The Power servant,[1] also called energy servant[2] or energy slave,[3] is a unit of input power equal to 100 watts. Input power is the amount of energy required to fulfill a system's needs before thermodynamic inefficiencies are taken into consideration (see megawatts thermal for more information).

Humans eat roughly 2000 food calories a day. A calculation shows that this roughly 100 W (rounding to make a more convenient number):

[math]\frac{2000 Calories}{1 day}\times\frac{1 day}{24 hours \times 60 minutes \times 60 seconds}\times \frac{4184 Joules}{1 Calories}[/math][math]=96.85 \frac{J}{second}=96.85 W \approx 100 W[/math]

This means that 100 W (100 joules/second) can be thought of as the amount of primary energy necessary to feed a person in a day. This energy/time gives a power (power is how fast energy is used) that is more personal than watts. Power servants are almost always used for per capita power consumption.

Another calculation shows that one tonne of oil equivalent per year of power use is approximately 13 power servants:

[math] \frac{1toe}{1 year}\times\frac{1 year}{365 days \times 24 hours \times 60 minutes \times 60 seconds}\times \frac{ 41868000000 J}{1 toe}\times\frac{1servant}{100W}[/math]=13.29 power servants[math]\approx 13[/math] power servants

One barrel of oil/year is 1.94 power servants.

The interactive graph below shows how much primary power per person different countries use. To convert to power servants divide the number on the graph by 1.94 (taking a half is accurate enough).

For further reading

For further information please see the related pages below:


  1. J.M.K.C. Donev noticed in 2015 that the more commonly used energy servant and energy slave were in fact units of power so changed the name to power servant
  2. R. Wolfson, "High-Energy Society," in Energy, Environment and Climate, 2nd ed. New York, U.S.A.: Norton, 2012, pp. 20–21
  3. This term is attributed to R. Buckminster Fuller's cover article of Fortune Magazine 1940.