Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Required switch capacity (amps)
84
Estimated installed cost
$980
Fits your generator's output? (1=yes)
0
Raw sum of circuit ratings
120
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How to use this
- 1Enter circuits to back up.
- 2Enter average circuit amperage (A).
- 3Enter transfer switch type.
- 4Enter generator max output (A).
- 5Read your required switch capacity (amps) on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
A transfer switch is the code-required way to connect a generator to your home's wiring without back-feeding the utility grid (which is illegal and dangerous to line workers). This calculator sizes the transfer switch based on the circuits you want backed up, checks that against your generator's output, and estimates installed cost by switch type — manual transfer switches for portable generators are far cheaper than automatic switches paired with standby generators, but automatic ones start the generator and switch over within seconds of an outage without you doing anything.
Worked example
Using the values the calculator loads with:
Inputs
- Circuits to back up: 8
- Average circuit amperage: 15 A
- Transfer switch type: Manual transfer switch (portable generator)
- Generator max output: 60 A
Results
- Required switch capacity (amps): 84
- Estimated installed cost: $980
- Fits your generator's output? (1=yes): 0
- Raw sum of circuit ratings: 120
What each field means
Inputs
- Circuits to back up
- The circuits to back up used in the calculation. Starts at 8 so you have a working example on load. Accepted range: 1–30.
- Average circuit amperage (A)
- The average circuit amperage used in the calculation, measured in A. Starts at 15 A so you have a working example on load. Accepted range: 15–50 A.
- Transfer switch type
- Pick the option that matches your situation — the maths changes per option. Choices: Manual transfer switch (portable generator), Automatic transfer switch (standby generator).
- Generator max output (A)
- The generator max output used in the calculation, measured in A. Starts at 60 A so you have a working example on load.
Results
- Required switch capacity (amps)
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Estimated installed cost
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Fits your generator's output? (1=yes)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Raw sum of circuit ratings
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why can't I just plug a generator into an outlet?
Back-feeding through a dryer outlet or similar without a transfer switch energizes your home's wiring and can flow back out to the utility line, which can electrocute line workers repairing the outage and is illegal in every US jurisdiction. It also risks damaging your generator or appliances by connecting incompatible circuits. A transfer switch physically isolates your panel from the grid before connecting the generator.
Manual vs automatic transfer switch — which do I need?
A manual switch (paired with a portable generator you wheel out and start yourself) costs a few hundred to a thousand dollars installed and requires you to be home and flip breakers. An automatic switch, paired with a permanently installed standby generator, detects an outage and starts the generator within 10-30 seconds with zero action from you, but the combined system costs $5,000-$15,000+ installed.
What's a 'diversity factor' and why apply it?
Not every backed-up circuit draws its full rated amperage simultaneously, so sizing the switch for the literal sum of every circuit breaker would badly oversize it. A 0.7 diversity factor is a reasonable planning estimate; your electrician's actual load calculation (similar to the whole-home load calculator) gives the precise number for your specific circuit selection.
Can I add circuits to an existing manual switch later?
Only up to the number of slots physically built into that specific switch model — most manual switches come in 6, 10, or 16-circuit versions. If you're likely to add an EV charger or heat pump backup later, size up now since swapping the switch itself later costs nearly as much as installing the right size initially.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Generator Transfer Switch Cost & Circuit Planner. Retrieved from https://www.revenuelab.fyi/toolbox/generator-transfer-switch
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/generator-transfer-switch" target="_blank" rel="noopener">Generator Transfer Switch Cost & Circuit Planner — RevenueLab</a> (2026).</p>
Source: [Generator Transfer Switch Cost & Circuit Planner — RevenueLab](https://www.revenuelab.fyi/toolbox/generator-transfer-switch) (2026).