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Sump pumps and inverters

Sump pumps are one of the most common backup requirements for Xantrex customers.

In order to help you plan, size and install your Xantrex solution for your sump pump backup requirement, you will probably need to do a little research.

Sizing the inverter

It is true that 1 horsepower equals 748 watts in all the conversion tables; this doesn't mean that a 1000 watt inverter will run a 1 horsepower motor. Motor loads frequently surge 4 - 6 times their rated running load; and the 748 watts is measured at the output shaft of the motor, not taking losses into account.

In our experience, customers should not try to run a one horsepower motor on anything smaller than a 2500 watt inverter, to account for both the surge and running loads.

As a rule of thumb:

1/4 horsepower motor = at least 1000 watts continuous inverter
1/2 horsepower motor = at least 1500 watts continuous inverter
1/2 - 1 horsepower motor = at least 2500 watts continuous inverter

More than 1 horsepower = speak to the manufacturer of the motor and see if you can get the locked rotor amps for the motor (or the wattage of the startup surge load), which will help a technician assist you in sizing the inverter.

Sizing the battery bank

To find out how big a battery bank you need, determine the following:

How many hours, minimum, you want the sump pump to run
The duty cycle of the sump pump (how many minutes per hour the sump pump is running in the worst case scenario)
The rated running load in watts or amps of the sump pump.


You want the sump pump to run for 4 hours.
It will be running 50% of the time - so effectively you need about 2 hours of battery capacity.
The rated running load of the sump pump is 7.1 amps ac.

Amps convert to watts with the formula 120 x #of amps, in this case 120 x 7.1, yielding 852 watts.

The battery calculator is located on the Xantrex website at

Your battery bank will likely be 12v. If you enter 200 amp hours of battery capacity, and enter 852 watts as the load, and click on "Calculate" you find to your surprise that you will not get 2 hours of run time. Doubling the battery amp hour capacity gives you just over 2 hours of run time, or 4 hours of service based on 50% cycle time.

Possible solution:

2500 watt inverter (Xpower 3000, PROwatt 3000)
Xantrex Truecharge 20+ battery charger
Automatic transfer switch
400 amp hour battery bank
Heavy duty cabling, 400A fuse and fuse block

Total cost (retail) between $1500 and $2000, not including shipping, taxes, inspection or installation.

Putting the system together

If you want the system to be automatic - in other words, you don't have to be in the house for it to start working in the event of a power failure - you will need to purchase an inverter/charger, or a standalone inverter with a transfer switch, or purchase the transfer switch separately. You will need to purchase an appropriately sized battery bank, and if you don't get an inverter charger, a battery charger which will charge the batteries when not in use.

Sump pumps do not require true sine wave to start and function properly.

Many sump pump manufacturers market backup power solutions tailored to their products and customers; however, please remember that turnkey solutions are generally more expensive than systems you put together yourself.
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