Pumps

I have a 12 V ..20 watt solar panel running a 12 v 15 watt fish pond pump ,I want to connect a battery between the two for continues running ,what would be the best size battery (amps ) and what sort of charge controller would be best ,thanks
firstly thanks for your imputs so far
when I say"continues running " I was meaning during periods of cloud through out the day instead of the stop start that happens at the moment
when it’s a very cloudy day then I don’t expect the pump to run but I want to make sure it doesn’t run the battery too low

If you are running the 15W pump continuously, the 20W solar panel won’t be big enough. 15W x 24 hours = 360 watt hours (wh). Since the solar panel is generating electricity only during sunny hours, it can’t keep up. Let’s use 5 sun hours as an example, which is the total number of hours the sunlight is equal to the standard test rated power. 20W x 5 hours = 100wh. You’re 260wh short. You need around 95W to do what you want. (360wh / 5 sun hours + ~30% inefficiencies) http://www.altestore.com/store/Solar-Panels/100-to-149-Watts-Solar-Panels/Carmanah-CTI-110-110W-12V-Solar-Panels/p6871/

For the battery, you’ll need a deep cycle battery that is designed to be drawn down to about half way and recharged, repeatedly. They are rated in amp hours, (ah), not cca. Since you only can use half the power in it, you need it twice as big as you calculate. If you need 360wh, that’s 30ah usable power, 360wh / 12v = 30ah. 30ah x 2 (only using half of battery) + 15% (battery losses) x 1.19 (50F temperature compensation)= 82ah battery. http://www.altestore.com/store/Deep-Cycle-Batteries/Batteries-Sealed-Agm/Universal-Ub12900-12V-90Ah-20Hr-Sealed-Agm-Batt/p2004/

The charge controller is rated based on the current output of the solar panel. That 110W 12V panel is rated at 7.5A. Add 25% safety margin, you need at least a 9.3A charge controller. If you get one with a low voltage disconnect (LVD), you can have the pump automatically shut down if the battery gets low, saving the life of the battery if you’ve had a day or two of clouds or rain. http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/PWM-Type-Solar-Charge-Controllers/Morningstar-Charge-Controllers-PWM/Morningstar-Sunsaver-Ss-10L-10A-12V-Pwm-Chgctrl-Lvd/p799/

You can run the numbers through a calculator like this to see for yourself, http://www.altestore.com/store/calculators/off_grid_calculator/

…….Update based on asker’s update…………. If you just want to run it for as long as you can with your 20W solar panel, you can size it based on your potential output from the solar. Let’s say 1.3A output, 5 sun hours, so 6.5ah. Multiply by 2 since we’re only using half of the battery, so at least a 13ah battery, figuring you’ll be using the power about as fast as it’ll be putting into the battery, so it’s really just acting like a buffer. I suggest rounding up if possible, since I didn’t figure in any inefficiencies or losses. http://www.altestore.com/store/Deep-Cycle-Batteries/Batteries-Sealed-Agm/MK-ES17-12-18-Ah-Sealed-Lead-Acid/p5925/ . Or if budget is more important than running duration, then http://www.altestore.com/store/Deep-Cycle-Batteries/Batteries-Sealed-Agm/Universal-UB12120-12V-12AH-20HR-Sealed-AGM/p1996/.

I still recommend a charge controller with a LVD to turn the pump off when the battery is low. So, while the panel requires only about a 2A charge controller, the smallest charge controller I can think of with LVD is the 6A, http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/PWM-Type-Solar-Charge-Controllers/Morningstar-Charge-Controllers-PWM/Morningstar-Sunsaver-Ss-6L-6A-12V-Pwm-Chg-Cntrl-Lvd/p804/.

12 volt Water Pumps

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