Power consumption from current and voltage

Power consumption from current and voltage Work out the power drawn from the current, the voltage and the duty cycle.

Current

Voltage

Duty cycle (0 to 1.0)

Power consumption

—mW

History

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Formula

Power: P = I[A] × V[V] × duty
(duty runs from 0 to 1.0; use 1.0 for continuous operation)

Design notes

Knowing the power drawn is what lets you size the supply, plan for the heat and work out battery life. For anything that runs intermittently under PWM, the average power — duty included — is the number that matters.

Where it comes up:
• PWM dimming of an LED: at duty 0.5 the power halves
• Battery design: back out the run time from the average power
• Deciding on supply wattage and whether a heatsink is needed

When you need this

For sizing a supply, or working out the average power of intermittently active equipment. The duty factor lets you express things like "3mA all the time, 120mA while transmitting 5% of the time" directly.

Using the duty factor

Duty runs from 0 to 1.0 and is the fraction of time that current flows. Continuous is 1.0; 10% of the time is 0.1.

For equipment whose current differs greatly between modes — radios, sensors — calculate each mode separately and sum. Sleep current has a duty close to 1.0, so even a small figure dominates over a year.

Worked example: a LoRa sensor node

3.3V, transmitting for 300ms once every 10 minutes.
Mode	Current	Duty	Average
Sleep	5 µA	0.9995	about 5 µA
Sensor read	8 mA	0.0003	about 2.4 µA
LoRa transmit	120 mA	0.0005	about 60 µA
Total	—	—	about 67 µA

3.3V × 67µA ≈ 0.22mW. On paper a 2000mAh cell lasts over three years. In reality you must add cell self-discharge and regulator quiescent current to the same table. In low-power designs, sleep current and regulator quiescent current almost always dominate.

Sizing the supply

Do not size on the average. Supply capacity follows peak current. An average of 67µA still draws 120mA during transmission. If the supply cannot deliver it the rail collapses and the device resets. The usual answer is local capacitance (see Capacitor sizing).
Do not forget efficiency. This is the load-side power. At 85% DC/DC efficiency, the input draws this divided by 0.85 (see Power supply efficiency).
Calculate per rail. Adding currents across 3.3V, 5V and 1.8V rails is meaningless. Work out power per rail, then sum.
Treat inrush separately. At start-up, charging capacitance draws many times the steady current. Check that supply protection does not trip.

Frequently asked questions

What if I enter a duty above 1?

It just multiplies through and gives a physically meaningless result. Keep it between 0 and 1.0 — if you are thinking in percent, divide by 100 (20% becomes 0.2).

How do I calculate battery life?

Capacity [mAh] ÷ average current [mA] gives hours. But rated capacity applies to a specific discharge condition, and less is available at low temperature or high current. With self-discharge at 1–3% per year, budgeting around 70% of nominal is prudent for long-lived equipment.

Does this work for mains equipment?

It approximates resistive loads with power factor near one, but in general apparent power (V×A) and real power (W) differ. Equipment with a switching supply can have a power factor of 0.5–0.6, so converting current to power requires that factor. Note also that breaker sizing depends on current, not power.

Standards and references

IEC 62301 — Measurement of standby power.

Last updated: 2026-08-29