higher wattage solar panels are designed for the grid-tie market
where array voltage is not as critical as compared to an
off-grid battery based system. A 200+ watt panel with an
operating voltage in the range of 28 to 30 volts will not
properly charge a 24 volt battery bank that is is intended to be
bulk charged to 29.6 volts, or even equalized to 31 volts. Warm
weather or weak sunlight will keep the panel from reaching full
voltage, thus keeping the battery bank from ever reaching a 100%
fully charged state. Using such a panel with a traditional PWM
controller to charge a 12 volt battery bank would lose half the
available power rendering the purchase of such a panel moot in
the first place.
So how can we make use of such panels in off-grid battery based
systems? The answer is to use a Maximum Power Point Tracking
charge controller and wire the array at a higher voltage than
the battery bank.
The original intended advantage of an MPPT charge controller is
that they yield 5-25% more power, depending on weather and
temperature, harvested from a given array compared to a
traditional PWM controller. A secondary effect that we can take
advantage of is the voltage regulation. An MPPT controller
doesn't care what the incoming array voltage is (within some
limits). It simply wants to know what the output voltage to the
battery bank should be.
Thus if an MPPT controller is used, a single panel at 29ish
volts can properly charge a 12 volt battery bank, getting full
usage of the wattage available. Two panels can be wired in
series at 58 volts to charge a 24 volt battery bank (or a 12 volt
bank), and at least 3 panels in a series string at 87 volts
would be needed for a 48 volt battery bank.
The other advantage of an MPPT controller is that having the
array wired at a higher voltage minimizes the amperage that has
to be transmitted from the array to the charge controller,
allowing smaller gauge wire to be used. The savings in copper
wire alone often can justify the higher cost of the MPPT
To summarize; MPPT controllers let us utilize the larger wattage
solar modules (that tend to have better price points than small
modules), they harvest more power over the life of the system,
and can offer a cost savings in the transmission wire needed
from the array.
Hours: Mon. thru Fri.
8 to 5 Pacific Time.
Visiting Hours: M-F 1:00-4:00 PM
24 Hr. Fax: 208-265-4788
Backwoods Solar Electric Systems
1589 Rapid Lightning Creek Rd
Sandpoint, Idaho 83864 USA