# Why does solar voltage drop under load

I have a floating solar pump with panels putting out 30 volts and 360 watts. The meter shows 30 volts at the pump end but as soon as I connect the 24 volt pump it drops to 3 volts and obviously doesn’t work. It’s not the pump as I have tested it on batteries and it works fine. Can anyone tell me what’s happening, maybe the panels or submerged cable is stuffed I don’t know

• are the panels fully lit? They will reach their rated open-circuit voltage even with very little exposure, but current will be very low. Commented Dec 28, 2021 at 7:28
• What are the specs of the pump and the panels? Is there some sort of regulator between the two or are the panels connected directly to the pump? Also, electronics.stackexchange.com/questions/531556/… could be relevant, depending on the (start-up) impedance of your pump. Commented Dec 28, 2021 at 8:43
• Starting current of the motor is huge, several times operating current, ,while the panels are strictly current limited. You need a source of power at fairly high current - like a small battery - to get the pump started, then it should run off the panels (assuming pump power <= panel's actual output power)
– user16324
Commented Dec 28, 2021 at 12:25

In theory,

1. W = I * V

2. 360 = I * 30 => I = 12A (Max at fully charged state)

3. 360 = I * 24 => Iload = 15A

4. 360 = I * 3 => Iload = 120A


(Thats too much current, there may be short somewhere but still depends on your system and I don't know it)

These are presenting the fully charged state.

But in most common cases, it's not working like that.

For example:

Your solar battery has charged 120W/s and your load is constant 5A,30V which means 150W/s. It will start to run at these parameters but the voltage will decay eventually.

Probably your load has higher power usage and dissipation than your solar system so it's not enough to run the system all the time.

This should be a comment but I don't have enough reputation to comment.

• Plus 1 to your rep for comments. And seems ok for an answer anyway. Commented Dec 28, 2021 at 8:04
• OP did not mention a battery in its system. Also, this is not how solar panel works, 360W at 30V translates to 288W at 24V (12A remains), not 15A. Commented Dec 28, 2021 at 8:54
• Yes, I might not express myself well enough. Those formulas are just for elementary guys. I am not a solar power expert, there are lots of miswritten expressions so I just want him to get some idea from basic stuff. Thanks for feedbacks anyway. Even I have learned so much knowledge from comments and other solutions. :)) Commented Dec 28, 2021 at 9:00

The batteries connected to the end of solar system might not be charged fully! Your meters gave out 30V is not real,it was in floating state. The panel to convert sunlight to electric needs time.

• I think you want to say that "charging the batteries requires time" in your last sentence.
– Syed
Commented Dec 28, 2021 at 7:22

Assuming that the solar panels were designed as a package with the pump, the panels should be operating at about 24V when connected to the pump. The collapse of the loaded voltage indicates (normally) that the panel, or part of the panel, is shaded, and can't supply the current required.

Abnormally, collapse of the voltage indicates that part of the panel is just not working: broken, corroded, disconnected, having the same effect as if it was shaded: not supplying the current required.

• I also have a similar issue of voltage drop. Can this happen due to wire not being of proper gauge. Commented Jun 2 at 1:01