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Community and Q&A

Sizing a Well Pump?

RICHARD EVANS | Posted in Mechanicals on

Building a Pretty Good House in NH. Well was just drilled and will have water pressure at 30 gallons/ minute @300 feet.

Well contractor wants to use a 1/2 HP pump and says it will use 5-6 amps. I asked about opting for a 1/3 HP pump (after reading Q&A here on GBA) and thought that was silly.

Will this constitute a major drag on electricity consumption? He said it will cost me $20 a month to operate. (17 cents a watt here in NH).

Should I push for the 1/3 horsepower pump? Or is a 1/2 hp okay? (Will it really be that expensive per month?)

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    It's hard to buy a 1/3-hp submersible pump these days. Most manufacturers don't sell anything smaller than a 1/2-hp pump, although there may be one or two manufacturers that sell 1/3-hp pumps.

    This only matters for off-grid homeowners who need to power their pump with batteries and an inverter. If your house is grid-connected, go with the 1/2-hp pump.

    1. Dustin_7022224 | | #9

      Any advice on how to properly size a pump? In particular, we are looking at a constant pressure pump and are using an open loop geothermal system for radiant heating and cooling.

      The well was just drilled to 65' and reportedly flows 65 gpm. I'm told the geo unit will require 6 gpm. The house is 3 bedrooms, 3 baths, about 2k square feet, under construction, as close to PGH as we can afford.

      The well driller doesn't seem too sophisticated - has offered us an 18 gpm or 25 gpm pump (for a little extra). Will be a Franklin motor w/ Grundfos pump and an FE Subdrive controller with 220v (I believe variable frequency).

  2. Expert Member
    RICHARD EVANS | | #2

    Thank you, as always, Martin!

  3. ranson | | #3


    Check out cycle stop valves. They're a cheap way to make an oversized pump work better with a lightly loaded system. They'll reduce pump cycling (increasing pump life), let you use a much smaller pressure tank, and potentially reduce your energy draw.

    The general idea is that they act as a constant pressure regulator for your well pump, matching flow to demand. If you turn on your shower, the pump will run continuously, rather than cycling on and off. This means that you don't repeatedly draw spikes of current when the motor is turning on, and your pump sees less wear too. You still need a small pressure tank, so the pump doesn't turn on every time you run your sink, but only in the 4-10 gallon range.


  4. GBA Editor
    Martin Holladay | | #4

    Not everyone is a fan of cycle stop valves. For more information, see this earlier Q&A thread: Cycle stop valves.

  5. calum_wilde | | #5


    As you can see I was the one that posted the question about the cycle stop valves previously. I found two things that were damning in my mind. 1) The installation of a cycle stop valve will often require you to install a device around the pump that will help keep it cool while it's running for long periods of time with often very little flow. Some pumps might not require it, and many manufacturers won't tell you if their product does need it. And 2) well pumps don't use much electricity.

    Let's say your pretty good house also has pretty good appliances. I'm going make a bunch of assumptions here, but they should all be in the ball park.

    Washing machine 15gal/load @ 5 loads per week = 45 gal/week
    Dishwasher 5 gal/load @ 7 loads per week = 35 gal/week
    Showers 2.5 gpm @ 8 minutes per shower x 2 people = 280 gal/week
    Toilets 1.6 gal/flush @ 4 flushes per day x 2 people = ~90 gal/week
    Random pot filling, hand washing etc. = 200gal/week
    All told that's about 650 gal/week. Or with 52 weeks in a year, 33,800 gal/year.

    A ½hp pump should be drawing about .373 kW.

    Let's say your pumping losses ends up giving you 7 gal/min flow rate.

    33,800 gal/year / 7 gal/min / 60min/hour = 80.5 hour/year that your pump will run.

    80.5 hour/year x .373kW = 215 kWh/year

    215 kWh/year x $0.17/kWh = $36.70/year or about $3/month.

    Now you can play with the numbers, add things I might have missed like water softener regen cycles, sprinkler systems, lower flow shower heads, etc., but really, the amount of power a well pump uses, if your on grid, isn't worth worrying about. Also, a lower power pump will use less power while it's running, it'll also run longer due to lower flow rate. I highly doubtful a lower flowing pump would end up using less kWh. And after researching the subject, I doubt very much that a cycle stop valve would save any significant amount of power with the possibility of reducing the life of your well pump. A lower flowing pump might also be a dissatisfier if you're trying to fill a soaker tub and it takes half an hour.

    If I were you, I'd go with the pump your chosen plumber recommends. They don't want to be replacing them under warranty so they're going to use a pump they can rely on and one they can source new parts or replacements for quickly when something does go wrong. That to me would matter more than anything else.

  6. Expert Member
    RICHARD EVANS | | #6


    Wow- Thank you so much! That is a big relief!

    John- I will look into these. Thank you for sharing the idea.

  7. SwitchgrassFarmer | | #7

    A 30 gallon a minute well, I wish!

    I run just a slightly shallower submersible pump, 1 HP on a variable frequency drive. Electricity here is $.12 kWh. My usage patterns are very similar to what Calum projected with the exception that often it's just me, so one shower a day. Less than $40 a year electric cost.

  8. ranson | | #8

    A pump sleeve is pretty common for large bore wells, lakes or cisterns. It's cheap piece of PVC, and not a particular downside for installation. Heating in the motor becomes moot. If you have a smaller pressure tank, the CSV will then prevent excess cycling, making it a net win for pump life.

    To be clear, a properly sized (larger) pressure tank is more efficient than a CSV. Period. An undersized pressure tank is a completely different story. A CSV has its place when you need to optimize for tank size, occupant comfort (pressure cycling), pump cycling, and price.

  9. this_page_left_blank | | #10

    The power of the pump will have negligible effect on operating cost. A higher power pump will be pumping more water. If anything, the higher power pump will use less electricity because a larger portion of the total power goes to water delivery and less to overcoming head pressure. Just size the pump based on the gpm you want and the parameters of the well.

  10. joshdurston | | #11

    I've used the grundfos SQE series of constant pressure well pumps in the past. Good results and nice and efficient. You can eliminate the large pressure tank (you use a small 2gallon one instead).

    The cycle stop valves may reduce instantaneous electricity usage but you will use more kwh per month, since the effectively make the pump work longer (it will be pumping less efficiently even though it using less power). You are simply trading flow for head. With lower flow rates the amperage goes down, but since the flow rate went down, your gpm per amp, or kwh per gallon will be worse.
    The only reason to consider one is that you have a mismatched tank and pump that is causing extreme short cycling.

  11. Jon_R | | #12

    Within reasonable limits, the pump curve is more important to efficiency than the pump HP. Eg, don't use a pump that is rated for 400' with a 100' well.

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