You cannot convert watts to amps, but if you know the voltage, which in this case is 12 volts, you can calculate to total amperage requirement. to do so, you divide the watts by the volts and the answer equals the amps. In this case, the bulbs are 35 watts times 4 = 140 watts. I don't know the watts draw of the motor, so for this calculation I will guess it to be 15 watts. Add the 15 to the 140 = 155 total watts. Divide 155 by 12 = 13 amps rounded up. 2 beacons on the same circuit would equal 26 amps. In this example, you would need a 30 amp fuse to protect the wiring to the beacons, since the fuse needs to have a safety margin of the 4 amps to allow for start up surge. A 30 amp circuit should have a 10 gauge wire going to the fuse to properly handle the amount of current required. Also, keep in mind that the finer the strands of wire, and the more strands of wire inside of the insulation, the better the current will flow in a particular application. This means that the finer stranded wire will work better than a solid piece of wire. Also, the composition of the wire will also have an effect on the flow of current. Aluminum wire doesn't carry the electrons as well as copper wire, and as such, you will need a heavier gauge of wire if you are not using a high quality wire product. Many people don't think of wire quality when they are purchasing wire for an automotive application, and purchase the wire where it is the least expensive. This may or may not be a wise choice, and I will leave that to the purchaser to decide. Another thing to consider is the distance from the power source (battery) to the appliance (beacon). The longer the distance, the larger the wire size that is required. Lucky for us this is only a consideration when we get into very large current draws, such as electromechanical sirens. I would use a 12 gauge wire to each beacon. It is up to you how you are going to power the beacons and if a solenoid or relay is in your best interest. Remember that you need to consider the total amperage draw of everything that is being used also. You need to know that your battery (storage box) has sufficient capacity to keep it all going, and that the alternator is able to replace the amount of amperage equal or close to equal of the total draw of the total package.
Another way to think of this, is that you are filling a bathtub with water from a facet, and the drain is open. Your bathtub represents the battery, and the drain represents the lights and siren. Your alternator is represented in this as the facet. If the water is going out of the bathtub faster than the facet can fill it with water, then you are eventually going to have an empty bathtub... i.e. a DEAD BATTERY, and possibly a DEAD alternator. You can only make the alternator work hard for a certain length of time before it can't continue. Alternators at maximum output get very hot, and tend to burn out...
Another way to think of this, is that you are filling a bathtub with water from a facet, and the drain is open. Your bathtub represents the battery, and the drain represents the lights and siren. Your alternator is represented in this as the facet. If the water is going out of the bathtub faster than the facet can fill it with water, then you are eventually going to have an empty bathtub... i.e. a DEAD BATTERY, and possibly a DEAD alternator. You can only make the alternator work hard for a certain length of time before it can't continue. Alternators at maximum output get very hot, and tend to burn out...
