Computer Wattmeter


1. Overview
2. Materials
3. Construction
4. Tests


With an ever-increasing number of users having high-performance computers, one aspect of the computer that is often overlooked is the power consumption. While on the short-term many would not notice this as a crucial factor, on the long term, with just a minute increase in amperage, the electricity bill costs for that tiny extra could add up to a few hundred. Thus, it is crucial that the power consumption of a computer is monitored, but the problem remains that it could not be easily determined. As such, I’ve decided to build my very own DIY wattmeter, which can tell me how much power my computers consumes.

Difficulty: 1 2 3 4 5 6 7 8 9 10

Cost: 1 2 3 4 5 6 7 8 9 10

Time: 1 2 3 4 5 6 7 8 9 10


1. Standard Computer AC Power Cord to cannibalise
2. Multimeter capable of measuring current
3. Penknife and/or wire strippers
4. Electrical tape or anything to join electrical contacts together safely


Begin with the AC Power Cord (duh).

The AC Power Cord was stripped using the pen knife to reveal the 3 inner wires within.

These are the Live, Neutral and Earth. With some basic physics knowledge, you’ll know that the brown wire represents the Live wire, which is what we’re looking for as the current to the computer passes through it.

A wire cutter and stripper was used to cut the Live wire and stripping it to reveal a portion of the wire.

Using a connector, the wires were held down for easy access using the multimeter.

As it is AC current, it does not matter where the black and red probes of the multimeter go to.

When the circuit is closed, there seems to be a small inaccuracy of the multimeter, displaying 0.02A before the power cord is connected to anything. This could be compensated by subtracting this value off the final reading.


The power cord is then connected to the main supply and the test computer itself, and the mains is switched on.

Without the computer switched on, the computer already requires 0.02A (0.04A minus off the 0.02A error reading) by the Power Supply Unit to maintain its memory such as the clock timing and to keep the various LEDs lighted up.

The computer is finally switched on. On average, without any load (no programs running intensively), the computer requires about 0.40A. When programs are running and loading, the computer occasionally takes up about 0.60A.

Assuming this computer if left idle for downloading purposes, we can calculate its power consumption by:

Power = Current x Voltage
As the current required when it is idle is 0.40A (possible error of 0.02A), theoretically, the computer will require 0.40A x 230V = 92W when running idle.


Disclaimer: All experiments and projects presented here are highly dangerous and purely intended for educational and experimental purposes only. Do not attempt them at any rate.