2.4 Power & Energy

Calculate electrical power, total computer power draw, and energy used over time.

Power

Electrical power is the rate of energy transfer. It is represented by P and measured in watts (W). [1, pp. 86-93]

P = V × I

One watt is one joule per second:

1 W = 1 J/s

Power supplies are rated in watts because they must deliver energy quickly enough for all of the connected loads.

Device labels give useful information

Grob uses appliance nameplates as a practical example: if voltage and power are listed, current can be calculated with I = P / V. [1, p. 98]

We can do the same thing with computer hardware and USB devices.

Example

A USB device operates at:

  • 5.00 V
  • 0.900 A

P = V × I

P = 5.00 V × 0.900 A = 4.50 W

The answer has 3 significant figures.

Checkpoint: plug in two USB devices

A computer is already drawing 338 W.

You plug in:

Device Voltage Current
USB A 5.00 V 0.900 A
USB B 5.00 V 1.50 A

Calculate:

  1. the power used by USB A
  2. the power used by USB B
  3. the combined USB power
  4. the new total computer power draw
  5. the final answers using the correct significant-figure or decimal-place rule
Check your work

USB A:

5.00 V × 0.900 A = 4.50 W

USB B:

5.00 V × 1.50 A = 7.50 W

Combined USB power:

4.50 W + 7.50 W = 12.00 W

New computer total:

338 W + 12.00 W = 350 W

To make the intended precision explicit:

3.50 × 10² W

Energy

Power tells us how fast energy is transferred. Energy tells us how much total has been transferred.

E = P × t

For two USB devices drawing 12.00 W for 2.00 h:

E = 12.00 W × 2.00 h = 24.0 Wh

That is also:

86.4 kJ

Power supply vs battery

Keep these ideas separate:

  • a power-supply rating in watts describes a rate of energy delivery
  • a battery capacity in watt-hours describes an amount of stored energy

Inside a computer

Modern electronics repeatedly convert and stabilize power.

Open Circuits shows that multilayer ceramic capacitors are commonly used for local power-supply stability, and it also shows compact DC-DC converter modules that change one DC voltage into another more efficiently than a simple linear regulator. [2, pp. 36, 249]

This gives us a path into later topics:

power -> voltage conversion -> capacitors and inductors -> real motherboard power circuits

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