Estimating solar requirements to run a water pump

Aragon · August 2014

I get my water from a well and pump it with a 1.5 HP/1150 watt compression pump to the house. I visited a local solar business and asked for an estimate for a backup solar system to run the pump in case of power outages, which have become more frequent in recent years. I was puzzled by how the seller calculated my needs to run the pump and wanted to get some input from you guys.

Although the pump is rated at 1150 watts, I assume that this is the wattage required to actually pump water. There are only two of us and we both work outside the home, so most of the time the pump is in a stand-by mode. Although I do not know how much wattage it uses in stand-by, it is hard for me to believe that it is the same as when it is kicked in and actually pumping water. Nevertheless, the seller has estimated my needs based upon a 1150 watt load 24-hours per day. Even when we both are home all day, I doubt that we pump water to the house more than 2-3 cumulative hours per day.

Can anyone explain to me why the seller is estimating 1150 watt load 24-hours per day?

Thanks!

Cariboocoot · August 2014

Re: Estimating solar requirements to run a water pump

Welcome to the forum.

I think there's some terminology confusion here. Watts is how much power the load uses while it's on. Watt hours is the quantity of power it uses in that time.

Here's where it gets complicated. Technically 1.5 HP is 1119 Watts (746 Watts per HP). What the pump really draws, especially on start-up, is likely quite different than that. If it wasn't measured as 1150 Watts that number is probably inaccurate.

What it uses in 'standby' is likely zero; a pump that is shut off draws no power.

Now about those Watt hours. If the pump does use 1150 Watts and runs for 1 hour per day then it uses 1150 Watt hours. If it runs for 30 minutes a day it uses 575 Watt hours. The difference is that an inverter needs to be sized to supply the running Watts (and start surge) whereas a battery bank has to be sized to supply the Watt hours (and withstand the surge current demand).

If the seller based his design on the idea the pump uses 1150 Watts 24 hours a day then he's a complete idiot.

BTW you are probably better off buying a small generator for back-up purposes than an inverter and battery bank (much less solar to recharge). It only makes sense to go off-grid if the outages are frequent and prolonged. The batteries will die over time whether or not they are used. A generator only needs an occasional run to make sure it's standing ready.

BB. · August 2014

Re: Estimating solar requirements to run a water pump

I am not quite sure I understand what kind of water pump you are using... But if you are using a lot of water and/or plan to spend longer periods without AC Grid power-Then you probably would want to look at pumps that are more "off grid power" friendly.

A typical AC water pump can easily have 5x or more starting surge current--Which takes either a large(r) genset and/or larger inverter+battery bank to support those large surge currents.

Pumps designed for off grid/low power use can have surge currents that are no higher than running current--plus they can use less power to pump water.

There are even pumps that are AC and DC compatible (run from AC mains and AC Backup genset, solar panels, or even a battery bank). These pumps are not cheap.

"Slow pumping", backup well pumps, and even manual pumps (for household water use) could all be options. Using a larger genset and well pump moving water to a cistern (run once a day, or couple times a week) and a small DC or AC pressurization pump + pressure tank for home water use.

And there are other options too... Using Variable Frequency Drives (VFDs) with 3 phase or some compatible single phase motors (usually those with external capacitors).

-Bill

westbranch · August 2014

Re: Estimating solar requirements to run a water pump

that is a pretty big pump... How deep is the static level and how deep is the well?
I assume that is not a variable speed pump that runs constantly.

ILFE · August 2014

Re: Estimating solar requirements to run a water pump

Aragon wrote: »

Can anyone explain to me why the seller is estimating 1150 watt load 24-hours per day?

Probably, because he / she wants to sell you an entire system much larger than what you need?

Or, as coot says, he / she is an idiot.

Aragon · August 2014

Re: Estimating solar requirements to run a water pump

It is pumping from a cistern next to the house. The seller recommended it and we followed his advice. I think that it can handle 5 water outlets simultaneously.

Cariboocoot · August 2014

Re: Estimating solar requirements to run a water pump

Aragon wrote: »

It is pumping from a cistern next to the house. The seller recommended it and we followed his advice. I think that it can handle 5 water outlets simultaneously.

But is this actually what you need?
Frankly I've lived in houses with no larger than 1/2 HP pumps for years and never had a problem with water pressure issues. Managing use can go a long ways to saving money.

westbranch · August 2014

Re: Estimating solar requirements to run a water pump

Likewise, we now have a 3/4 horse submersible pump that pumps from a well that is 300 feet deep into the house . It gives us 5 GPM and will supply 2 garden hoses at the same time at a min 30 max 65 PSI...??

Estimating solar requirements to run a water pump

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