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  • SPDT - switches between 2 devices, one pole. [1]
  • DPDT - switches between 2 devices, 2 pole [2]. Electromechanical example [3]
  • SPST - 50A UL listed $35 [4]

Mechanical vs Solid State

  • Mechanical, 15A, draws 1W [5]


  • common relay and pinout - [6]

Power Draw

A solid state relay will draw .1W on the driving side, but the load side loses significant power (1V drop). 20W loss when running a 20A load - significant through the day. If solar, that is good, but if we are night-time converting, power loses are not so good at this level.

May want a mechanical relay. Sainsmart relay is 15-20mA at 5v to energize. And no voltage drop on the power side, as it's not going through a transistor like in a solid state relay.

5v-15A 120v relay boxes - [7]


  • Life of a relay is decent. [8]
  • Can be a good long term solution where cycling happens only once or few times per day in off-grid power control automation