Off Grid Battery-based Solar
Electrical Installations
By Scott Carlson
Living in a home and providing power to that home, through
an off grid solar electrical system is a dream for many people.
Today there are already thousands of people doing just that.
Solar electrical equipment though is typically very expensive,
therefore prudent use of electricity itself is necessary if
your budget is limited. The average home in Southern California
uses approximately 1000 kWh per month on average. By slightly
modifying ones lifestyle and choosing energy efficient electrical
appliances or moving some appliances to a power source such
as propane you can easily reduce the electrical consumption,
to approximately 500 kWh per month or much less. Typically,
spending a little extra money on an appliance, so that it is
more energy efficient, thereby decreasing the size of the solar
electrical system necessary, is a more cost-effective way to
go.
There are three major necessary components to an off grid solar
electrical system.
Inverter
Most off grid solar electrical systems utilize the larger Xantrex
or Outback inverters, costing approximately $2500. They are
highly reliable and approximately 90% efficient. Therefore,
when building a house, the electrical wiring is done in the
traditional fashion and no longer needs special DC wiring. One
of the most critical considerations on inverter selection is
starting surge capabilities. Most electrical loads utilize a
consistent amount of electricity when they are turned on. But
electrical loads that use a motor can require a large surge
of electricity, for a fraction of a second upon startup. This
starting surge can be as much as 11 times the electricity needed
to continue to run the motor. Generators can typically handle
large starting surges because; they have the flywheel, crankshaft,
pistons, and etc. to provide the extra mass that for a fraction
of a second to get the motor loads started. Inverters do not
have this capability except for what was designed into the inverter
initially. For example a Xantrex SW- 4048 inverter can continuously
run at 33 amps and handle a starting search of 78 amps. If the
motor load starting surge plus all other loads presently being
utilized exceed 78 amps, the inverter in order to protect itself,
will turn off and display an error message. Typically all that
is needed to restart the inverter, is for the homeowner to turn
off the loads, then turn the inverter off and back on. Beside
the cost of the inverter itself there are typically many other
smaller components necessary for the inverter to be installed
in a code compliant manner.
Batteries
There are many choices of batteries homeowners can select from.
Over half of all battery-based solar electoral systems are utilizing
Trojan T 105 or Trojan L 16 batteries. This is because they
are made in large quantities for the golf cart industry, are
readily available and have a known reliability factor. Larger
systems are utilizing 48 volt inverters therefore requiring
the 6 volt batteries to be installed, using 8 batteries per
string (6 x 8 = 48). For the average house we normally see 24
Trojan T 105 batteries being utilized. This would give the homeowner
approximately 24 K. W. worth of electrical storage. The downside
is that these batteries need to be maintained and will last
only 7 years. Distilled water needs to be added, and equalizing
charge needs to be done approximately once a month. An equalizing
charge is doing a controlled overcharging of the battery bank.
This stirs up the electrolyte within the battery and to knocks
sulfation off the battery plates. Batteries that are maintenance-free
typically cost 2.5 times the amount of lead acid batteries.
A typical bank of 24 Trojan T 105 batteries will cost approximately
$2500. This will typically allow the homeowner enough storage
to cover an occasional cloudy day or two and overnight usage.
Modules
Solar modules are the most expensive components within a solar
electrical system. But fortunately they are the easiest to add
on in order to grow your system size. A 200 W solar module will
provide approximately 25 kWh of electricity per month. Therefore
a homeowner utilizing 300 kWh per month of electricity will
need approximately 12-200 W solar modules. Depending upon customer
usage of solar electricity and geographical location additional
modules may be necessary for Winter usage when the days are
shorter providing less electricity and the necessity for heater
fans and additional hours of lighting increase.
