Watts Law

WATT'S LAW is a means of calculating the amount of work an electrical appliance is doing. It can be a light bulb, an electric motor or a speaker. The most common item on a automobile measured in Watts is in fact the light bulbs. Bulbs are not sold or ordered by the amount of Amps they use. It is generally the Watts
A marker light will be around 6 to 10 Watts a Headlight around 60 Watts or more. As the Headlight does a lot more work or uses more power than the marker light the amount of Watts used is greater. The formular is very simple it is:- Amps x Volts = Watts So the headlight at 12 Volts using 5 Amps will be producing 60 Watts of power.

Now combining what we know about, Series/Parallel circuits, circuit rules and then using WATTS LAW we can work out the Amperage draw on a circuit with marker/sidelights and headlights. Calculating the sizes of fuse required.

This diagram is a Parallel circuit and each branch is a Parallel circuit. So using Ohms Law the quick way is to add the Watts in each branch and divide by 12 Volts that will show the Amperage in each branch. (Check by figuring the Amp's on each bulb, then use KIRCHOFF'S LAW to do the calculation.)
Note:- Don't add in both the High and Low beam together for the complete Amperage as they are not used at the same time. Use the one with the higher draw.
The marker light branches are 5 Amps each. The Main beam branch is 10 Amps so we add together for a total of 20 Amps for the complete circuit.