Why These Calculators Are Different
Most generator sizing tools you'll find belong to a manufacturer or a dealer network, and their job is to move you toward a specific product line and installer — not necessarily to show you the full picture. Ours don't sell generators.
We Show the Math
Every result comes with the running watts, the largest starting-watt surge, and the safety margin that produced it — not just a single number. You can see exactly why the recommendation landed where it did, and change any input to see how the answer moves.
No Email, No Sign-Up
You get your answer before we ask for anything. There's no gated PDF, no "enter your email to see results," and no dealer contact form required to use any calculator on this site.
Ranges, Not False Precision
Real installed cost depends on local labor rates, site conditions, permitting, and the length of a gas or propane run — factors no online calculator can see. We give you a defensible range and explain what moves it, instead of pretending to know your exact quote.
We're Not a Dealer
GeneratorSizingPro is an independent tool built by Berean Defense L.L.C. We are not owned by, or affiliated with, any generator manufacturer, dealer, or installation company. See our About page for our full editorial approach.
⚡ Generator Wattage Sizing Calculator
Check off exactly what you want to run during an outage. We factor in running watts AND the surge/starting watts that motors need to kick on.
Quick Wattage Estimate
Select your most demanding loads for a fast estimate. For the full 30+ appliance database with quantities, use the dedicated tool below.
🏠 Whole-House Standby Size by Panel
Not sure where to start? Get a fast estimate from your home's electrical service size and central AC.
Whole-House Quick Sizing
A quick-start estimate — the full appliance calculator above gives a more precise number.
⛽ Fuel Runtime & Cost Calculator
See how long your generator will run on a tank and what it actually costs per hour to operate.
Fuel Cost Estimate
Enter your generator size and fuel setup for a runtime and cost estimate.
All Free Generator Calculators
Five tools to help you buy the right generator the first time.
Wattage Sizing Calculator
Check off every appliance you want to run — get exact running and surge wattage, plus a recommended generator size.
Use Calculator →Whole-House Sizing
Size a standby generator from your panel size and central AC — with altitude derating and transfer switch guidance.
Use Calculator →Fuel Runtime & Cost
Estimate runtime on a tank and real operating cost per hour across gasoline, propane, diesel, and natural gas.
Use Calculator →Standby vs. Portable Cost
Compare total cost of ownership — unit, installation, fuel, and maintenance — over 5 to 20 years.
Use Calculator →Installation Cost Estimator
Get an itemized breakdown of equipment, pad, transfer switch, gas line, and labor costs before calling installers.
Use Calculator →Which Calculator Should You Use First?
If you're not sure where to start, use this order — each tool builds on the one before it.
- Start with the Wattage Sizing Calculator if you already know roughly which appliances and systems you want to keep running. It's the most precise of the five tools because you're entering your actual loads rather than a proxy like square footage.
- Use the Whole-House Sizing Calculator instead if you'd rather start from your electrical panel size and central AC tonnage — a faster, rougher estimate that's a reasonable starting point before you get into individual appliances.
- Once you have a size in mind, run it through the Fuel Runtime & Cost Calculator to understand what actually operating it will cost you per hour and per day of an outage, in your fuel type.
- Still deciding between a portable and a standby system? The Standby vs. Portable Calculator compares total cost of ownership — not just the sticker price — over your expected years of ownership.
- Ready to talk to installers? Run the Installation Cost Estimator first so you know what a fair itemized quote should look like before anyone gives you a number.
Every calculator states plainly that its output is an estimate, not an electrical design or a quote — a licensed electrician or qualified installer needs to verify final sizing, fuel supply capacity, panel capacity, code compliance, and site conditions before anything gets installed.
From the Blog
Guides to help you buy, size, and run a backup generator with confidence.
Running Watts vs. Starting Watts: What Generator Sizing Really Means
Why the number on the box isn't the whole story — and how motor surge changes everything.
Read Guide →Do You Need a Transfer Switch? A Complete Guide
What the electrical code actually requires, and why a "suicide cord" can kill a line worker.
Read Guide →Standby vs. Portable Generator: Which Is Right for Your Home?
A honest breakdown of cost, convenience, and maintenance for both paths.
Read Guide →