# Solar panel for irrigation?

Hey guys

I am not sure if I am doing this correct.
My total device consumption is 8 Watt (even less because my solenoid will work for 10 min).
I am thinking to buy 12 V 20 watt solar panel and lead acid battery 12v 7ah.

Here is my calculations
12v*7ah= 84 watt
My device = 8watt

So I will get 10 hours 84/8

Is my calculation correct ? Will I be getting more hours during sunlight times?

12v*7ah= 84 watt

watt hours.

You should not assume the battery will supply the total charge and should never discharge the battery below 10 V. Normally it is best to buy a battery with twice the estimated required capacity.

You need a charge controller for the solar panel.

Yes, manufacturer's tend quote the best capacity is it possible to achieve (new battery, at the right
discharge rate, at the right temperature, over-charged). Allow for the capacity to fall substantially over
the life of the battery.

Over-discharge is a very easy way to ruin the battery, so protect against it. If you can find a latching
solenoid valve that would be much less consumption of course, but I suspect they are not easy to
source.

Allow for 5 days of low solar to recharge and max discharge of 50%.
This means a battery of about 10 times your daily use. Select a panel that will recharge in a couple of days including having enough to run your load.

The greater the death of discharge, the shorter the life of most batteries.

Weedpharma

Was thinking how am I able to select solar charge controller? On the basis of battery or solar panel?

Regards Saif

This depends on the battery type chosen.

Weedpharma

And the solar panel. Check the charge controller data sheets.

Consider using a latching solenoid, that way you only need to provide a burst of power to set or reset the solenoid. Many of the common irrigation valves have a latching version or solenoid replacement.

E.g. Hunter solenoid:

Warning these latching solenoids are typically made with a very fine wire (coil) which will smoke if full voltage and current are applied for very long. I like to charge a capacitor and then use that to latch them.

One other note is that DC voltages around water can have corrosion problems.

My total device consumption is 8 Watt (even less because my solenoid will work for 10 min).

How often will that 10 min cycle run?

The battery is probably adequate, depending on the duty cycle, and depending on how much energy you can really get from the solar panel. 8W at 12V is 670mA, so the battery should be capable of supplying the current.

Here is my calculations
12v*7ah= 84 watt
My device = 8watt

So I will get 10 hours 84/8

All of the energy has to come from the solar panel. So, it's a matter of charging and discharging the battery.

I'd be skeptical of that 20W rating... That's probably at noon on the equator in the summer. In any case, you won't get full-maximum power all day long. ...You'll probably just have to experiment to see if you can deep the battery charged under real-world conditions.

Will I be getting more hours during sunlight times?

Since the solenoid only requires 8W, theoretically you could power the solenoid all day with some energy remaining to charge the battery.

Thanks a lot guys

One last question of mine is that.7amp from my battery will fry my device?

Where do you get 7A from?

You do need to use drivers (or transistors) to switch any motor or coil, or, any device that needs more than about 20mA.

Weedpharma

There is no 7A from your battery. It is rated at 7Ah. The current drawn will depend on the load applied.

Weedpharma

weedpharma:
Where do you get 7A from?

You do need to use drivers (or transistors) to switch any motor or coil, or, any device that needs more than about 20mA.

Weedpharma

I will be using relays to switch solenoid

weedpharma:
There is no 7A from your battery. It is rated at 7Ah. The current drawn will depend on the load applied.

Weedpharma

Thanks a lot Weedpharma for clearing the the doubt. So it's like the higher the current drawn from battery the higher the watt consumption.which lower the runtime of battery .am I correct?

Correct. Like having a 100-ltr tank of water and having a small tap to fill a cup, it will take a long time to empty the tank. If you have a big tap to fill a bucket, the tank will empty a lot quicker.

Weedpharma

Hey guys

I bought the solar panel and charge controller now. The issue here I am facing is that my solar panels is of 18v and my battery is 12v .so can I charge it with 18 v solar panels?

The 18v is the reason you use a regulator. It converts and controls the voltage to the battery.

Weedpharma

weedpharma:
The 18v is the reason you use a regulator. It converts and controls the voltage to the battery.

Weedpharma

By regulator's you mean charge controller ? Wouldn't there power loss? Due to charge controller?

Hi,
The solar panel needs to be rated higher in voltage than the battery to produce the charging current, the charge controller takes care of all of this.
The 18V will actually be the OPEN CIRCUIT voltage of the panel, it will drop when the charge controller is working to charge your battery.

Tom....

OK another issue am facing while using solar panel is that on dmm on battery terminal it's showing 2V only (without battery). is it due to system Voltages automatic identification?