- SPDT - switches between 2 devices, one pole. 
- DPDT - switches between 2 devices, 2 pole . Electromechanical example 
- SPST - 50A UL listed $35 
Mechanical vs Solid State
- Mechanical, 15A, draws 1W 
- common relay and pinout - 
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 - 
- Life of a relay is decent. 
- Can be a good long term solution where cycling happens only once or few times per day in off-grid power control automation