A colocation provider I am interested in quotes €0.15 per “kWh” for power usage. They also mentioned “1 ampere equals about 166 kWh”. I wondered how they calculated these numbers and how I can calculate power usage in kWh myself.

The first thing you might want (or need) to calculate is how much Watt (W) or Ampere (A) a device (like a server) uses. Power usage can greatly depend on the components used and the load on the server (idle or full). The simple formulas are as follows:

Watt = Volt * Ampere
Ampere = Watt / Volt

So if you have a device that uses 1 Ampere at 230 Volt (default voltage in the Netherlands) then it’ll use 230 Watt. Per hour it will use 230 Watt-hour (Wh).

The k in kWh refers to “kilo” or 1000, so 230 Wh equals 0.23 kWh. And 166 kWh (see quote in first paragraph) is actually 166000 Wh. But where did they get that figure? The answer is that they calculate power usage per month. There are 720 hours in a 30 day month (24 hours * 30 days). So their calculation was (rounded up):

1 Ampere * 230 Volt * 24 hours * 30 days = 166 kWh per month

Anyway, so to calculate the monthly kilowatt-hour (kWh) usage of a device (assuming you already know how much Watt it uses; this can be easily found out using a Wattmeter) the formula is:

kWh per month = Watt * 24 hours * 30 days