An energy meter measures the amount of electrical energy consumed over time using kilowatt-hours. There are two main types: electro-mechanical and electronic. Electro-mechanical meters use a rotating disc to measure usage, but have errors, while electronic meters use digital circuits for more accurate and tamper-resistant readings. Future meters will have remote reading capabilities and allow time-of-day pricing to encourage off-peak usage. Meters are tested using specialized equipment and procedures to check for accuracy and compliance. Tampering methods can be detected by modern meters' sensors and digital components.