Wednesday, October 22, 2008

Electrical Calculations Are Shocking

Back a few posts I mentioned helping clients save electricity. See Making Money When Clients Want to Save.

Peace_Country added a comment and wanted detail on the electrical calculations.

Be careful what you ask for.

- - - - -

There are three ways to do this.

1) Ask the building maintenance people to isolate your circuits and put a clamp meter on them. They'll need to do this three times at three different times of day to get a good average.

2) Just look at the monthly electrical bills, take a baseline, and believe that you'll make a real difference. Look at it again after you've made changes and show the client what a difference you've made.

3) Get your own clamp meter at any electrical store and measure every device plugged in at the office, plus lights, air conditioners, and other equipment that is "hard wired."

3.a. You'll need to create a special extension cord for most of these devices. Take a good but short extension cord and isolate one of the outside wires (black or white, not bare or green). A flat cord is best for this. Do not expose any wires, but separate one of them so you can clamp around the one isolated wire to measure total amps.

Google "Clamp meter" for pictures, etc.

In Sacramento, electrical prices vary based on time of year, time of day, and whether there's an emergency due to overusage by the greedy people in Southern California who are the cause of all evil in the universe.

The average, average, average in Sacramento is $0.115 per KWH (kilowatt hour).

3.b. Add up the number of watts being consumed by servers, workstation, etc. Create an excel spreadsheet.

Electric usage is measured in kilowatt-hours.
1 watt-hour is the equivalent of 1 watt of power used for 1 hour.
1 kilowatt-hour is the equivalent of 1000 watts used for 1 hour.

Three values are needed to calculate the cost to use an device:
The power rating or wattage of the appliance. This is found on the appliance e.g. 50W or 1800W.
The time the appliance is switched on in hours (or minutes ÷ 60)
The cost per KWh.

Here's how we do the calculation:
1. Device wattage ÷ 1000 = kilowatts (kW)
2. Kilowatts × (time appliance is switched on in hours) = kilowatt hours (kWh)
3. Kilowatt hours x cost per KWh is what you pay for that appliance to run.

Example: 100W light bulb on for 24 hours where the cost per KWh for electricity = $0.10
(100 ÷ 1000) × 24 = 2.4 KWH used
2.4 kWh × $0.10 = $0.24
This means that it costs 24 cents to use a 100 Watt light bulb for 24 hours.

Computer Example: Desktop that draws 1.5 amps at 120 volts, draws 180 watts. If it is on 24 hrs/day for 31 days, (that is 180 x 24 x 31) then it uses 133,920 watts = 133.920 kilowatt hours.

If you have ten workstations and they each draw about 130 kwhs per month, then the desktop portion of your electrical bill is about 1300 KWH. If your average price is $0.10 per KWH, then the cost is $130 for those ten machines.

You can see where cutting this by 60% saves $52/month.

If you have 50 machines instead of 10, then it's $260/month.

If you also manage to turn off monitors, servers, laser printers, etc. it can really add up.

- - - - -

Additional Considerations

You can also measure electrical usage at power-up (will be higher) and when asleep (better be lower). Many of the early "energy star" machines are about as green as a 1957 Chevy.

We have found that a rack with three older servers can easily use 800-1,000 KWHs per month.

Tiny little split air conditioner for that server room? 700 KWHs.

Note: For every server you virtualize, there's a significant electrical savings.

New desktop PCs with new monitors can be less than one amp total, which is less than 90 KWHs per month.

Older PCs with CRT monitors can draw 2.5 amps, which is more like 225 KWHs per month.

Don't forget old switches, old routers, etc. If it's old, it's inefficient.

Your mileage will vary.

- - - - -


I mentioned earlier that return on investment (ROI) calculations can be very difficult for small businesses. So don't start with the Excel spreadsheet. Start with their electrical bill and make sure it's all for things they can control inside their office. Sit down and agree on the numbers that matter (total kilowatt hours).

Note, please, that many utilities reduce rates in the winter. So if you can start with July, August, and September bills, you may be comparing a different cost per KWH for October, November, December. That's why you need to focus on the total electrical usage.

And don't forget to measure those space heaters!

The advantage of the clamp meter is that you're guaranteed to know exactly how much electricity is used by the machines you control.

Seems like the WORST you can do by turning off machines at night is about $250/month for an office of ten.

Good luck!



  1. Anonymous6:43 PM

    Some great ideas, hopefully you really can bill for it as well (maybe in larger environments).

    Zenith recommends leaving all PC's on everynight so the scripts and whatnot can run. We typically set monitors to turn off and hard drives to spin down, but the PC as a whole really needs to stay on. How do you get around that problem?

    Some HP BIOS's I've seen have auto power on at a certain time--maybe that can be leveraged. Just typing out loud now, but maybe instruct all employees turn off PC's when they leave and then have the BIOS turn them back on a few hours before the office opens to run scripts. Systems would be powered off most of the night and scripts can still run just before people come in. Possible!

    Thanks for any other ideas.

    matt jurcich, invisik corporation

  2. The easiest answer is to use Wake on LAN to wake up the machines when scheduled maintenance needs to take place.

    There are also appliances that will do the job for machines with no WOL capabilties.

    We very often charge for these services, especially with new clients.


Feedback Welcome

Please note, however, that spam will be deleted, as will abusive posts.

Disagreements welcome!