Volts, amperes, ohms and watts

Calculate a DC load and distinguish electrical input power from RF output power.

Fundamentals2 min readReviewed 15/10/2026

HB3YAQ · Ajdin HadzicLearning to take awayhttps://hb3yaq.ch/en/knowledge/voltage-current-power

WHAT YOU WILL LEARN

Apply Ohm’s law correctly.
Distinguish power from energy.

Three connected quantities

For a resistor, U = R·I. Applying 12 V to 100 Ω therefore produces 0.12 A. Dissipated power is P = U·I = 1.44 W. The same power can be calculated as U²/R or I²R; all approaches must agree for the same inputs.

A radio is not a constant resistor, however. Reception, display, volume and transmission change its current demand. An equivalent load calculated at one operating point therefore does not predict behaviour at another voltage. Use documented operating ranges and measured or specified currents when planning its supply.

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Watts are not watt-hours

Power describes energy transfer per unit time. A constant 10 W load uses 30 Wh in three hours. Capacity in Ah describes charge instead; an energy estimate also needs voltage. Two batteries with the same Ah rating but different voltage do not store the same nominal energy.

RF transmit power is only part of electrical input power. The amplifier, processor, display and other circuits use energy and generate heat. A 5 W RF output therefore does not automatically mean 5 W input or an actual transmit current of 5 W/U.

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Use the calculator within its scope

The Ohm’s law calculator considers constant voltage across a resistance. It suits fundamentals and simple loads. AC reactance, dynamic loads and protective-device sizing need additional information. For a real resistor, also check its rated dissipation and cooling conditions in the data sheet.

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From reading to experimenting

Ohm’s law & power

12 V across 100 Ω gives 0.12 A and 1.44 W. A resistor rated at 0.25 W would be undersized.

Open example with starting values →Voltage: 12 V · Resistance: 100 Ω

Then change exactly one value. Compare the result and use the formula to explain why it changes. The model limits below the calculator also apply here.

TRY IT YOURSELF

Got the idea?

A load draws a constant 2 A at 12 V. How much energy does it use in three hours?

Show answer & explanation

24 W of power and 72 Wh of energy. Charge drawn is 6 Ah.

Sources & further documentation

Keep exploring.

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