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Ok - total newbie question here. I'm trying to work out the size or number of solar panels required to run a DC pump for use as a solar heating system for a swimming pool.

I've located what looks like a suitable pump with the following specs

  1. Voltage: 8V-24V DC (Standard:12V DC), 4.5amps
  2. Max Flow Rate for 15W model : 11.5 L/Min - Max Water Head: 3M
  3. Min start-up power consumption less than 2 Watt

The aim is to direct connect the pump to the solar panel allowing for it to operate whilst sufficient sunlight in order to pump water into the heating elements of the pool (think black plastic tubes on roof of house).

Now the question. Given 24v, 4.5amps I think it equates to approximately 100watts. The solar panels from a local supplier give specs such as:

  • Maximum power: 120W
  • Rated voltage: 12V
  • Open circuit voltage: 21.8V
  • Short circuit current: 7.14A

The other alternative is the 200w panel which has specs like:

  • Maximum power: 200W
  • Rated voltage: 24V
  • Open circuit voltage: 45.3V
  • Short circuit current: 6.15A
  • Voltage at max power: 37.8V
  • Current at max power: 5.31A

Thus which panel would work; 120W or 200W ? Also do I need to worry about the open circuit voltage and short circuit current ?

thanks for you help grant

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First of all, are those numbers for the pump all from the same model? Because the range of the numbers is rather large: you say that it starts with 2 watts, nominally runs at 15 watts(?), might sink 4.5 amps at 12 volts, which gives 54 watts. And even larger numbers are calculated in your question, which probably aren't relevant.

Looks to me that even the 120 watt panel is overkill, but it can be connected directly to that pump if the pump can inded handle 24 volts, which is more than the open circuit voltage, which is the maximum voltage the panel can ever generate.

The thing to realize here is that it's the amps that turn the motor, not the volts. Volts are something that have the capability of giving amps to a load. And pushing amps through a load generates volts across the load. In your case, there's a solar panel, which is more like a constant current source, unlike, say, a battery, which is more like a constant voltage source.

When you connect the solar to the motor and there is sun, the panel pushes amperes to the load. The 120W panel looks like it might push something like 6 amps to the motor. That would perhaps generate something like 16 volts over the motor, although this number is only based on a rough feeling. That would mean that some 96 watts are dissipated somewhere in the pump - and that seems quite a lot for a pump that nominally takes 15 watts. I don't know; maybe the pump is electronically regulated somehow so that it never draws so many amps...?

All in all, I have a bit uneasy feeling about this. It would help if you could provide some more info about the pump. A link to a datasheet or a product page, maybe?

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  • \$\begingroup\$ Yes, there are some flags there. Would feel better if we could see some more data about the pump. I have seen pumps specifically designed to mate with solar panels, with no battery, and this could be one of those. If so, then the startup spec may mean that it will start when at least 2W of power is available. \$\endgroup\$ – mkeith Dec 21 '14 at 6:47
  • \$\begingroup\$ One of the type of pumps I had looked at can be seen at topsflo.en.made-in-china.com/product/aqLngyJxuNrY/… \$\endgroup\$ – Grant Sayer Dec 22 '14 at 9:31
  • \$\begingroup\$ Ok, the link clarifies it a lot - the model numbers indicate the solar panel size: model 5PV is meant to be connected to a 5 watt solar panel, 10PV to 10 watt panel and model 15PV to a 15 watt panel. Nominal voltage of panel is 12 volts. \$\endgroup\$ – PkP Dec 22 '14 at 13:51
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EDITED TO CLARIFY VOLTAGE LIMIT

It looks to me like you can probably direct connect the 120 W panel to that pump. If you have a link to the pump specifications, that would help.

You need to worry about the open circuit voltage only in the sense that you don't want the operating voltage to be higher than the pump maximum. For the 100 Watt panel, since the Voc is lower than pump max, it is not an issue. I don't think you need to worry about the short circuit current.

The open circuit voltage of the 200 W panel is quite a bit higher than the pump max. It is not clear that the pump would load it enough to bring the voltage down to a safe level. So I would not use the higher power panel in this application.

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    \$\begingroup\$ I'll disagree with your comment on open circuit voltage. Open circuit voltage occurs (be definition) when there is no load. Therefor it cannot occur when the motor is connected to the panel. However, I do agree with your overall recommendation. The key is the spec on voltage at max power - this is 50% over what the motor is rated for. \$\endgroup\$ – WhatRoughBeast Dec 21 '14 at 4:27
  • \$\begingroup\$ You are right. It is not absolutely necessary to have the open circuit voltage be lower than the max voltage of the pump. But if the open circuit voltage IS higher than pump max, then the decision process is more involved. If it was over by a substantial amount, you would probably want to study the VI curve of the motor under various conditions, and compare with VI curve of panel at high illumination, and make sure the actual operating point will not be over the pump max. \$\endgroup\$ – mkeith Dec 21 '14 at 6:28
  • \$\begingroup\$ I reworded my answer. \$\endgroup\$ – mkeith Dec 21 '14 at 6:42

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