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**green**View PostFor a given amount of sunlight, the panel will be close to a constant current source. That is no matter how far down the pump pulls the voltage, the current will not increase enough to give the pump more power. The pump, on the other hand, will try to pull more current as the voltage goes down to deliver the same load. So you have to really decrease the voltage and increase the current but by a smaller factor to get the pump to do its best.

Here is an example with made up numbers:

Panel in full sun gives you 20 volts Vmp and 15 amps Imp. That is a 300 watt panel.

But in half the sunlight, you get 20 volts Vmp and 7.5 amps Imp. Delivering 150 watts.

Let's say that the pump would like to run on 24 volts, 10 amps for maximum output.

With full sun on the panel, it would be OK, drawing maybe 12 amps since the voltage is only 20 volts, and it would deliver close to full output.

With half sun on the panel, the pump would try to draw 12 amps but could not. The voltage at which the pump will draw only 7.5 amps may be as low as 10 volts.

Since the panel is a current source, it will deliver 7.5 amps at 10 volts, for only 75 watts getting to the pump.

A device called a linear current booster will go between the panel and the pump and convert that 20 volts at 7.5 amps to 15 volts at 10 amps and both the pump and the panel will be delivering the most output they can for that amount of sunlight.

This is somewhat simplified, but still should show you why a Linear Current Booster is important if you want the pump to deliver as well as possible on cloudy days and for more than just the two hour period centered on solar noon.

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