View Full Version : OT Power Inverters Question

06-23-2008, 05:43 PM
im trying to decide if a small 12v to 110 volt power inverter to run an existing 110v sump pump would be better that just buy a marine belge pump and a float switch that already runs on 12v.
i don't know the power draw on the existing 110v pump but im thinking 3-5 amps max, and i guess that should be the deciding factor.
being without power in a rain storm and needing a sump pump is a real drag.
as you can understand, im trying to get the best amount of time off a deep cycle or marine battery that i can.
do i have enuf information to get an opinion from an electircal wizzard ??
thanks in advance.

06-23-2008, 07:01 PM
Watts (power) = Volts * Amps. 5 Amps @ 110 Volts = 550 Watts.
Amps = Watts / Volts. 550 Watts @ 12 Volts = 45 Amps. (at 100% efficiency).

Adding a 70% efficiency factor for the inverter, about 65 Amps.

Car batteries are between 80 and 100 Amp Hours.

This would run the pump for (80/65 = 1.23) about 1 hour 15 minutes.
In reality it will be less - perhaps a lot less - batteries don't like loads over 1/3 of their rated amp-hour capacity.

Size the batteries accordingly.

The battery backup pumps around here use 2, 6V Golf Cart batteries in series.

J Tiers
06-23-2008, 11:08 PM
As a general rule, figure that you have 12 times the draw at 12V as at 120. I usually figure the watts, and then rough in the battery draw assuming 10V battery..... i.e. 120W is 12A at the battery.

A few seconds of thought suggests that the 120V pump will suck 36 to 60 amps (3 to 5 amps at 120V), which will really suck..

A small 12V pump may do well on as little as 3 or 4 amps...............

06-24-2008, 01:18 AM
Go with the marine pumps; they work really well and are quite reliable.
Use a good battery (like a replacement one for a computer UPS) and battery charger to match.

Do look out for the available lift, though; most marine pumps are optimized for short lifts since most bilges aren't that deep.

06-24-2008, 01:51 AM

What kind of UPS are you talking about. If it is an APC or whatever, 550 VA, you got a couple little sealed lead acids, I think one is 7 AH and the other is 12 AH.

They are not gonna run any kind of pump very long. Well, maybe an aquarium pump.

Hell, look on the box, it will tell you you have 7 to 15 minutes to save your data before the comp and monitor shut down.

Not pushing a computer ( depending on CPU ) mebbe a 200 watt PS would run it.

1/2 HP sump pump would probably draw 7 amps, 840 watts, AC.

being without power in a rain storm and needing a sump pump is a real drag.

I think you should buy a 2 KW gen and plug the sump pump and whatever else it can handle into it. Cheaper and simpler than your idea.



06-24-2008, 10:17 AM
IN a computer UPS, the 12 volt battery unclips.

I saw a nice bank of them under a friends house where he has implemented a emergency lighting system for his home using the compact flourescent lights.

J. R. Williams
06-24-2008, 12:20 PM
A good pump for sumps where a continuous water supply is available, such as city water, is a Water Powered Ejector Pump. The system is turned on and off by a simple float. The battery operated pump requires maintenance of the battery and charging system.


06-25-2008, 03:36 AM
I think the battery powered pump is going to be more efficient, plus you avoid the losses incurred by using an inverter. A very useful thing to know is how many gpm is it going to have to pump to keep up-

The water powered pump sounds interesting. I don't know about the rest of the country, but here I can't recall any time when there was a 'pressure failure'. We're lucky in that it's gravity fed, but there must be many places where water presure relies on electrical power. Obviously the homework on setting up such a sump pump system would not be complete without checking into this in your particular area.

I can see a HSM project here, the triple toilet plunger positive displacement water pressure powered sump pump, with slow flow high area intake pre-filter, of course.