what size battery,timer?

ron17571ron17571 Registered Users Posts: 19 ✭✭
I have a 20 watt panel and a controller,i want to power a pump to water a garden,so im trying to figure out what size battery to get,also i need this to work automatically,any suggestions?ive seen 12 volt boat pumps but i dont know how to hook up a timer to 12 volts,if i use a 110 with an inverter it uses some juice in standby.the pump would work two times a day for like a minute each time.ideas?


  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: what size battery,timer?

    Lets look at it this way, how much/many gals. of water do you need.
    May be easier to get a very small pump and run it all the time as long as the panel has sun. Just like some of the solar water fountains work.
  • ron17571ron17571 Registered Users Posts: 19 ✭✭
    Re: what size battery,timer?

    I just want to water the garden twice a day when im not there.a small pump to spray with mist nozzles.ive read about inverters that draw very little on anouther post,sounds ok for 110.now how much battery can 20 watts keep powered up?
  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: what size battery,timer?
    ron17571 wrote: »
    I just want to water the garden twice a day when im not there.a small pump to spray with mist nozzles.ive read about inverters that draw very little on anouther post,sounds ok for 110.now how much battery can 20 watts keep powered up?

    5 watts is a trickle charge maintainer for a car size battery, so 20W to move water, is not very much. That might pump enough for a 3GPH drip emitter. It all depends on hight of the water source, and the hight of the emitter (ow high the water has to be pumped)
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  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: what size battery,timer?

    i think ken's suggestion of the small solar fountain pump might be the answer unless your garden is more like a mini farm. the fountain pump won't have a large fast flow, but that may not be needed as the smaller flow will add up over time and will probably keep it watered well enough. you may need to figure out the best way to distribute the water though. it would be more of an automatic thing too as it will operate during sunny times and those are the times it'll need it the most. the 20w panel, if it is the same voltage as needed by the pump, would insure a good supply of current in addition to the regular pump's panel which are usually made small.
    if you try the solar fountain pump let us know how well it did and how much of an area it was able to keep watered. in fact anything you try let us know what it was you tried and how well it did. i don't know what your knowledge of gardening is or your soil condition, but lots of compost and possibly some vermiculite mixed well into the soil does help to reduce the need for watering.
  • BB.BB. Super Moderators, Administrators Posts: 32,925 admin
    Re: what size battery,timer?

    In the end, you need to understand your loads... If all you need is a pump to run twice a day for 1 minute a time...

    Assuming 4 hours minimum of sun, and 52% efficiency from solar panel to battery to AC Inverter output:

    4 hours * 20 watt * 52% / (2min/60min per hour) = 1,248 watts for two minutes per day...

    There are some other issues--practically speaking, that is a whole lot of watts from a smallish battery. You may need somewhat large panels a batteries to handle that much "surge" current.

    If, however, you will need more than two minutes per day of electrical power:

    4 hours * 20 watt * 52% = 41.6 watt hours per day of available power...

    A small pump (which can pump against pressure)--is probably going to be at least 70-100 watts (~1/10 hp). And you can run that for maybe 15-20 minutes per day.

    You said you want to do "misting"... A true misting nozzle (if I recall correctly) is going to need a minimum of 20-40 PSI--which requires power to move any volume of water.

    If you want to move water under pressure--then you might look at a Shurflo 12 volt pump like this one.

    You can skip the inverter (save money and a bit of power there)--and the pump drawing 5.6-9.5 amps is around 120 watts maximum.

    Since it is a positive displacement pump, you can pair it up with a pressure switch (and pressure tank--if you want) and it will only run when there is a demand of water (open water valve). And you can use a battery based water timer+irrigation valve to manage the timing and water flow desired.

    In the end--if you have a cistern/water tank mounted high enough above the garden (30+ feet of elevation? Perhaps even less if you just need flood irrigation instead of pressurized)--you would be better off with a solar panel + water pump to cistern and gravity feed the application. Saves battery costs and losses/maintenance. You just have a small pump that raises the water to the cistern when the sun is up--and you draw down water any time of the day or night.

    A system that avoids batteries+inverter will pump almost twice as much water with the same solar panel costs.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SlappySlappy Solar Expert Posts: 251 ✭✭✭✭✭✭
    Re: what size battery,timer?

    Ron....something your might wont to try/check out is the "the redneck timer" ....hunters use it for their automatic feeders, it is strictly 12v, it runs off of the 12v battery and controls a 12v motor for the hopper..... this way your watering system is a complete 12v system instead of a hybird type(12v/110); and the wiring will be simplefied! solar panel--->charge controler--->battery--->12v timer--->12v pump.... but for the battery size is very dependent on pump draw/how many days of cloud cover. That part I can not answer, but I can give ya an idea, to go by(maybe). I have a 12v 7.5 amp/hr, B&D weed wacker and it last right at 30 minutes of wacking weeds. So maybe a 20 amp/hr battery and a small bilge (boat) pump might work. scense it will be used for about 1 minute twice a day, and still have juice left to cover those cloudy days? hope this helps, and its just an "Idea".
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: what size battery,timer?

    it is questionable at this time if he will need or want to have water pumping beyond the sunny periods of time. that will add expense to the project, but would be doable if that dc timer is used and all loads are known along with proper batteries and pvs to supply the power are addressed. you won't know your requirements for any of those things until you choose a pump and when it's to be run.
  • ron17571ron17571 Registered Users Posts: 19 ✭✭
    Re: what size battery,timer?

    I haul water and want to pump from a tank,about twenty feet.i actually was thinking about a bilge pump for a boat.i think it draws like 1.2 amp.direct 12 volt hookup,its the timer thing i need to figure out,and i guess i hadnt figured out the pressure the mister system would need so thats out.i really dont need a high flow,here in the desert i just need to keep the plants from drying out.hence the twice a day watering.so i guess i need to figure out a timer to hook up to 12 volts.
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