We sell Datcon, Stewart Warner, ISSPRO and VDO voltmeters. All voltmeters are electric. They require no sensor or sending unit. If you are buying for an installation in a typical personal car or truck, you probably want a 12V DC voltmeter, which should have a range between 8-10V or 16-20V. For an installation in heavier equipment, you may want a 24V DC or 36V DC voltmeter, with ranges up to 32 or even 50 volts. Our voltmeters are 2" inch in diameter, which is the standard hole size for aftermarket fuel, volt, pressure, amp and temperature gauges (speedometers and tachometers are typically 3 3/8").