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I am looking to install a DC 12V 6W submersible aquatic pool pump for use in a small scale hydroponic setup. The location of the beds will be too far away from mains electrical, and I would like to power this pump via a charged (via solar panels) battery.

I have a very limited knowledge on electrical power generation and discharge in a setup like this, and all the research I can find tells of calculators for larger home-scale setups.

Can anyone please point me in the right direction? Thanks.

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  • \$\begingroup\$ Hopefully this helps until a more formal answer comes along... Essentially, you need to be able to constantly provide 12V at 0.5A, (p = vi >> 6W = 12V * 0.5A). You need a solar panel that can supply at least 6W with a controller to level the output to 12V. Many controllers have hookups for a battery and a load, so the panel will power the pump when it can, and keep the battery(ies) charged with the excess current. The higher the panel output (in wattage) the quicker it will charge the batteries. \$\endgroup\$ – Kurt E. Clothier Mar 27 '14 at 2:46
  • \$\begingroup\$ You also need a battery(ies) that can output (at least) 6W for (at least) 12 hrs of no sunlight, like a 12V, 6Amp-Hour battery. High battery charge will be better, but take longer to charge if fully depleted. \$\endgroup\$ – Kurt E. Clothier Mar 27 '14 at 2:47
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    \$\begingroup\$ Actually, I imagine you would need significantly more than 6W of solar panels. 6W of solar panels would not be able to charge the battery and run the pump at the same time, so you would need at last 12W. Also, solar panels will not put out their full capacity all day long in all conditions (e.g. clouds, dawn, dusk, etc.) so you might want to at least double that again to be safe. \$\endgroup\$ – alex.forencich Mar 27 '14 at 3:19
  • \$\begingroup\$ Very true, more panel output would be necessary for charging and powering under bad weather conditions. \$\endgroup\$ – Kurt E. Clothier Mar 27 '14 at 5:46
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Since you want to have the pump running 24 hours a day, you need to choose a panel that can output enough power during daylight to power the pump all the time. It might surprise you how big the panel will need to be.

1) Analyse the pump requirements. The power use is 6W. Multiply 6W by 24 hours and you get 144 Watt-hours.

2) Choose a battery that will store enough power. It's wise to over-spec the battery so that it can provide a few days (or even weeks) of power in the case of bad weather. At standard lead-acid battery is a good solution. A lead-acid battery is considered a "12V nominal" battery, although they like to be charged to a higher voltage (13.2V to 13.8V is common).

Battery capacity is measured in Amp-hours. So, for one day, we have 144 W-h divided by 12V, which gives 12 A-h. Choose how many days you want in reserve, and select you battery accordingly. This site is a good reference of what's available, and they also have low prices on the batteries they sell. If you want a 3-day buffer, for example, you would want a 36 A-h battery.

Also, lead-acid batteries don't like to be deeply discharged. If you let the battery drain to almost empty, then the energy storage capacity will quickly diminish. This is why automobile batteries get ruined if you leave your headlights on for a long time. A rule of thumb is to buy twice the capacity you need, so the battery won't become more than 50% discharged. (Thanks, Filek!)

3) Decide if you want to buy a solar charge controller. It'll cost extra money, but it really stretches the power you get from a given panel. With a good controller, you may get 90-95% of the panel's rated output into your battery. If you just hook up the solar panel to the battery, you probably will average only around 40-60% over the course of the day.

4) Calc your required solar panel. There are a lot of options here, depending on what you want. At one end of the spectrum, you wouldn't need solar, because you could charge the battery regularly. On the other end, with a lot of solar cell output, you could plug in a discharged battery and have it charge up rapidly (even with the pump running!).

To provide enough power for the pump to run 24/7, you would need 144 W-h per day from your panel. Assume you get 6 hours of good, high sun per day. 144 W-h divided by 6 hr = 24W. Adding your inefficiencies, that would be a 48W panel without a charge controller, or 26W with one.

Of course, after a few days of bad weather, you want extra power to top off the battery again! So get a slightly bigger panel than what you calculate. I personally would choose 20-30% bigger.

Here is a handy chart that shows how many "sun-hours" you can expect per day depending on your location. If you want more precise numbers, you can do a web search for "solar insolation". (yes, that's spelled correctly!)

Good luck!

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    \$\begingroup\$ Excellent answer. When buying batteries, keep in mind that the lead acid batteries will not last long if they are deeply discharged often. I.E. If the batteries have just enough capacity for your purpose and thus are drained to almost empty on a regular basis, they will soon die. It is recommended that the capacity of the lead acid battery be twice what you need so that you regularly discharge them to only 50%. Then they will last years. The manufacturer of the particular battery you buy can advise you better on this as some lead acid batteries can take a deeper discharge, but most are 50% \$\endgroup\$ – Filek Mar 28 '14 at 3:30

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