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The power triangleHow much current does a 20 watt lamp really draw?

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Power is the rate at which energy is used. It is measured in watts, W, and 1 kW = 1000 W.

P I V
P = watts · I = amps · V = volts

The three forms

P = I × V     watts = amps × volts
I = P ÷ V     amps  = watts ÷ volts
V = P ÷ I     volts = watts ÷ amps

Example 1: current drawn by a 20 W lamp

I = P ÷ V = 20 ÷ 230 = 0.087 A
  → round to 2 decimal places: 0.09 A  (≈ 87 mA)

Example 2: power of a load drawing 16.92 A

P = I × V = 16.92 × 230 = 3891.6 W
                         = 3.892 kW
  • Power is used by the loads, not by the cables. To get total power, add up every load. Three 20 W downlights = 60 W.
  • Work to 2 decimal places. If you round, round properly (0.087 → 0.09). Otherwise use the full number.
Which triangle? Read what the question gives you.
Given volts and ohms or amps, or asked for resistance → Ohm's law (V, I, R).
Given watts, or asked for power → power triangle (P, I, V).

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