I have 1/3 horsepower sump pump currently plugged into my home's 120 volt outlet. I have a 105 amp hour 12-volt battery and a 1200 watt pure sine inverter ready to go when the power goes out.

The only issue is if the power goes out, I would need to be home to physically unplug the sump pump plug from the wall outlet and plug it into the power inverter and also switch the power inverter on which is connected to that 12 volts 105 amp hour battery.

Is there a switch of some sort that I could get to automate the process where the inverter could be switched on and the sump pump automatically get power from the battery without me having to be home to physically move the plug over?

  • 3
    With a mostly-on AC supply, a battery charger, inverter, and automatic switchover, you're describing an Uninterruptible Power Supply. (apc.com/shop/us/en/tools/ups_selector) Note: They don't work well for inductive loads like motors...
    – spuck
    Commented Nov 25, 2019 at 18:06
  • You may want to look into a Venturi-style sump backup. This would be triggered when the well/hole fills to a certain depth. You also won't be dependent on any power source.
    – sirjonsnow
    Commented Nov 26, 2019 at 14:04

6 Answers 6


Another answer by Fresh Codemonger suggests having two pumps, one on regular power and the other on your inverter.

I would like to suggest a similar option that I think is more efficient.

As the other answer says, you should have a second pump. However, instead of connecting it to your inverter, your second pump should be one that is designed to run directly from 12VDC and have a battery charge controller. That way, the pump itself maintains the battery charge and automatically turns on when needed. That removes the need for the inverter, resulting in a simpler and more energy-efficient setup.

There are also a number of sump pump companies that actually make a 2-in-1 pump which has both a 120VAC pump and a 12VDC pump in the same unit. It automatically switches between them as needed.

  • You can even a 3-in-one system: AC for the main pump, battery backup, and water powered secondary backup.
    – JimmyJames
    Commented Nov 25, 2019 at 17:48
  • 2
    Side note: make sure the battery is elevated and all connections are elevated or insulated!
    – Doktor J
    Commented Nov 26, 2019 at 17:15
  • 1
    Boats have a constant need to pump water out, they are called bilge pumps, but are the same as sump pumps. Bilge pumps are normally kept below water level when running. They have upto 4 wires - 2 for power, one to indicate that it's running, and the last to force it on. The only problem may be the head - how high it can pump the water, this may be in 3 to 5 metres range. A good chandlers (boat supply shop) will be able to advise you on the best choice.
    – CSM
    Commented Nov 26, 2019 at 21:26

I think the easier thing to do is to have two sump pump connected. The first on household power and the second on your battery. The first would have a start sensor at a lower spot than the battery sump sensor. This way the battery one only runs if the water reaches above a high spot. This will also give the added benefit that if the household sump pump fails independent of power the battery based one will still take over.

  • 1
    This is how most consumer battery backup pumps work. The water sensor is just slightly above the main one.
    – MadHatter
    Commented Nov 25, 2019 at 1:28
  • 4
    The wonderful thing about this approach is that it covers three different situations: 1) Power failure. 2) Pump failure. 3) Heavy water flow that exceeds pump capacity. If the pump on regular AC fails, then when the water reaches the emergency pump switch, it engages the second pump. What you might want to add to this is some kind of alarm to engage if the second pump is running and regular AC is on, which indicates condition 2 or 3 is in effect. Commented Nov 25, 2019 at 21:56

You don't want to do that.

An inverter powerful enough to start a sump pump will also have high standby losses. It will burn your battery down in short order.

When you're dealing with a 12V system this small, you want all the loads to be DC.

It might be tolerable if you only spun up the inverter for the second per day the pump was actually in operation. However, leaving it spinning 24x7, the inverter will account for 95% of the battery's capacity; very little will actually pump water.

Pumping water is cheap (energywise). Get a 12VDC sump pump that is 100% off when it's off.

One option I have considered is to have a supervisory system (like an Arduino) spin up the inverter on some event (hourly?) then monitor until current draw goes below what a fridge draws, then shut off. shut down the inverter on low current draw. If the fridge or sump pump was needed, they'd run until done, then the inverter would see they've stopped drawing, and shut off.

By the way, you should seriously consider augmenting your battery with a modest sized solar system. A solar panel can keep that battery topped up.


Depending on how handy you are with electrics and/or basic electronics, you could build a relay circuit that switches your pump over to the inverter (and a second relay to connect the battery, to avoid the inverter idle drain sucking your battery dry before it's actually needed).

(I don't yet know how to add diagrams to a post here, else I'd draw up a minimal circuit for that function..)


I accomplished the almost exact setup you are looking for. 1 - 1/6 hp ac sump pump that switches sources when power goes out. I wired up a 2 pole relay that has the sump connected to the common terminals. Normally open contacts connect to the city power and normally closed contacts connect to the inverter. With the coil connected to the normally open contacts(city power), it stays energized until power goes out then switches power sources. I have a smart charger connected to the batteries to maintain the battery life.


Why not use a smart inverter with all this built-in? They charge the battery with city power, then automatically switch to inverter power when city power goes out. The sump pump is plugged directly into the inverter.

  • 1
    Basically, that's what a UPS is.
    – keshlam
    Commented May 9, 2023 at 19:13

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