# battery backup for sump pump?



## ilyaz (Sep 12, 2010)

We are not quite ready to install a generator, so in case we lose power the most important piece of equipment that we might want to keep working is the sump pump. Can I buy a battery backup for it? How do I determine how long this backup will last? In other words, if I have specs for my pump, how do I calculate what type of backup I need for the pump to operate, say, for 24 hours?

Thanks


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## Nestor_Kelebay (Sep 13, 2010)

I think the answer to your question boils down to recognizing that:  1 watt (AC) = 1 watt (DC) = 1 volt X 1 amp

Find out how much amperage your sump pump draws.
Multiply that by 120 volts to get the wattage of your pump.

The amperage rating of your pump is "real" power, but your sump pump get's it's power from an electric motor, which has a high inductive reactance, meaning that a lot of the power just goes into creating magnetic fields (which doesn't actually pump any water).  So, boost the wattage of your pump by a guestimated 35% to account for power factor.

Multiply that by 24 hours to know how many Watt-hours you need to run your pump off batteries.

Multiply that number by two because you don't want to drain your batteries down by more than 50%.

If you're using 12 volt deep cycle batteries, then divide the above number by 12 to get the amp-hours of battery life you need.

Deep cycle batteries are rated according to the number of amp-hours they can deliver at different rates of discharge; typically over 8 hours, 20 hours and 100 hours.  Use the 20 hour rate cuz it's closest to 24 hours.

So, let's say you work out that you need 800 amp hours of battery power, and your 12 volt deep cycle battery delivers 200 amp-hours when discharged over 20 hours.

Then you'd need FOUR 12 volt deep cycle batteries wired in parallel to meet your needs.

And, besides the four batteries (probably costing about $100 each) you also have to buy an inverter to convert the 12 volts DC into 120 volts AC so your pump can use that power.

For $500 (guessing) for the battery back-up system, you could buy a gasoline powered generator to keep the sump pump running.

That is, the cost of a battery back-up system to tie you over until you buy a generator would cost you darn near as much as the generator.

You'd be better off buying the generator.


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## triple D (Sep 13, 2010)

Depending on how much water you need to move, and how fast, you could put a bildge pump in there. Then cut the float switch off and wire to a standard sump float. Use two deep cycle batteries, and go to that one store that has everything, and get a small solar battery charge pad. This can maybe get you through extended outages. Providing the pump runs for 45 seconds, every 20 minutes or so. This is just a thought from my tired head after midnight, good luck..


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## kok328 (Sep 13, 2010)

I don't think you calculations would need to be based on the pump running constantly for 24hrs.  Like Triple D says, 45 secs. every 20 mins.  However, if your not on a well that would require power to deliver water to the home, you could consider installing a water powered sump pump.  These basically harness the power of city water to run and impeller on a unit to evacuate the sump crock.


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