The operation of electrical appliances receiving electrical power from an electrical system may be indirectly monitored using monitoring units engaged with outlets on branch circuits of the electrical system. Electrical systems providing power to appliances to be monitored in accordance with the present invention may comprise split phase alternating current systems, tri-phase systems, or any other type of electrical system. Known loads may be applied to calibrate the monitoring system. The monitoring system may measure the power consumption of appliances operating on the electrical system and/or detect possible fault conditions. Fault conditions may be detected by comparing obtained voltage or power signatures of appliances to expected voltage or power signatures. Expected voltage or power signatures may be obtained, for example, using historical data, a database providing typical voltage or power signatures, or through other means.