Generator sizing calculator
Buying a backup generator by guessing either leaves the fridge and sump pump offline during an outage or wastes money on a unit far larger than your essential circuit list.
Run — free
Runs in your browser. Free, unlimited — your data never leaves this page.
This generator sizing calculator turns a plain list of appliances—each with continuous running watts and optional starting surge watts—into a deterministic peak load, a recommended wattage with headroom, and an equivalent kVA figure for comparing nameplates. The classic residential method adds every running watt, then adds only the largest motor start extra (starting minus running for the hungriest starter), multiplies by a safety factor (one point two five by default), and ceilings to whole watts so purchase capacity is never understated. Use the free browser tool when planning a transfer switch shortlist, then call the same pure function from scripts when you generate contractor quotes, product filters, or preparedness checklists. Results always match for the same inputs: no network, no randomness, and no model drift between the widget and the paid API path.
How to use it
Enter your values in the form above. The tool checks them before calculating and shows the result on the same page.
Check your inputs
Use the labels and units shown next to each field. If something is missing or outside the allowed range, the page points to the field to fix.
Use it again or automate it
Use the browser tool for individual checks and the API when you need the same capability in an automated workflow.
What you can do with it
Get an answer now
Enter one set of values and see the result without building a spreadsheet or script.
Compare scenarios
Change one value at a time and rerun the calculation to understand what affects the result.
Automate repeated work
Use the API when the same calculation needs to run inside your product or workflow.
FAQ
How do I use this capability?
Complete the fields above and run it on this page. The form highlights anything that needs attention.
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/home/generator-sizing \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"appliances":[{"name":"Refrigerator","running_watts":200,"starting_watts":1200},{"name":"Sumppump","running_watts":800,"starting_watts":1300},{"name":"LED lights","running_watts":150,"starting_watts":150},{"name":"Modem/router","running_watts":30,"starting_watts":30}]}'const res = await fetch("https://api.kit.forhosting.com/home/generator-sizing", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"appliances": [
{
"name": "Refrigerator",
"running_watts": 200,
"starting_watts": 1200
},
{
"name": "Sumppump",
"running_watts": 800,
"starting_watts": 1300
},
{
"name": "LED lights",
"running_watts": 150,
"starting_watts": 150
},
{
"name": "Modem/router",
"running_watts": 30,
"starting_watts": 30
}
]
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/home/generator-sizing",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"appliances": [
{
"name": "Refrigerator",
"running_watts": 200,
"starting_watts": 1200
},
{
"name": "Sumppump",
"running_watts": 800,
"starting_watts": 1300
},
{
"name": "LED lights",
"running_watts": 150,
"starting_watts": 150
},
{
"name": "Modem/router",
"running_watts": 30,
"starting_watts": 30
}
]
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/home/generator-sizing", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"appliances":[{"name":"Refrigerator","running_watts":200,"starting_watts":1200},{"name":"Sumppump","running_watts":800,"starting_watts":1300},{"name":"LED lights","running_watts":150,"starting_watts":150},{"name":"Modem/router","running_watts":30,"starting_watts":30}]}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"appliances":[{"name":"Refrigerator","running_watts":200,"starting_watts":1200},{"name":"Sumppump","running_watts":800,"starting_watts":1300},{"name":"LED lights","running_watts":150,"starting_watts":150},{"name":"Modem/router","running_watts":30,"starting_watts":30}]}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/home/generator-sizing", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"appliances": [
{
"name": "Refrigerator",
"running_watts": 200,
"starting_watts": 1200
},
{
"name": "Sumppump",
"running_watts": 800,
"starting_watts": 1300
},
{
"name": "LED lights",
"running_watts": 150,
"starting_watts": 150
},
{
"name": "Modem/router",
"running_watts": 30,
"starting_watts": 30
}
]
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "home.generator_sizing",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |