Watts: volts times amps, in your head

Volts, amps and watts: a garden hose

Volts are the pressure: how hard the water is pushed. Amps are the flow: how much water comes out. And watts are the work the water does.

A little pressure and a little flow barely fills a cup. A lot of both, and you’re washing a car. So to get watts, you multiply pressure times flow: volts times amps.

The formula: P = I × E

On the exam it’s written P = I × E. P is power, in watts. I is current, in amps. E is voltage, in volts. Power equals current times voltage: P, I, E, easy as pie.

So 13.8 volts times 10 amps is 138 watts. And 12 volts at 2.5 amps is 30 watts.

• The wrong choices are what you get from the wrong move: 23.8 watts is adding, 3.8 is subtracting and 0.7 is dividing. Watts are always a multiply.

Need amps? Cover I

Picture the pie as a triangle: P on top, I and E side by side on the bottom. Cover the one you want, and the other two tell you what to do.

Cover I and you see P over E: watts divided by volts. So 120 watts on a 12-volt system takes 120 ÷ 12, which is 10 amps.

Picking a power supply

Radios aren’t perfect. Some of the power going in turns into heat instead of signal, the way a car engine gets warm. So a 50-watt radio needs more than 50 watts going in.

That’s why the right power supply for a typical 50-watt mobile radio is 13.8 volts at 12 amps. Run the pie: 13.8 × 12 is about 165 watts, with room to spare.

• 13.8 volts at 4 amps is only about 55 watts in: not enough once the heat is counted. The 24-volt choices are wrong too, because mobile radios are built for a car’s 13.8 volts.

How long a battery lasts

Think of the battery as a water tank. It’s rated in amp-hours, which is how much it holds. Your radio is the faucet.

Divide the tank by the flow and you get hours: the battery’s amp-hour rating divided by the radio’s average current draw. 20 amp-hours at an average of 2 amps lasts about 10 hours.

• Average is the word to watch. Mostly listening is a trickle, so the tank lasts. Lots of transmitting opens the faucet wide, so it drains faster.

Three things to remember

1. Watts are volts times amps. Easy as pie.
2. Need amps? Divide watts by volts.
3. Battery hours are amp-hours divided by amps.

Every question this covers

6 questions from the 2026–2030 Technician pool, word for word. On the exam the choices may come in a different order.

1. T4A01 Which of the following is an appropriate power supply rating for a typical 50-watt output mobile FM transceiver?

   1. A 24.0 volts at 4 amperes
   2. B 13.8 volts at 4 amperes
   3. C 24.0 volts at 12 amperes
   4. D 13.8 volts at 12 amperes Answer

2. T4A09 How can you determine the length of time that equipment can be powered from a battery?

   1. A Divide the watt-hour rating of the battery by the peak power consumption of the equipment
   2. B Divide the battery ampere-hour rating by the average current draw of the equipment Answer
   3. C Multiply the watts per hour consumed by the equipment by the battery power rating
   4. D Multiply the square of the current rating of the battery by the input resistance of the equipment

3. T5C08 What formula is used to calculate electrical power (P) in a DC circuit?

   1. A P = I x E Answer
   2. B P = E / I
   3. C P = I2 x E
   4. D P = I / E

4. T5C09 How much power is delivered by a voltage of 13.8 volts DC and a current of 10 amperes?

   1. A 138 watts Answer
   2. B 0.7 watts
   3. C 23.8 watts
   4. D 3.8 watts

5. T5C10 How much power is delivered by a voltage of 12 volts DC and a current of 2.5 amperes?

   1. A 4.8 watts
   2. B 30 watts Answer
   3. C 14.5 watts
   4. D 0.208 watts

6. T5C11 How much current is required to deliver 120 watts at a voltage of 12 volts DC?

   1. A 0.1 amperes
   2. B 10 amperes Answer
   3. C 12 amperes
   4. D 132 amperes

Keep going

• Practice every question on HamStudy
• How to get your license, step by step
• A simple emergency go-kit
• Program your first radio

Questions from the NCVEC 2026–2030 Technician question pool, with the errata of February 19, 2026.