A $5 DIY Power Meter

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My first measurements were on a system composed of the following:

P4 System

  • P4-1.6A CPU overclocked to 2125 MHz
  • Gigabyte GA-81RXP motherboard with built-in LAN and sound card
  • 2 Seagate Barracuda IV 40G hard drives
  • 1 CD burner
  • 1 DVD drive
  • 512 meg DDRAM
  • Enermax EG365P-VE(FCA) 350W power supply
  • 1 Panaflo 80mm low power fan at 5V

At idle, the multimeter read 0.49V. This translates directly to 0.49 Amps. I had already measured the voltage across the AC outlets in the house; it reads within 0.5V of 120V. I assumed 119V, taking into account the effect of the 1-Ohm resistor. This is the voltage drop across the PSU. Multiply 0.49A by 119V = 58.31W. This is the power consumed by the system at idle.

At maximum load (achieved by running SiSoft Sandra CPU burn-in while opening and closing both CD drive drawers), the mutimeter read 0.87V steady, and 0.92V momentarily a few times. It is possible that there were peaks which couldn't be registered by the multimeter, but I would think these peaks are short enough not to pose any problem for most PSU. Calculated the numbers: the steady maximum came to 103.5W while peaks reached 109.5W.

The numbers seem amazingly low, don't they? I checked the multimeter's accuracy against a more expensive multimeter and a calibrated test power supply. At this voltage range (0-2V), they correlated within 1%, so there appears to be no instrument error.

Manufacturers rate power supplies by adding up all the power that can be delivered on each voltage rail. However, few ordinary PSUs can deliver full power to all the voltage rails simultaneously. The Enermax EG365P-VE FCA used in this system, for example, is rated for 350W. The fine print shows that the maximum power deliverable on its 3.3V and 5V lines is limited to 185W. The 12V line is rated for 17A, for a maximum of 204W. The total is actually 389W, so the unit is conservatively rated.

The ability to delivery stable current on each voltage line up to its maximum without being affected by activity on other voltage lines is probably the biggest benefit obtained with a higher rated PSU. Instability and unwanted ineractions between different voltage rails is the likely cause of most PSU-related PC misbehavior. This Builder's Guide for Desktop/Tower Systems by AMD provides interesting details about system configuration and power requirements.


The Enermax PSU is rated for ~70% efficiency. This means that 70% of the electrical energy it draws from the AC supply is converted into DC power. What happens to the remaining 30%? It is lost as heat inside the PSU, caused by losses in components, contacts, wiring, etc. So at idle, the PSU in this system generates 17.5W of heat; at maximum, the heat goes up to 32.4W.

The remaining power is delivered to the actual system components: 41W at idle; 77W at maximum. The lion's share of this power is dissipated as heat by the P4, which can hit ~55W maximum. The Barracuda IV hard drive, when working hard, generates up to 12.8W, most of which translates to heat as well. The work that is done consists of spinning motors in drives, motors and other electromechanical devices, and pushing electrons around in circuits. Most of it converts to heat.

This simple power meter will be used as an analysis tool for reviews of power supply units, CPUs, systems and other hardware.

If you decide to build one for yourself, please note the following CAUTION: While modifying an unplugged AC cable is not dangerous, the fact that it is used to carry live AC current makes this device dangerous. Make and/or use this device at your own risk. Your health at risk from electrical shock, and warranties for any equipment with which the modified AC cable is used may be voided.

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Discuss this article in our Forums.

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June 1, 2002 -- Reader Peter Nudin wrote me a thoughtful email about the limitations of this DIY Power Meter:

"I stumbled across your DIY-Powermeter article and found a little problem with it...

"The calculations you made are perfectly fine for DC circuits. This will also work for an AC circuit if your load is a simple resistor (e.g. a lightbulb).As soon as you have a capacitor or a coil in your circuit (and in your PC there's many) things start to go off the rails.

"Why's that?

"Think of the AC voltage as a changing DC voltage (which it is). In a DC circuit nothing changes, so at a given voltage you will have a specific current flowing. When conditions change (e.g. the voltage rises) the current change will depend on the type of components in its way. Resistors just follow, so if the voltage reaches its maximum the current will do the same. Simple.Coils or capacitors however respond with a timeshift, so at any time the current can be higher or lower than you would expect from a resistor.

"The next factor is your multimeter. Most DMMs are made to measure AC voltages only if they have a perfect sine shape. The number displayed in the end is the result of some calculations. Again these calculations go wrong if you have anything else than a sine. (Which is the case here) If you want real numbers you need a DMM that is prepared for the job. It needs to be equipped with a feature called "True RMS". Unfortunately many multimeters on the market today labeled with "True RMS" do not actually have it, but make funny calculations again, so look closely. True RMS DMMs have a tendency to be expensive."

I responded: The question is how much of an error is introduced. Any idea? Also, isn't the effect of the coils and caps buffered somewhat?

Peter answered:

"Typically the real power consumption will be somewhere around 70-80% of the readings that you get. Normally I wouldn't expect the factual power consumption to be higher than your results. The exact effect is hard to tell because of the unknown characteristics of your DMM and also because the behaviour of the power supply will usually change depending on the load. (BTW: The efficiency of your power supply will usually also change depending on the load)

"Your method is OK to get a rough idea of how much energy you're blowing through the chimney - for comparisons I would suggest getting a real powermeter. Maybe you can borrow one somewhere, I would suggest contacting your electricity company or a local environmental agency or initiative."

The good thing, if Peter is right, is that my device probably reads high. To check, I have decided to take his advice: I found an AC power meter called Kill-A-Watt (groan...) for just US$40 and decided that was within my budget. There are some nice reports about this device. Will report how well it works and compare results to my cheap and dirty power meter.

For the electronically savvy, here is a DIY article for a really nice AC power meter at Circuit Cellar: A PIC-Based AC Power Meter by Rick May. The article is also downloadble as a PDF.

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