There are a range of systems you can use to monitor your energy use. Some of these just measure ambient conditions (air temperature and humidity for example) while others will measure actual current, volts or kWatts used.
The HOBO Energy Logger has been purpose built to monitor energy usage in buildings, with a range of selectable inputs including temperature, humidity, AC or DC current (True RMS), kW, gauge or differential pressure, volts plus many more.
The ZW Data Nodes allow wireless monitoring of a wide range of parameters critical to the indoor environment and energy efficiency
HOBO UX100, U12 and U14 dataloggers are economical ways to monitor indoor temperature and humidity
HOBO UX90 State loggers monitor when motors are on/off, doors open/closed or lights on/off. These can then be analysed to see if efficiencies could be made
Customised systems can also be built around Campbell Scientific dataloggers and suitable sensors