# Physical sizes and units

Electrical characteristics of an appliance

Instantaneous data

P: electrical power in watts (W) 1 kW = 1000 W = 100,000 mW

I: current consumed in amperes (A) 1 A = 1000 mA = 100,000 pA

V or U: operating voltage in volts (V) 1V = 1000 mV

P = U x I
(W) = (V) x (A)

Example: For appliance consuming 4 A at 12 V, electrical power P = 48 W.

Integrated data over a period of time

E: Energy consumed over a period of time

E = P x N in watt-hours for a period of N hours (Wh) = (W) x (h)

or for a constant system voltage (for example, 12 V)

E = I x N in ampere-hours for a period of N hours

(Ah) = (A) x (h)

Note: This value in Ah is equivalent to the capacity (of a battery, for example). Example: Energy consumed by a 4 A appliance at 12 V for 5 h:

E = 4A x 5 h = 20 Ah at 12 V or E = 48 W x 5 h = 240 Wh

Note: Divide the duration by 60 if it is expressed in minutes and by 3600 if it is expressed in seconds, to convert it into hours.

p x n

E = ^ in watt-hours for a duration of n minutes

60 Example: Energy consumed by a 4 A appliance at 12 V for 10 min:

Note: The values W/h and A/h are meaningless.

Updated: August 25, 2015 — 3:50 am