John Drake Services, Inc.
1427 E. 68th Street
Long Beach, CA 90805
electricity from the sun by John Drake II | home
About Us | About this website. | Getting Started | Solar Panels / Photovoltaic Modules - Read Before You Buy | Make A Plan | PV System Layout and Parts | Start with the loads you are going to operate | Battery Types and Sizing | Battery Safety | Battery Charging Voltages and Temperature | Battery Trouble Shooting | Charging Batteries - You can't have everything. | Battery State of Charge and Measurement | Safety Devices Fuses and Circuit Breakers | Series & Parallel Wiring | Wiring and Power Distribution Connections | Photovolatic Module Specifications - Real or a Pipe Dream | Photovoltaic module solar panel location and positioning | Photovoltaic Module and Solar Panel Information | Charge Controller Types | Wire and Cable Types | Wiring - Doing it Right | Connections for Wiring | Voltage Drop - Wire Loss, What are they? | Outdoor Connections | Wire loss - Voltage drop charts | D.C. to A.C. Inverters | Low Voltage D.C. Lighting & Color Temperature of Light | Battery System Monitoring | Dont fool yourself - Spending your money wisely. | Solar Converters GS-1AC | Linear Current Boosters for Water Pumping | Utility Grid Intertie Systems | Solar Insolation Chart | More Information | Alternative Energy Expectations | Power Needs Worksheet | Reference Sources | Contact Us
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
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
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)
Everyone shortly followed in their footsteps.
The early solar panels usually had only 30 to 33 cells which was okay in
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
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".