I am wiring up a circuit of a couple of 20 watt mini bulbs for an architectural project for uni.What amount of battery voltage would you need to power a 20 watt mini bulb (almost the size of an led)?You can't get the voltage from just knowing the watts. You also need to know the current or the resistance of the bulb in its intended operating range. With a variable power source that tells you the voltage, V, and an ammeter to measure curent, I, you could start with a small voltage and increase until VI = 20watt then you would know V.

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