It is relatively easy to measure amps to determine the amount of power an appliance or load uses, but the measurement must be done accurately to avoid damaging your multimeter. Multiplying the voltage in a circuit, with the current flowing in the circuit, will give us the total power in the circuit, represented in watts. This is important in determining electricity usage.

Locate the power source in the circuit. With the power flowing in the circuit, adjust the dial on the multimeter to measure voltage (AC or DC). Then touch the red positive probe on the multimeter to the positive terminal on the power source and the black negative probe on the multimeter to the negative terminal on the power source. If you were measuring a car battery, it should read about 12 volts DC.

Measure the current flowing in the circuit. This is done by first disconnecting the power or turning off the circuit, then breaking the circuit and attaching the black lead from the meter to one side of the broken circuit, and the red lead to the other side, allowing the current to flow through the meter. Turn the selector wheel to measure current (amps, AC or DC). Turn the power back on or reconnect the power. Before doing this step, check to ensure your meter can handle the expected amount of current. Hand-held digital multimeters are not capable of handing very much current. If you expect there to be a fair amount of current flowing (more than your meter's maximum capability), get a clamp-on meter that will measure current by the electromagnetic field around the power cable, enabling you to measure very high current flow.

Multiply the measured current by the measured voltage to get your power rating in watts. For example, if you measured a car battery to have 12 volts, and the current flowing at 5 amps, there would be 60 watts of power flowing in the circuit, enough to light a 60-watt light bulb.