Electrical power calculator
12 volts at 2 amperes is 24 watts, and you also see the 6 Ω that pair implies. Enter any two of voltage, current and resistance to get the power and the missing third value. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Voltage V in volts
- 12
- Current I in amperes
- 2
- Resistance
- 6 W
- Power
- 24
- Inputs used
- voltage, current
- Precision
- 10
This electrical power calculator returns the power dissipated or delivered in a simple resistive circuit when you know any two of voltage, current, and resistance. The core identities are P equals V times I, P equals I squared times R, and the equivalent form P equals V squared over R when resistance is non-zero. Ohm's law V equals I times R is used to fill the missing electrical quantity whenever the algebra is defined, so a voltage-and-current pair yields resistance, a current-and-resistance pair yields voltage, and a voltage-and-resistance pair yields current. Classic checks you may already know include twelve volts at two amperes giving twenty-four watts, two amperes through six ohms giving twenty-four watts, and twelve volts across six ohms giving twenty-four watts again. The engine uses only finite arithmetic with no network calls, no sampling, and no date-dependent shortcuts.
What electrical power means and what you get
Electrical power is the rate at which energy is transferred by voltage and current. In a pure resistive model the product of voltage and current is the instantaneous power in watts when voltage is in volts and current is in amperes. Substituting Ohm's law yields the two other textbook forms: power equals current squared times resistance, and power equals voltage squared divided by resistance when resistance is not zero.
This calculator accepts any two of the three quantities and returns power together with the third quantity when it can be derived without division by zero or an infinite open-circuit resistance. Zero current with zero voltage leaves resistance indeterminate and returns null for resistance rather than inventing a value. Zero resistance with non-zero voltage is rejected because P equals V squared over R would divide by zero.
Negative resistance is rejected because ohmic R in these identities is non-negative in this tool.
Where V-I-R power checks fit in study, design, and software
Students meet P equals VI and P equals I squared R when a circuit unit introduces power dissipation in resistors, when a lab asks how many watts a load draws at a measured voltage and current, or when a contest problem gives two of V, I, and R and asks for power without a full SPICE run. Teachers can pre-generate answer keys for modest voltages and currents without trusting opaque calculator apps that silently mix milliamps with amps or switch between peak and RMS without saying so.
Developers call the same contract from scripts to seed deterministic test vectors for power meters, to normalize user-entered load estimates before a bill-of-materials sketch, or to satisfy programming-challenge statements that demand watts rather than a hand-waved estimate. Field technicians and makers use it for quick checks on twelve-volt and twenty-four-volt DC rails, heater elements, and simple DC motor loads when only a multimeter reading and a known resistance are available.
What you can do with it
Homework and lab checks
Compute power from any two of V, I, and R before grading resistor-power worksheets or DC lab write-ups.
Design and maker checks
Estimate load power on low-voltage DC rails when only two of voltage, current, and resistance are known.
FAQ
What does the electrical power calculator cost?
It is free in your browser on this page.
Which formulas does it use?
P = V·I, P = I²·R, and P = V²/R when R ≠ 0. Ohm's law V = I·R derives the missing quantity when defined.
What if I only supply one of V, I, or R?
You must provide at least two of voltage, current, and resistance.
Can I send all three values?
Yes. Power is computed as V·I and checked for consistency with I²·R and Ohm's law; inconsistent triples are rejected.
What units should I use?
Use a consistent set such as volts, amperes, and ohms. Then power is in watts (W). The tool does not convert prefixes for you.
Related tools
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/power-vi \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"voltage":12,"V":1}'const res = await fetch("https://api.kit.forhosting.com/elec/power-vi", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"voltage": 12,
"V": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/power-vi",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"voltage": 12,
"V": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/power-vi", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"voltage":12,"V":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"voltage":12,"V":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/power-vi", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"voltage": 12,
"V": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.power_vi",
"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.
Limits
max_abs_value | 1000000000000000 |
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. |
Technical notes
Fewer than two of the three inputs is rejected with invalid_input, because a single measurement does not determine power without another independent value.
What electrical power means here and what the response contains
The response includes normalized voltage, current, and resistance fields (null when a value cannot be determined), the computed power, an inputs_used list naming which fields you supplied, a method tag such as P equals VI or P equals I squared R, a short formula reminder, the precision used, and a unit hint W for SI-consistent inputs. There is no AC phasor path, no reactive power, and no Monte Carlo sampling: the engine either evaluates the closed forms inside the supported numeric range or rejects the input with invalid_input.
How voltage, current, and resistance are read and how P stays reproducible
Each supplied value must be a finite decimal within the documented magnitude ceiling; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a clear message so a bad cell does not become a wrong wattage. When voltage and current are both present, power is V times I and resistance is derived as V over I when current is non-zero. When current and resistance are both present, power is I squared times R and voltage is I times R. When voltage and resistance are both present, power is V squared over R for non-zero R, and current is V over R. If all three arrive together, the primary power is still V times I, and the engine checks consistency against I squared R and Ohm's law within a tight relative tolerance so contradictory triples do not silently return a misleading wattage. The preferred field for voltage is voltage, but aliases V, v, volts, and potential are accepted so spreadsheets, query strings, and typed forms can reuse the same contract. Current prefers current, with aliases I, i, amps, amperes, and amp. Resistance prefers resistance, with aliases R, r, ohms, and ohm. You must provide at least two of the three fields: a lone voltage, a lone current, or a lone resistance fails with invalid_input and a message that names the two-of-three rule. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose, defaulting to ten and capped at fifteen so JSON goldens stay stable. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline.
Where V-I-R power checks fit in study, design, and software
Product and education tools use the endpoint when a learner types any two electrical values and the UI must show power and the derived third quantity. On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result. Pair it with unit converters or other physics helpers when you need related energy steps, and with math endpoints when you are building a small lab of free, deterministic electrical calculators for study and software tests.
Integration use cases
Deterministic test fixtures
Generate reproducible wattages for unit tests, multimeter demos, and CI assertions on P=VI and P=I²R.