Size it right, the first time.

Calculators for the "what size do I actually need?" questions: generator capacity, heater running costs, whole-home heating. Every tool shows its math and tells you when bigger is not better.

Winter ready

Generator Size Calculator

Pick your appliances, get running watts plus the one surge that actually decides the size. Storm-prep presets included.

Run the numbers → Winter ready

Space Heater Cost Calculator

What a 1,500 W heater really costs per hour, day, month, and season, at your own electricity rate.

Price your winter → Winter ready

Heating Cost Comparison

Electric vs gas vs propane vs oil vs heat pump. The same heat, priced five ways per million BTU.

Compare fuels → Live now

BTU / AC Size Calculator

Cooling load from room size, sun, and insulation — in BTU and tons, with the math shown. Built for spring so it's aged by summer.

Size my room →

Winter tip: if you're sizing a generator, only the single biggest motor's starting surge has to fit. Stack every appliance's surge together and you'll talk yourself into a much bigger (and pricier) machine than you need. The generator calculator does this the right way.

Why this site exists

Most sizing calculators are built by stores, and stores earn more when you buy the bigger unit. That's why "add 20% just in case" shows up on every tool online, even when it's wrong. Oversizing a generator wastes fuel and is hard on the engine. Oversizing an AC chills the room without pulling the humidity out, so it ends up cold and clammy at the same time.

So the math is the whole sales pitch here. Each calculator shows the formula behind every number and links its source (DOE, EIA, manufacturer sizing guides), and when "bigger is safer" isn't actually true, the tool comes out and says so. Everything runs in your browser. There's nothing to sign up for and nothing being sold.