WireGaugeCalc Free calculator · NEC cited · works offline
Home Generator Sizing Calculator
Home generator sizing calculator is simple with WireGaugeCalc, since you list the loads you want to run and it totals the watts.
The tool adds running watts to the largest motor starting watts, so the result reflects how loads actually start.
You end up with a generator size in kilowatts that matches the circuits you plan to back up.
Open the calculatorEnter any two values and read the rest.
Ohm’s law: V = I × R and P = V × I. Field aid only, verify against the current NEC and your local authority having jurisdiction.
Home Standby Generator Calculator That Counts Starting Watts
Motors like a well pump or air conditioner draw a surge at startup, so the calculator separates starting watts from running watts.
It sums all running loads, then adds the single largest starting load, since two big motors rarely start at the same instant.
The total gives a realistic generator rating so you do not buy a unit that stalls under inrush.
Home Generator Size Calculator Inputs
Enter the appliances and circuits you want during an outage:
- Running watts for each appliance
- Starting watts for motors and compressors
- Refrigerator, well pump, and furnace blower
- Air conditioner or heat pump load
- Lighting and outlet circuits
- Total kilowatts for the home standby generator
One app for every NEC calculation
WireGaugeCalc keeps the calculations you reach for most in one place:
- Voltage drop and wire sizing for any run
- Conduit fill for EMT, PVC, and RMC
- Box fill and junction box sizing
- Ampacity and temperature derating
- Motor circuit and load calculations
- Conduit bend offsets and saddles
Built for the field, works offline
The whole app runs on your phone, so it keeps working in a basement, an attic, or a job site with no signal.
There is no account to create and nothing to load. Open it, run the number, and get back to work.
- No signup and no signal needed
- Answers in a tap, not a spreadsheet
- Same tool on phone, tablet, and desktop
Every result cites the NEC article
Each answer shows the table or formula it came from, so you can check the method and learn the code as you go.
That makes the app useful on the job and during exam prep, since the reasoning is right next to the number.
Switch the code year your job runs on
Jurisdictions adopt the NEC at different times, so you can match the calculation to the code in force:
- NEC 2017, 2020, and 2023 tables
- Copper and aluminum conductors
- 60, 75, and 90 degree C terminations
- Single-phase and three-phase systems
Run the number, then get back to work
Stop flipping through a paper book or hopping between calculator sites. Enter your values, read the code-cited answer, and move on. Free to use, no signup.
Open the calculatorFrequently asked questions
How does the home standby generator sizing calculator handle motor startup?
It adds the largest motor starting watts on top of the running total, since a motor pulls several times its running current for a moment. This keeps the generator from dropping voltage when a compressor kicks on.
Should I size for the whole house or only key circuits?
Either works, and the calculator totals only the loads you enter. For a permanent standby unit on a transfer switch, confirm the install and load calc against the current NEC and your local authority having jurisdiction.
Is WireGaugeCalc free to use?
Yes. Every calculator is free to run with no signup. A paid tier adds saved projects, PDF reports, and extra code years, but the core math stays free.
Does it work without internet?
Yes. The app runs on your device, so it keeps working with no signal on a job site, in a basement, or in an attic.
Are the results code accurate?
Results follow published NEC tables and standard formulas, and each answer shows the article it came from. It is a field aid, not a stamp of approval, so verify against the current code and your local authority before you wire anything.
Which NEC code year does it use?
You can switch between NEC 2017, 2020, and 2023, since jurisdictions adopt the code at different times. Pick the year your job runs on.