Question 1

How is "required starting watts" actually calculated?

Accepted Answer

The tool adds up the running (continuous) watts of every appliance you select, then adds the starting/surge watts of only the single largest motor-driven appliance minus its own running watts. This matches the standard rule generator manufacturers publish: totalRunningWatts + max(startingWatts − runningWatts). It assumes you stagger appliance startup — turning one motor on at a time — rather than starting every motor simultaneously.

Question 2

What is the difference between running watts and starting watts?

Accepted Answer

Running (rated) watts is the power an appliance draws once it is already operating. Starting (surge/LRA) watts is a brief spike — often 2-3x the running watts — drawn for a fraction of a second when a motor or compressor first kicks on, as in refrigerators, well pumps, sump pumps, and air conditioners. Purely resistive loads like lights, toasters, and space heaters have no surge, so their starting watts equal their running watts.

Question 3

Why does the calculator only add one appliance's surge, not all of them?

Accepted Answer

In practice you don't start every motor in your house at the exact same instant — you plug in or switch on one appliance, let it stabilize, then start the next. Only the single largest surge needs to be absorbed on top of everything else that is already running steadily. Adding every appliance's full surge together would badly oversize the generator you need to buy.

Question 4

Why is there a safety margin added on top of the required watts?

Accepted Answer

Running a generator at 100% of its rated capacity for extended periods shortens engine life, causes voltage sag, and leaves no headroom if a motor takes slightly longer to start than expected. A 20% safety margin (the default here, adjustable from 0-100%) is a commonly recommended buffer for continuous backup use.

Question 5

Should I size for a portable generator or a standby (whole-house) generator?

Accepted Answer

Portable generators (2000W-10000W) are cheaper and movable but require manual setup, fuel refills, and typically a transfer switch or extension cords to specific circuits. Standby generators (7500W and up) install permanently, run on natural gas or propane, and start automatically during an outage — but cost significantly more. Use this calculator's recommended wattage to shop within either category.

Question 6

Can I add an appliance that isn't in the preset list?

Accepted Answer

Yes — use the "Add a custom appliance" section to enter your own running watts and starting watts (found on the appliance's nameplate or owner's manual) along with a quantity, and it will be included in the total alongside your preset selections.

Question 7

Is this calculator accurate enough to buy a generator with?

Accepted Answer

This calculator uses standard industry wattage figures and the sizing formula recommended by generator manufacturers, and is a solid starting point for comparison shopping. For a permanent whole-house standby installation, have a licensed electrician confirm your home's actual circuit loads and local code requirements before purchase.

Class	Rated (Running) Watts	Surge Watts	Typical Use
2000W Portable/Inverter	2,000W	2,200W	A few lights, phone/laptop chargers, a small fridge — camping/RV or minimal backup
3500W Portable	3,500W	4,000W	Fridge, lights, sump pump, and a small window AC
5000W Portable	5,000W	6,000W	Fridge, well pump, furnace fan, and several household circuits
7500W Portable	7,500W	9,000W	Multiple major appliances at once, or a small central AC
10000W Portable/Standby	10,000W	12,500W	Most of a home’s essential circuits, including a central AC
12000W Standby	12,000W	15,000W	Large homes needing broad circuit coverage plus HVAC
16000W Standby	16,000W	20,000W	Whole-house backup for larger homes with multiple HVAC/well systems
20000W+ Standby	20,000W	25,000W	Whole-house backup for very large homes or homes with multiple large motors

Home Backup Generator Sizing Calculator

Select Your Appliances

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Heating & Cooling

Water & Laundry

Lighting

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Add a Custom Appliance

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Generator Wattage Class Reference

Frequently Asked Questions

How is "required starting watts" actually calculated?

What is the difference between running watts and starting watts?

Why does the calculator only add one appliance's surge, not all of them?

Why is there a safety margin added on top of the required watts?

Should I size for a portable generator or a standby (whole-house) generator?

Can I add an appliance that isn't in the preset list?

Is this calculator accurate enough to buy a generator with?

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