Turn off the power flowing into the circuit.
Disconnect the connectors between the negative terminal of the power supply to the circuit.
Connect the ammeter between the circuit and power source, making sure to connect the negative connector of the ammeter to the negative terminal of the power source and the positive connector to the circuit.
Turn on the power and measure the amps of the circuit.
Turn off the power flowing to the circuit. Never turn on the power while the ohmmeter is connected.
Zero your ohmmeter by setting it to its lowest range and touching the leads together. Turn the calibration knob until the needle is at zero. This will compensate for battery deterioration in the ohmmeter.
Disconnect both ends of the parallel circuit.
Connect the ohmmeter to both ends of the circuit and measure the resistance in ohms.
Use this formula-Power = amps squared divided by resistance-to calculate the watts of the circuit.