Solar Panels / Photovoltaic Modules

electricity from the sun by John Drake II | home

Solar Panels / Photovoltaic Modules - Read Before You Buy

A cautionary note on photovoltaic module voltages and connections.

Over the past few years, most, if not all photovoltaic module manufactureres

have shifted their production of pv panels from ones designed to charge

batteries to ones specifically designed to be used with utility grid-tie

inverters.

This is because, with government and utility company rebate programs,

the largest demand for pv panels has been in this market.

As a result, many modules are being offered for sale at very attractive

dollar per watt prices.

Many of these are brand new left overs from projects and in some cases

where projects have been cancelled.

For the foreseeable future it is highly recommended that you look carefully

at the specifications on any pv modules you are planning to purchase.

Most of these panels have odd ball voltages specifically designed for a

particular model or family of grid-tie inverters.

If you try to use these panels with a conventional photovoltaic charge controller

they will work poorly or not at all.

This is not decrying the panels, they were never designed for this application.

We get calls on a regular basis form folks who got a "great deal" on some

solar panels but they will not charge their batteries.

Solar Converters, Inc. in Canada has made many charge controllers for our

customers that are set for use with their particular panels.

To avoid this you want to look for these two specifications to see if the modules

are designed for charging batteries.

First is the Voc, or open circuit voltage. It should be about 19.5 to 22.0 volts

for a 12 volt battery system or 40.0 to 44.0 volts for a 24 volt system.

Second is the Vmp, or voltage maximum power. It should be about 17.0 to 18.5 volts

for a 12 volt battery system or 34.0 to 36.0 volts for a 24 volt system.

In the old days, pre-utility grid-tie systems, most panels had a "J" or

junction box on the back.

This was a weather tight enclosure where you connected the wires from

the panel to the charge controller.

Nowadays most modules have MC connectors which are designed for

higher voltage rooftop connections for use with a grid-tie inverter.

If you purchase panels with these connectors you would be best off

picking up some adapter connections for your wiring.

A quick comment on pv module manufacturer's specifications.

If you look at the specifications on pv modules some manufactureres show

two columns.

The first is the specifications at 25 degrees C and the second is at

45 degrees C.

You will notice there is quite a difference between the two.

The 25 degree numbers are great for advertising and the 45 degree

numbers are more along the lines of real life.

As the cell temperature in the panel increases, the voltage decreases.

The overall power output also decreases.

This is why we recommend having an air space behind the modules

in roof mounted arrays.

In a ground or pole mount installation, this is not a concern.

I remember when Arco Solar came out with their 36 cell modules and

acclaimed their higher power output in elevated (read that as real life)

temperatures.

Everyone shortly followed in their footsteps.

The early solar panels usually had only 30 to 33 cells which was okay in

cool weather.

When the weather got hot, those panels with fewer cells would barely

charge a battery and in some cases would not work at all.

Some of the manufacturers even made panels with fewer cells and

promoted them as self-regulating so they would not need a charge

controller.

From personal experience, this did not work well at all.

I am looking at two of them right now in the display in our office.

And yes, I am getting old, losing my hair and my joints hurt - but I was

there in the "good old days".

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