1. Mathematics of electric power
Circuits
Electric power, like mechanical power, is represented by the letter P in
electrical equations. The term wattage is used colloquially to mean
"electric power in watts."
In direct current resistive circuits, electrical power is calculated using
Watt's Law:
P=V.I
where P is the electric power, V the potential difference, and I the electric
current.
Joule's law can be combined with Ohm's law (V = RI) to produce two
more equations:
P=I2.R
And
P=V2/R
where R is the electric resistance.
So, to calculate loading..:
Unit for Power is Watts or Wattage. Therefore we can say that
Power =Load=Voltage X I (Current)
V, the Potential Difference in this case will be our operating voltage.
In most parts of Asia, the operating Voltage is 220-240V except for
Philippines, Japan and North America, the operating voltage for these
countries is 110V.
I, Current will be the ampere for that particular power point.

2. For example,
To calculate the loading for a single domestic power point,
We have:
230(Voltage) X 13 (Amps) = 2990 Watts
 We use 230V because it is mid range.
 To be more conservative, we can use 220V.
 We can also try to push for max loading using 240V, provided we
know that it is an isolated point. (Means that this point is not used by any
other appliances or eqpt)
We usually factor in a 10% safety range which works out to be:
2990W X 90% = 2691W (app 2700W or 2.7KW)
* We know our eqpt output. Eg, Redhead has an output of 800W, so we
can only connect max 3 Redheads to a single 13A point.
Total load of 3 Redheads is 2400W (800 X3)
Please note that these calculations are only theory, actual situations might
vary.
If we’re using power tapped from HDB void decks, staircase landings or
multi-storey carparks, usually they’re 15A points.
Difference is the socket/plugs. 13A has 3 plugs that are rectangle shaped,
while 15A has plugs that are rounded.
13Amp 15Amp