Electric power (voltage and current) needs to measured in a cost-effective way.
Both AC as well as DC currents can be measured using affordable integrated Hall-Effect sensors such as the ACS712 or similar devices. These integrated devices can be used to measure non-fault currents up to 50A (1.2 kW at 24V) and are quick enough for active current limiting, see ‘DC and Transient Current Capability/Fuse Characteristics of Surface Mount Current Sensor ICs’.
Power is simply the current times the voltages, a calculation which is easily performed by any of your favorite microcontrollers, like Arduino, with built-in analog to digital convertors.
Practically a no-brainer, (DC) voltage can be measured accurately using a voltage divider. For a nice example, refer to a howto on instructables here.