Help figuring AH/Day for my water pump.

rp3703rp3703 Registered Users Posts: 61 ✭✭

Hi Again,

I’m trying to figure out how much AH of battery storage per day I will need to run a Red Lion RL12G05-2W1V submersible pump. I should start by saying that this pump will supply water to a four bedroom cottage, located on a lake in Ontario Canada. I plan to pull water right from the lake at a depth of maybe 10-15’ and the floor level of the house is probably 20-30' above the top of the water, so 30-45' of head height. Now the pump I want to use states that at 40' of head height, it will pump 13 GPM @ 50 PSI. I am thinking about pairing the pump with a 40 gallon pressure tank and a regulator to keep the tank pressure between 30-50 PSI. Even though I have not chosen a specific model of pressure tank, the draw down specs for a Wellmate 40 gallon tank states that it will deliver 12.5 gallons of water between 30/50 PSI. So to me that would mean that the pump would cycle on for 1 min every time the tank drops bellow 30 PSI to bring the tank back up to 50 PSI. 

Using this information, if I figure a shower head will put out around 2 GPM and if each shower lasts 10 min. that would be a total of 20 gallons used for the entire shower and the pump would cut on after the initial 12.5 gallons of the 20 gallon shower have been used. Now, it would make sense to me that if the pump supplies 13GPM that 2 of those gallons would go to the shower and the remaining 11 gallons would go to fill up the tank. So the pump would run around 1.2 minutes for one complete 10 minute shower. If my logic is correct, then at 8 showers a day, The pump will run around 10 minutes a day for all the possible showers that could be taken in a day.

Now to the washing machine. I have not picked out a model yet but I plan on using a newer model front loader so that it plus a dryer can be stacked. According to info I found online, high efficiency washers use 15-30 gallons per load. I’m guessing based on my current washing machine, that there are at most 4 fill cycles per load or at 30 gallons, 7.5 gallons per fill. That would mean that the pump would cut on maybe 2 times per load at around a minute each time. So that’s 2-3 minutes the pump will run per load. At 3 loads a day, that’s 6-9 minutes total.

Other than that, there will be a kitchen sink and vanity. I would guess both of those would run at most an hour combined each day. At 2 GPM per faucet, that’s 120 gallons total. It would be impossible for me to guess how many times the pump will cycle on and off so I’m just going to divide the 120 by the 12.5 gallon draw down which is 9.6 times. I’m sure that’s way high but it’s probably good to figure worst case scenario. So we are talking 10 min of pump time for the kitchen sink and vanity.

So we have 10 + 9 + 10 for a total of 29 minutes the pump will run each day. The pump I chose runs at 10 Amps at 115V or 1150 watts x .5(30 min.) = 575 WH/Day. Using the formula for figuring AH/Day that I got from Bill, 575 x 1.18 = 678.5 x .04(24V)= 27.14 x 2 = 54.28 x 2 = 108.56 AH/Day. 

So my question is, does this sound right? If not, what do you think would be the correct AH/Day? 

Comments

  • EstragonEstragon Registered Users Posts: 1,416 ✭✭✭✭
    I think head for the pump is from the lake surface, not the pump depth plus elevation to cabin. Shouldn't make much difference though. My setup is about the same, though I also have a gravity storage tank up the hill that supplies toilets and lawn service taps. I didn't see toilets listed in you water use - composting toilet or outhouse presumably.

    If possible, I'd recommend more/bigger pressure tank. I have a pair of 30gals which is all I had space for, but would have upped this if I had room. As well as being more likely to last overnight, more storage cuts down on pump cycling which should help with lifespan.

    The ah/day sounds order of magnitude right. Timing of showers etc makes a big difference though. Showers taken while the sun's shining and the batteries are charged don't count.

    I hope you aren't planning on using an electric clothes dryer unless it's as an opportunity load.
    Off-grid.  
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  • rp3703rp3703 Registered Users Posts: 61 ✭✭
    Ha, electric clothes dryer. Funny. No, planning to pick up a gas dryer. Yes we have outhouses at the moment. May add a toilet in the future but the only reason would be to keep from getting annihilated by the mosquitos while peeing after dark. Other than that, the outhouses work fine for now. Good thought on showers during the day. With most of them being taking in the late afternoon, the system should be on float by that point in good sun. 
  • HorseflyHorsefly Registered Users Posts: 250 ✭✭✭
    edited August 13 #4
    We have a similar 40 gal pressure tank at our cabin. When the pressure gets down to about 10psi, the well pump turns on. Our 1/2 HP well pump is about 50 ft below the cabin, and an additional 120 ft or so on the level to the cabin and the tank. It runs for about 2 minutes and 30 seconds each time to get the pressure back up to about 45psi. All the data you gave about how many gallons that might be is stuff I've never computed. I will tell you that a "normal" shower of about 7 minutes (that would be a shower for me, but not my wife  :)) is not enough to cause the pump to run if it is full when the shower starts. Two showers does require a run of the pump.

    That's all just to give you some anecdotal info. Like I said I don't have the stats you are using of how many gallons we use from the tank before it runs again, or what the GPM flow rate for our pump is.
    rp3703 said:

    So we have 10 + 9 + 10 for a total of 29 minutes the pump will run each day. The pump I chose runs at 10 Amps at 115V or 1150 watts x .5(30 min.) = 575 WH/Day. Using the formula for figuring AH/Day that I got from Bill, 575 x 1.18 = 678.5 x .04(24V)= 27.14 x 2 = 54.28 x 2 = 108.56 AH/Day. 

    So my question is, does this sound right? If not, what do you think would be the correct AH/Day? 

    That's a bit of a different way to compute the AH/Day, but I can also make some sense of it.

    Assuming 575WH/Day is correct (you've got some substance to say it is), then to convert that to AH per day you divide it by the voltage of your battery bank, which looks to be 24V.  So, 575WH/Day / 24V = 24AH/Day.

    However, your inverter introduces some inefficiency. A good assumption is 85% efficiency, you would divide 24AH/Day by 0.85, giving you 28.23AH. This is how many amp hours your batteries need to provide to your inverter.  

    Assuming you are using Lead Acid batteries (FLA or AGM), you don't want to run the State of Charge (SoC) below 50% before they get some PV charge again, so you have to size the battery for twice that many AH per day. 28.23AH * 2 = 56.46AH.

    Then you need to consider that you may not get much PV charge now and then. If you size your battery bank so that it can take the load for 2 days before getting recharged (2 "days of autonomy") you would double this again; 2 * 56.46AH = 112.92AH.

    That's kind of close to what you got, but I'm not exactly sure what all went into your equation, since you didn't mention days of autonomy or what efficiency you were assuming for the inverter.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
  • rp3703rp3703 Registered Users Posts: 61 ✭✭
    The formula that I used I got from Bill(BB.) on this forum: WH/Day x Inverter efficiency(85% or 1/0.85 = 1.18)(My inverter's efficiency rating is actually in the 90's but I still use 85% just to make it simple) x 1/Battery Voltage(1/24V = .04) x 2 days of storage x Maximum Discharge(50% or 1/.5 = 2). Either way, both our answers are pretty close and like you said, my 575 WH/Day number is probably way high so I'm just going to figure I need 100AH worst case scenario.
  • jonrjonr Solar Expert Posts: 844 ✭✭✭✭
    I encourage you to make the system more efficient.   For example, carefully select the pump for the expected head, use a lower HP pump to pump slower (less dynamic head), larger pipes (also less dynamic head), use lower pressures, use 1 GPM shower heads, locate the pressure tank near the pump, etc.
  • mike95490mike95490 Solar Expert Posts: 7,249 ✭✭✭✭
    I've heard the pressure tank pressure should be set to 50% of the system pressure switch pressure, the pump has to be able to displace the diaphragm in the pressure tank.
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  • jonrjonr Solar Expert Posts: 844 ✭✭✭✭
    Pressure tank pressure should be set to 2 psi below the lower pressure switch pressure.
  • AguarancherAguarancher Solar Expert Posts: 263 ✭✭✭
    jonr said:
    Pressure tank pressure should be set to 2 psi below the lower pressure switch pressure.
    While that is basically true for a bladder type pressure tank, at or just below cut-in pressure is fine, in my 150gal air-over(captive-air) system it is just the opposite. You set it the pressure at the cut-out pressure. In my case that is 50psi as i have old plumbing and run a 30-50 pressure switch.
  • jonrjonr Solar Expert Posts: 844 ✭✭✭✭
    edited August 16 #10
    I've never heard of setting pressure in a bladder-less tank - only volume based adjustment (ie, completely full of air at cut-in pressure).  But I'm curious how your adjustment process works.  
  • AguarancherAguarancher Solar Expert Posts: 263 ✭✭✭
    edited August 16 #11
    jonr said:
    I've never heard of setting pressure in a bladder-less tank - only volume based adjustment (ie, completely full of air at cut-in pressure).  But I'm curious how your adjustment process works.  

    It’s pretty simple. The tank is vertically mounted, fill with water 2/3 of the way and then open the ball valve on the top of the tank which has a Schrader valve threaded into it, pressurize the tank with an air compressor to your cut-out pressure, close the ball valve and you’re good to go for 4mo. Repeat the process when the air saturates into the water and you notice short cycling of the pump.

    This tank has been in continuous operation for close to 40yrs. The main reason I use it over a bladder type (which is more efficient) is sand. The plumbing attaches 12” from the bottom that feeds out from the tank. It also has a valve attached to the bottom of the tank for draining down the water level in the tank and flushing out built up residue (occasional sand particles), Saves all the water valves in the house, irrigation run and barn from sand and no water filters req.
  • jonrjonr Solar Expert Posts: 844 ✭✭✭✭
    edited August 18 #12
    Interesting.  For anyone using this method, note that the higher pressure water level changes with the hi/low pressure values, elevation and even barometric pressure.  For example, for optimal 30/50 drawdown: (30+14.7)/(50+14.7) = .69 or 69% air, 31% water at the higher pressure.  No calculation or measurement needed if you use the lower pressure:  ~100% air, 0% water.
  • MangasMangas Solar Expert Posts: 537 ✭✭✭✭
    edited August 17 #13
    Thought I'd share our bladder less system.

    We have a bladder less 500 gallon potable water pressure tank plumbed with a small Gast air compressor which runs 12 minutes p/week to maintain the tank's air bladder controlled by an Intermatic programmable digital timer.  The pressure switch tells the underground tanks Grundfos 1/2 hp pumps to start until 60 psi is reached in the pressure tank. We run them simultaneously (about 3 to 5 minutes p/cycle) to shorten pressurization time but the they can also be run independently if we lose one.

    At tee, 4" Hydrant on a ball valve too.

    Ranch Off Grid System: 2 x pair stacked Schneider XW 5548+ Plus inverters (4), 2 x Schneider MPPT 80-600 Charge Controllers, 2 Xanbus AGS Generator Start and Air Extraction System Controllers, 64 Trojan L16 REB 6v 375 AH Flooded Cel Batteries w/Water Miser Caps, 44 x 185 Sharp Solar Panels, Cummins Onan RS20 KW Propane Water Cooled Genset, Rastra House Construction, 2 x High Efficiency Variable Speed three ton Central A/C 220v compressors, 2 x Variable Speed Air Handlers, 2 x HD WiFi HVAC Zoned System Controllers
  • rp3703rp3703 Registered Users Posts: 61 ✭✭

    I appreciate all the input from everyone here but as was stated earlier, a bladder less pressure tank is less efficient than a bladder tank. I would assume efficient refers to the amount of power used to maintain pressure which would be my biggest concern. So Jonr, since you seem to know all the formulas for this stuff. Are my numbers about right for pump cycling? How should I go about making this system more efficient? Any reference material you want to suggest? I used specs from the lowest powered submersible pump Red Lion makes to get my numbers. The reason I chose a submersible is due to the anticipated head height. From what I've read, a regular water pump is not made to push water above around 20' of head height. Besides a pump, pressure tank and regulator are there any other components I should be looking into? I should also say that I plan on pushing this same water through a 3 stage filter and a UV light before it hits the tank. 

  • EstragonEstragon Registered Users Posts: 1,416 ✭✭✭✭
    You'll likely want a stand that keeps the pump off the bottom so it isn't sucking mud etc.

    If you might want to use in winter, there's a guy in Toronto who sells a pressurized drainback type of system, which avoids having to run heat tape.
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  • jonrjonr Solar Expert Posts: 844 ✭✭✭✭
    edited August 18 #16
    Any water pump is fine for pushing water - it's pulling that you have to be concerned about.

    A 24V, 100AH battery bank looks correct to me.

    Put filters after the pressure tank where the flow is typically much slower.  

    I like the idea of a submersible pump and a drainback system if you want to keep the system operational in freezing weather.  Not hard to setup, just a few extra valves and an air vent.
  • rp3703rp3703 Registered Users Posts: 61 ✭✭

    So you think it’s alright to store unfiltered lake water in the pressure tank? Brother in law suggested we get one of these as well http://www.flowsleeve.com

  • EstragonEstragon Registered Users Posts: 1,416 ✭✭✭✭
    The drainback I use also has an air pressure tank. It pressurizes the line so water is forced down below the ice, which would otherwise only drain to the top of the ice and require heat tape from top to bottom of ice layer.

    I use a coarse whole house filter ahead of the pressure tanks, and the tanks still fill in a couple of minutes.
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  • jonrjonr Solar Expert Posts: 844 ✭✭✭✭
    edited August 18 #19
    > water is forced down below the ice

    Nice.

    Here is a picture of a DIY sleeve that won't draw in  mud (but it also needs screens).

    http://www.laketravispumps.com/Images/Lake_Travis_Hudson_Bend___Water_Level_Dropped_Below_Pump_Sleeve_and_Pump_Burned_up_WEB.jpg


    I think it's OK to store lake water in a tank that is only filtered by an intake screen.
  • EstragonEstragon Registered Users Posts: 1,416 ✭✭✭✭
    Looks like that would get the job done. Simple and cheap.
    Off-grid.  
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    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • rp3703rp3703 Registered Users Posts: 61 ✭✭
    Thanks for the info on the drawback system but I won't be anywhere near this place when the water freezes over. With no insulation in the cabin, this is strictly a summer vacation home. Thanks for the pic of the DIY sleeve. Like that it both acts as a cooling sleeve as well as keeps the pump from sucking off the bottom.
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