Is an energy efficient power supply (80 PLUS) worth it for my computer?

The other day I was customizing a new desktop computer, and one of the first options was about the power supply.

I had to decide between the default option and an energy saving one rated as “up to 88 percent efficient” PSU (power supply units). The energy saving option costs $20 more.

How do you decide if the power savings merit the cost? Here is a bit on how I did the math.

Background: 80 PLUS

Computers consume a lot of power. In the age of energy consciousness, an initiative was announced in 2004 to promote efficiency. The standard is known as 80 PLUS, and it certifies products that have at least 80 percent energy efficiency at specified work loads.

What does 80 percent efficient mean?

Efficiency here refers to how well a computer power supply can turn power that it draws (input power) into power that is used (output power). No power supply is perfectly efficient as some power is lost and converted to heat. An 80 percent efficiency roughly means 20 percent or less is lost to heat at specific work loads.

For instance, a 100-watt power supply running at 80% efficiency when at full load would draw 125 watts from the main power supply and lose 25 watts to heat. In contrast, the scenario at 60% efficiency would need to draw 167 watts from the main, losing 67 watts to heat. Thus it requires more electricity to run and is more expensive.

The question of whether an efficient PSU is worth it depends on how much you can expect in energy savings.

Details you’ll need

To figure out energy savings, you’ll need to know the following details:

1. How much energy your computer uses
2. The gain in efficiency from an 80-PLUS PSU
3. How much energy costs in your area
4. How often you use your computer

Here are some ways you can can estimate these values. Note I am not an expert, so if you’d like to give some references it would be much appreciated!

1. How much energy your computer uses

Computers can vary in how much energy they require.

This article lists some average computer energy usage, the range being 65 watts to 500 watts.

The computer I was looking at was middle of the line, and that would be more like the average of 120 watts–needing a little bit more juice than an incandescent light bulb.

2. The gain in efficiency from 80-PLUS

You can get an idea of the 80-PLUS efficiency ratings in this handy table. The system offers higher medals, like bronze, silver, gold, and platinum, for more efficient ratings.

I am not sure how you can look up efficiency ratings of older models, but that same Wikipedia article lists a range: “Older ATX power supplies were typically 60% to 75% efficient.”

The one thing to note is the ratings are dependent on the computer operating and running at higher work loads. When your computer is idle, the power consumption is going to be about the same. It all depends on the computer usage, but the efficiency gains could range from nearly 0 percent (if your computer is mostly idle) to something like 25 percent (computed as 80%/65%).

As a rule of thumb, going from a standard supply to an 80-PLUS can mean about a 15 percentage point improvement when the computer is running at decent work loads. It could be even more if you are running a top of the line processor and monster graphics card which eat a lot more power, in which case you might be getting a 20 percentage point improvement or more.

3. How much energy costs in your area

You can find this out by looking at your energy bill and seeing how much you are being charged per kilowatt-hour. In my area it is something like 10 cents per KW.

You can also get a baseline estimate by looking at the EIA electricity data by state. Pay attention to the column labeled “Residential.” The U.S. average is about 11 cents, but in Hawaii it was a whopping 29 cents per KW in December 2010.

4. How often you use your computer

This is completely individual. I run my computer most of the day as I take care of virus scans and large downloads (like GPS map updates) during the night. I’ll use an estimate of 12 hours a day of my computer running at a decent work load.

A sample calculation

Here’s my calculation on the energy savings of the efficient power supply

I’ve taken the liberty of converting some units–like cents into dollars and watts into kilowatts–for a proper calculation.

Annual savings = Energy usage (watts / 1,000) x Efficiency gain (%) x Electricity cost ($ per KW hour) x Time used (hours per year)

Annual savings = 120/1000 KW x 0.15 x $0.10/KWh x 12 hours/day x 365 days/year

Annual savings = $7.88 / year

This makes the $20 for the more efficient PSU something of a toss-up: the energy savings recover the cost in about 3 years. That’s good enough for me, but perhaps not everyone. Plus it will pay back sooner if electricity prices rise.

A power user could definitely benefit from a more efficient PSU. Under the same assumptions, if the person literally ran the computer 24/7, then it would be an energy savings of over $15 per year. Or if the computer used 360 watts, then it would be a savings of over $23 per year. At that point, it might be worth considering the higher grades of gold and platinum 80-PLUS.

In conclusion

The math suggests an 80-PLUS certified power supply can be worth the extra cost if you run your computer for a majority of the day.

Of course, do the math in with your specific circumstances to be sure.