance has two power numbers. Running (rated) watts are what it draws continuously; starting (surge) watts are the extra burst it needs for a second or two to start — motor-driven appliances like refrigerators, sump pumps, and AC units can need 2–3× their running watts to start. To size a generator, add up the running watts of everything you'll run at once, then add the single largest starting surge (only one motor typically starts at a time). For example, a fridge (700 running / +1,500 starting), sump pump (1,050 / +1,100), furnace fan (800 / +1,300), and four lights (1,200) total 3,750 running watts; adding the fridge's 1,500 W surge gives a 5,250 W peak. Apply a 25% safety margin and you'd choose about a 7,500 W generator, aiming to run at 50–75% of capacity. Portable generators typically range from 2,000 to 8,500 watts; standby (whole-house) units run from about 10 kW to 26 kW.
Generator Sizing Formula
Generator size = (sum of running watts + the single largest starting surge) × 1.25 safety marginHow to Use This Generator Sizing Calculator
Start with a scenario (Essentials, Storm backup, Whole house) or build your own.
Add the appliances you want to run and set the quantity of each.
Optionally enter your generator's fuel tank size to estimate runtime.
Read the recommended generator size and whether it's portable or standby.
Check the running watts, peak (starting) watts, and the per-appliance breakdown.
Shop for a generator rated at or above the recommended size, and verify appliance nameplates.
Common Use Cases
Storm & Outage Backup
Size a generator to keep the fridge, sump pump, furnace, and lights running during a power cut.
Whole-House Standby
Estimate the kilowatts needed to run central AC and major appliances automatically.
RV & Camping
Find a quiet inverter or portable unit that runs your AC, fridge, and devices.
Job Site Power
Size for power tools like a circular saw and air compressor with their startup surge.
Tailgating & Events
Power lights, a TV, and small appliances without overloading a small generator.
Before You Buy
Compare your load against a generator's rated watts before purchasing.
Why Use a Generator Sizing Calculator?
Avoid Buying Too Small or Too Big
Size to your actual load so the generator starts everything without overloading — and you don't overpay.
Handles Starting Surge
Accounts for the surge watts motors need to start, the step most people miss.
Appliance Picker
Choose from common household appliances and tools with realistic wattages built in.
Portable vs Standby
Tells you whether a portable, inverter, or whole-house standby generator fits your needs.
See the Breakdown
A per-appliance table and chart show exactly what's driving your wattage.
Free and Neutral
No brand funnel or signup — just the number you need to shop confidently.
Frequently Asked Questions
It depends on what you want to run. For essentials (fridge, sump pump, furnace fan, lights, Wi-Fi), a 5,000–7,500 watt portable generator is usually enough. To run a whole house including central air conditioning, you typically need a 10–22 kW standby generator. Add up your appliances above for a tailored number.
Running (rated) watts are the continuous power an appliance needs to keep operating. Starting (surge) watts are the extra power needed for a second or two to start a motor — a fridge or pump can need 2–3× its running watts to start. Your generator must cover the running total plus the largest single surge.
A refrigerator uses about 700 running watts but needs roughly 2,200 watts to start. On its own, a 2,000–2,500 watt generator can run a fridge, but if you're powering other things too, size for the fridge's surge plus everything else's running watts.
Running an entire home — including central AC, well pump, water heater, and major appliances — usually requires a 10 kW to 26 kW standby generator wired to your electrical panel. The exact size depends on your AC tonnage and which large appliances run at once.
Add the running watts of every appliance you'll run simultaneously, then add the single largest starting (surge) wattage among them. Multiply by about 1.25 for a safety margin, and choose the next standard generator size. This calculator does all of that automatically.
Portable generators (about 2,000–8,500 watts) are cheaper and cover essentials, but you set them up and refuel manually. Standby generators (10–26 kW) are permanently installed, run on natural gas or propane, and power your whole house automatically — but cost more and require professional installation.
It depends on the load and tank size. A portable gasoline generator with an 8-gallon tank running a moderate load often lasts 8–18 hours. Higher loads burn fuel faster. Enter your tank size above for a rough estimate; standby units running on a gas line don't run out.
A 2,000–4,000 watt inverter generator runs most RV essentials, including a single air conditioner, fridge, and devices. Larger RVs with two AC units may need 4,000+ watts. Inverter generators are popular for RVs because they're quiet and produce clean power for electronics.