Solar panels voltage? Confused [Archive]

Rob Beckers

9th June 2019, 07:19

Hi John,

Welcome to the forum!

The panel Voltage is determined by the number of cells that it is made of. Each cell is about half a Volt when it's producing power. Your Hanwha panel has 60 cells, and so it runs at about 30 Volt. A larger solar cell will produce more current, but its Voltage will still be around that half Volt. On the label on the back of the panel you'll find two Voltages: Voc and Vmp, the first is "open circuit" when the panel is not delivering any current and that would be around 39 Volt for a 60-cell panel, the other one (Vmp) is "maximum power" and that is the Voltage when it is producing maximum output power, around 30 Volt.

There is a bit of history to the whole 12 and 24V panel thing: Decades ago solar panels were really only made to charge batteries, and then mainly 12 Volt batteries (which actually charge closer to 15 Volt). Someone figured out that to do so reliably you needed 36 solar cells, and with a half Volt per cell that made those panels run at about 18 Volt. The extra Volts were needed because very cold batteries need a higher Voltage to charge, and very warm panels produce a lower Voltage.

So, a "12 Volt panel" had and still has 36 cells in it, and produces around 18 Volt under load. They are still made, but now mainly for small sizes of around 100 - 150 Watt output power.

Solar panels slowly got cheaper over the years, and people started putting them on roofs to feed power back to the grid. They found the little 36-cell panels inconvenient, you needed too many of them, and the industry standardized on 60-cell panels for that purpose. Because many more panels are used to feed power back to the grid these days than charge batteries you will see 60-cell panels as the most common ones around now. they are mass-produced (and in far larger numbers than 36-cell), and have become a commodity. Prices of 60-cell panels in terms of dollars-per-Watt have plummeted, they have become very cheap (we sell them at 79 ct/Watt Canadian!). Nowadays they are much cheaper in dollars-per-Watt than 36-cell panels. As mentioned they run at about 30 Volt.

For larger systems, mainly ground-mounted utility scale projects, the 60-cell panels were found too small and they standardized on 72-cell modules. Think of them as 60-cell ones with an 'extra two rows of cells' on top, making them the same width but taller (nearly 2m tall). Those too are produced in vast quantities and are cheap in dollars-per-Watt, possibly even cheaper than 60-cell modules.

As it happens 72-cell is exactly twice the number of cells in a 36-cell panel. The 36-cell panel is great for 12 Volt battery charging, so (you guessed it), 72-cell works fine for charging 24 Volt batteries! It runs at around 36 Volt, just what is needed for a 24-Volt battery bank.

All these different cell-sized panels can be used to charge batteries. A 60-cell panel (running at 30 Volt) can charge a 12V battery bank, if we convert the Voltage down. That is what an MPPT type charge controller does (stands for Maximum Power Point Tracking). They have electronics that can convert Voltage from higher to lower, convert from a higher Voltage and lower current, down to a lower (battery) Voltage and higher current. Much like a gear box in a car matches the speed of the wheels to the engine RPM.

For a 36-cell panel you don't need to convert the Voltage, it's fine as it is for charging batteries (if the batteries are charging at 14 Volt they will simply pull down the panel Voltage to match that, while the current from the panel stays the same). That means a simpler charge controller can do the work, a PWM type controller (stands for Pulse Width Modulation). Those are cheaper than MPPT controllers because they are much simpler devices.

It used to be that MPPT charge controllers were very expensive, but these days they are only a little more than a PWM controller. Many MPPT controllers in fact allow for 100 or even 150 Volt on the input side, and are able to convert that down to 12 or 24 Volt for the battery bank. That means you can wire two or three 60-cell panels in series (positive of panel one to negative of panel two etc.), and run low current high Voltage through the wiring to the charge controller. That allows for smaller wire sizes. The controller (near the batteries) then converts it down to battery Voltage with a much higher current.

Hope this helps!

-RoB-