Watt-hours from power and time calculator
This watt-hours calculator converts a device's power in watts and its running time in hours into an energy requirement in watt-hours and kilowatt-hours.
Run — free
It is useful when estimating how much energy an appliance consumes, choosing a battery capacity, or checking whether a solar system can cover a planned load. Enter average operating power rather than a peak label value when possible, then use decimal hours for partial hours such as ninety minutes.
Turn power and running time into energy
Power and energy describe related but different quantities. Watts state how quickly a device uses energy at a particular moment, while watt-hours state how much energy is used over a period. This calculator multiplies power in watts by running time in hours: energy in watt-hours equals watts multiplied by hours. A device drawing 60 watts for 8 hours therefore uses 480 watt-hours. The result also includes kilowatt-hours, obtained by dividing watt-hours by 1,000, because electricity bills and larger system plans commonly use that unit. Use decimal hours when the duration is not a whole hour: 30 minutes is 0.5 hours, 90 minutes is 1.5 hours, and 15 minutes is 0.25 hours. Both inputs may be zero, which correctly produces zero energy. The calculation is deterministic and does not add conversion losses, standby demand, startup surges, or changing power by itself. Those factors belong in the power or runtime estimate you provide.
Choose a realistic power value
The quality of the estimate depends mainly on the watt value. A nameplate may show maximum input power, not the average draw during normal operation. For a steady resistive load such as a traditional heater, the rated value can be a useful approximation while the heating element is active. For refrigerators, pumps, air conditioners, computers, and tools, power can change with cycling, workload, temperature, or speed. A plug-in energy meter or a monitored smart outlet can provide a better average. If you only know several operating states, calculate each state separately and add their watt-hour results. For example, treat active use, idle time, and sleep time as individual power-and-duration segments. When planning conservatively, choose a plausible upper average rather than silently treating a brief startup surge as continuous demand. Surge power matters for inverter output rating, but battery energy sizing normally depends on the load's energy across the full operating period. Document the source of your watt figure so the estimate can be revised when better measurements become available.
Apply the result to batteries and solar planning
The calculated watt-hours describe energy at the load before real-system allowances. A battery or solar design usually needs extra capacity because inverters, wiring, charge controllers, temperature, battery aging, and charging processes introduce losses or reduce usable storage. Begin with the calculator's watt-hour result, total the energy for every device, and then apply efficiency and reserve assumptions appropriate to the actual equipment. Do not compare the result directly with a battery's amp-hour rating unless voltage is also considered: watt-hours equal amp-hours multiplied by volts under the stated conditions. A 12-volt battery and a 24-volt battery with the same amp-hour label do not store the same nominal energy. Also respect the battery chemistry's recommended depth of discharge and the manufacturer's usable-capacity specification. For solar planning, daily watt-hours are only the demand side; available sunlight, panel orientation, seasonal weather, controller limits, and system losses determine the generation side. This calculator supplies a clear, auditable starting value, while component ratings and safety margins should come from manufacturer documentation and qualified design guidance.
What you can do with it
Size portable battery energy
Estimate the watt-hours a laptop, light, router, or medical device needs for a planned period away from mains power.
Plan daily solar loads
Calculate each appliance's daily energy demand before totaling loads and applying system-efficiency and weather allowances.
Estimate appliance consumption
Convert a measured average watt draw and typical runtime into watt-hours and kilowatt-hours for comparison or budgeting.
FAQ
What formula does the calculator use?
It uses watt-hours = power in watts × running time in hours. Kilowatt-hours are watt-hours divided by 1,000.
Can I enter minutes?
Convert minutes to decimal hours first by dividing by 60. For example, 45 minutes is 0.75 hours.
Should I use rated power or average power?
Use average operating power when available. Rated maximum power can overstate consumption for devices whose demand varies or cycles.
Is the watt-hour result the battery capacity I should buy?
Not by itself. Account for usable depth of discharge, inverter and wiring losses, temperature, aging, and the reserve required for your application.
Does this calculator include startup surge power?
No. Surge power affects whether an inverter or supply can start a load, while this calculation estimates energy from the average power and runtime entered.
What does an API request cost?
Each API request costs $0.002. The same deterministic calculation can also run in the browser.
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/elec/watt-hours-power-time \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"power_watts":60,"running_hours":8}'const res = await fetch("https://api.kit.forhosting.com/elec/watt-hours-power-time", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"power_watts": 60,
"running_hours": 8
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/watt-hours-power-time",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"power_watts": 60,
"running_hours": 8
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/watt-hours-power-time", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"power_watts":60,"running_hours":8}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"power_watts":60,"running_hours":8}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/watt-hours-power-time", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"power_watts": 60,
"running_hours": 8
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.watt_hours_power_time",
"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. |