seedguy10seedguy10 Registered Users, Users Awaiting Email Confirmation Posts: 3
I am trying come with a solar system to power a sprinkler clock. The requirement is 24v, 1.8 amp .
I haven't been able to find a prepacked one so I need help. Any ideas?


  • BB.BB. Super Moderators, Administrators Posts: 30,490 admin
    Welcome to the forum Seedguy,

    Generally, the place to start is to understand how much energy (amps, volts, just running clock, cycling/holding water valves, etc.).

    For example, if you needed 1.8 amps @ 24 volts for 24 hours per day:
    • 1.8 amps * 24 volts * 24 hours per day = 1,037 Watt*Hours per day
    That is a fair amount of energy... My guess is that the timer (if LCD, not LED which would draw more power), would only pull a fraction of that power (24 hour a day load). And if you only run the solenoids are few hours a day (at perhaps 0.25 to 0.50 amps or so), the actual WH/AH per day is much less than would be suggested from my above "dumb" equation with 100% power draw assumption.

    Then the other is how much current the solenoid takes... There are those, full size, that pull current whenever they are "on"... And there are others that are a small solenoid that channels water pressure to the main valve and pull much less current, or possibly only pull current when cycling off to on and on to off.

    Here is an example of a single solar valve that uses almost no energy (solar panel only several inches in size):


    To full blown commercial timers (not cheap):


    And latching valves (only take energy to change state--I think):


    If you have a known sprinkler system and want to compare setting up a solar power system to run them, tell us how much energy per day (Watt*Hours or Amp*Hours @ 24 volts) you need.

    Note--Check the power output from the 24 volt power cube... Is it 24 VDC or 24 VAC -- Many times, running an AC coil on DC can cause them to draw too much current and overheat. Also, the switching controls is usually different between AC vs DC loads.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • seedguy10seedguy10 Registered Users, Users Awaiting Email Confirmation Posts: 3
    Thank you for the reply. The system will be running 4 hours per day max. 
    Here are the specs : https://www.rainbird.com/sites/default/files/media/documents/2018-02/ts_ESP-LXBasic_en.pdf
    Looks like 24v Ac. Maybe it's not even possible:/
  • BB.BB. Super Moderators, Administrators Posts: 30,490 admin
    Just to say there are a lot of pretty cost effective solar powered irrigation systems out there (many are 1 circuit systems--4x may not be cheap, or meet your needs for ease of programming):


    However, I am more than happy to give you an idea of what full sized AC solar power system may look like.

    For a better estimate, get a Kill-a-Watt meter to figure out what the "real loads" are per day:


    First, lets assume worst case of 1,037 WH per day (this will actually give you more than enough power for some LED lighting, cell phone charging, laptop power, etc... -- My guess). So this is a good sized off grid power system for a cabin (no refrigerator).

    First size the battery:
    • 1,037 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery bank * 2 days storage * 0.50 max discharge = 406 AH @ 12 volts
    That is the equivalent of 2x 6 volt @ ~220 AH golf cart batteries in series (12 volts) by 2x parallel = ~440 AH battery bank for a total of 4x $100 golf cart batteries (flooded cell, AGM will be more expensive).

    The first choice you need to make... For base load design (24 hours per day x 7 days per week), I would make the battery bank (and supported load) 2x larger--But lets not bother now--I believe that would be terribly oversized for your needs and make this proposed system way too expensive... To continue

    Next size the solar array to the size of the battery bank... 5% to 13% rate of charge suggested. 5% for weekend/summer usage, 10%+ for full time off grid:
    • 220 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 207 Watt solar array minimum
    • 220 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 414 Watt array nominal
    • 220 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 539 Watt array "cost effective" maximum
    And then there is sizing for the amount of sun you get in a day for your area (and is this 3 or 4 season irrigation system). Say fixed array near St. George Utah:

    Saint George
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 53° angle from vertical:
    (For best year-round performance)


    Assuming worst case (through winter), a December "break even" array (you may need a genset during bad weather to keep battery bank >50% state of charge):
    • 1,037 WH per day * 1/0.52 AC inverter off grid system eff * 1/4.32 hours of sun = 462 Watt array (Dec break even)
    And, in reality, I would be suggesting 2x that amount to reliably supply a 1,037 Watt*Hour per day Base Load 12 months a year (minimal genset usage).
    • 2 * 462 Watt array = 924 Watt array
    Hardware wise, you would need an MPPT controller of:
    • 924 Watt array * 0.77 panel+controller derarings * 1/14.5 volt charging = 49 Amp minimum MPPT controller
    https://www.solar-electric.com/motr60ampmps.html (nice controller, not cheap)

    AC inverter:


    Solar panels (3x in parallel):


    The above is a very capable AC off grid power system, probably 3x larger than you would need (after you measure your irrigation controller energy usage)--But gives you the math behind the design, and some samples of good quality hardware and what questions you have to answer vs my guesstimates.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PNW_StevePNW_Steve Registered Users Posts: 79 ✭✭
    Way back before Moses.... I  used to repair irrigation timers. 

    The vast majority of them were 24v.  The devil is in the details...  They ran on 24 volts AC.  They would not run on DC. 

    The solenoids in the valves were 24vac as well. I  imagine that they may work on DC but may have some heat issues. 
  • seedguy10seedguy10 Registered Users, Users Awaiting Email Confirmation Posts: 3
    Thanks for the info. The spinklers will be running for 4 hours per day max. Here are the specs:

  • BB.BB. Super Moderators, Administrators Posts: 30,490 admin
    Depending on how far you want to go down the Rabbit hole... Say the clock will run on battery (you could even bring out the coin cell connections to a little AA battery holder--Just find the operating voltage for those batteries--Depending on how long they can support the timer without power).

    Use an AC inverter with a timer to only run 4 hours per day... 4x 7 VA (basically 7 Watt solenoids worst case), your energy needs could be down to:
    • 7 Watts * 4 solenoids * 4 hours per day + 6 Watt AC inverter Tare losses * 4 hours per day = 136 WH per day
    • 136 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 2 days "base load" fudge factor 1/12 volt battery bus = 107 AH @ 12 volt battery bank 
    Sizing of battery bank--Suggest 10% rate of charge minimum:
    • 106 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 200 Watt array "nominal"
    And based on loading (December):
    • 136 WH per day * 1/0.52 off grid system eff * 1/4.32 hours of sun (Dec) = 60 Watt (Dec break even)
    • 2 x 60 Watt panel for base load fudge factor = 120 Watt minimum (base on loads)
    • 200 Watt based on battery charging > 120 Watt based on load => 200 Watt array minimum suggested for "reliable system"

    This inverter is "Over Kill" for your needs, but it has a remote 12 volt on/off input (put a 12 volt timer that turns on when you need power), and it also has search mode (might be able to use this instead of timer--Inverter of "off" and only turns on when it sees > 6 Watt load on 12 VDC line--Aka, when timer turns on solenoids, inverter turns on).

    Also, some sprinkler controllers have a remote on/off line to control a well pump... You could possibly connect this to the inverter's remote on/off input--And do without the second timer.

    Not suggesting that this is "the way"--But it is an option that could meet your needs (cost, ease of maintenance, possible other local solar 12 VDC or 120 VAC uses--Like motion detector lighting, security alarm, laptop/tablet/phone charger, etc.).

    -Bill "where there is a will, there is a way" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,754 ✭✭✭✭✭
    I'd tried the battery / solar timers for a orchard, 6 stations.   After several failures, and tree losses, I bit the bullet and dug a 1,000 feet of trench and ran power for a conventional irrigation timer.   5 years and no troubles since. (ok, one valve didn't drain and cold weather froze it)  But no power / timer issues since using AC power.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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