# Two voltage rails from one photovoltaic system

I'm working on designing a photovoltaic well water pumping system.

I will have two 12V 120W PV panels connected in series, for a 24V 240W output (during full sunlight).

Is it possible to connect a load in parallel to one of the panels? In practice, will this reduce the power output of the system?

simulate this circuit – Schematic created using CircuitLab

What are the reprecussions of hooking up loads like this two a multi-panel PV array?

• That's actually a very bad idea. The lower battery will discharge much faster than the upper. Keeping them properly charged and balanced will be difficult. Jun 2, 2015 at 17:21
• @WhatRoughBeast If they were batteries, but they are photo voltaic panels. The loads are directly connected to the panels. There are no batteries in that setup. Jun 2, 2015 at 17:33
• Ok. My apologies. I just assumed a PV system would have a battery associated with it, and "we all know what happens when you assume". The big problem is that the lower PV panel must provide as much current as both motors draw, while the upper panel only needs to provide current for the 24 volt motor. If it can't, the voltage will drop and you'll have difficulties with both motors heating up. Jun 2, 2015 at 17:38

There are situations where that set-up works just as well as connecting only 12V or only 24V motors.

That said, it's not a great idea for reliability to connect motors directly to PVs, so "just as well" is a bit of a limited qualifier there. If the pumping system needs to work some of the time and the motors are sturdy in the sense that they can handle any voltage for longer periods of time, all you need to do is make sure the bottom PV can handle the total drain of both motors.

If you want more reliability it's better to put the panels to work together, connect them to a box that converts a wider range of voltages and currents to a stable 24V and 12V or turns off when it can't any more. For best lower-light performance look for one with a term similar to "Maximum Power Point Tracking", but after an engineering and marketing department that's likely to cost 5 times as much (the chips for it are quite affordable now, though).

So it's a bit dependant on what you want to happen in all cases. The trouble is, a PV isn't an absolutely guaranteed supply. And I couldn't even tell you whether your specific manufacturer or reseller says 12V for its peak open voltage, or for its optimum voltage, or because it can charge a 12V car battery with 14V optimum voltage. I assume you do know, or actually hope you do.

One certainty is that without any regulation there will always be a point where the 12V motor will use all that the bottom panel can supply, then the added strain of the 24V one will make the bottom one sink to a situation where the 12V uses little enough to let the 24V one operate as well. (And some motors might have you end up at both not running, but getting hot.)

Whether this is all day except for one hour of high sun or it is only 15 minutes at dawn and dusk depends entirely on the specifications of the motors and what the insolation is like over there in Nova Scotia.

((A solar panel just converts the light that falls on it with efficiency X. I don't know if there's rules as to whether a manufacturer should define the power output at earth-average, or if they can just do it right on the equator.))

If they are 5W motors, then the tiny bit of time that the panels supply more than nothing, but less than 5W will be so short they can't even overheat if they want to. But somehow, I doubt that's the case.