I’m setting up a backup system for my home’s sump pump. Rather than deal with the installation/maintenance of a separate backup pump I’ve decided to go with an inverter & 12v battery setup. My sump pump claims 9A draw, so I purchased a 2000W inverter & a battery. What I didn’t realize is that the inverter is only for providing constant power, but does not switch between power sources. The inverter has a standard AC outlet on it for plugging in a load. What I think I’m looking for is a relay that switches between two 120V power sources and sends the power to a single load. I’ve tried searching, but I feel like I might be missing the right terms to narrow down to what I’m looking for. Any tips on finding a relay like this? Does such a thing exist?

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    \$\begingroup\$ Instead of inverter, look for Standby Power Supply or Uninterruptible Power Supply... \$\endgroup\$ – Solar Mike Nov 26 '17 at 14:31
  • \$\begingroup\$ Or get a beefy enough charger to charge the batteries and run the pump via the inverter. (in essence, a DIY double conversion UPS) \$\endgroup\$ – ThreePhaseEel Nov 26 '17 at 15:09
  • \$\begingroup\$ This is one case where a bad installation can cause harm to other people by back feeding power to the supply. Because of this some areas demand that installation of a transfer switch (which is what this is) is done by a a licensed electrician. You might want to check your local codes. \$\endgroup\$ – RoyC Nov 26 '17 at 15:29
  • \$\begingroup\$ Forgot to mention it’s an inverter/charger, so it’s going to charge the batter while not in use. \$\endgroup\$ – Joe Fiorini Nov 26 '17 at 17:15


simulate this circuit – Schematic created using CircuitLab

Figure 1. Automatic transfer circuit.

Figure 1 shows one way of doing the job.

  • RLY1 needs to be a contactor (heavy-duty relay).
  • Coil voltage = 120 V or 240 V AC, depending on your supply.
  • Contact rating 10 A inductive or motor start rated at the AC voltage you are switching.
  • Contact arrangement: 2-pole, changeover.

When mains power is lost (as shown in the schematic) power will be supplied from the inverter. When power is restored the relay will energise and power the pump from the mains.

Normal safety precautions apply. Install in box that requires tools or key to open. Gland all cables to prevent chaffing. Add a fuse to cut supply to the coil in the event of a solenoid fault. It may prevent a fire.

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    \$\begingroup\$ I am wondering if the unavoidable phase shift between the mains and the inverter can cause any problems to the pump. \$\endgroup\$ – Vladimir Cravero Nov 26 '17 at 15:13
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    \$\begingroup\$ Very good point and it had completely slipped my mind. I'd be more worried about the inverter on mains failure as switching on to the pump when running 180° out of phase would cause large surge currents. Switching back to mains would have the same problem but the mains should handle it provided the circuit breaker is slow enough. This complicates things! We need some way to prevent inverter startup until the pump stops. \$\endgroup\$ – Transistor Nov 26 '17 at 15:20
  • \$\begingroup\$ A UPS doesn't have that problem does it? \$\endgroup\$ – Solar Mike Nov 26 '17 at 17:27
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    \$\begingroup\$ It shouldn't. A good one would sync with the mains and transfer without much of a bump. \$\endgroup\$ – Transistor Nov 26 '17 at 18:02

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