Specifications

AT 52
would recharge the battery after just two hours,
while a cheaper 62.5W panel would require
eight hours.
Solar panels are available in many different sizes
and power ratings. Sometimes it is not possible
to get a single solar panel with sufficient output
power. In this case, two or more solar panels can
be connected together in parallel. The example
above required a solar panel rated at 125W.
A single 125W solar panel would do the job
nicely, but other combinations are possible. Two
60W solar panels wired in parallel will produce
120W. Likewise, four 30W solar panels wired in
parallel will also produce 120W. (Both of these
examples represent a shortfall of 5W per hour,
or a total of 20W during the four-hour peak
sunlight recharging period. This is acceptable
because the panels will almost certainly be
exposed to more than four hours of sunlight per
day, allowing them to make up the difference
– a 120W panel will produce the extra 20W in
10 minutes of additional peak sunlight.)
The ability to combine solar panels has another
benefit if the budget is tight. For the example
above, it may be possible to start with a single
60W solar panel and add a second solar panel
when the budget permits. The downside is
that a 60W solar panel will take longer to
recharge the battery. If, however, you lived in
an area that got considerably more than four
hours of peak sunlight per day, a 60W solar
panel might be sufficient. Likewise, if the
audio technology was only used on alternating
days (e.g. Monday, Wednesday, Friday, etc.), a
60W solar panel would have plenty of time to
recharge the battery. If the audio technology was
only used one day per week (e.g. Saturdays), an
inexpensive 30W solar panel would have plenty
of time to recharge the system during the week.
One final note: The effectiveness of a solar
panel is affected by its physical positioning in
relation to the movement of the sun across the
sky. Careful positioning is required to maximise
the output power. This is where the voltage
and current indicators provided on the more
expensive charge controller/regulators come in
handy – by monitoring the output voltage of
the solar panels, it’s possible to fine-tune their
position for optimum output power.
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Now that we know the battery and solar panel
requirements, we must consider the devices
that interface them together and to the audio
technology they are powering: the charge
controller, the regulator, the inverter and the
UPS power board.
The charge controller must be able to withstand
the highest current the solar panel is capable of
producing, which is known as its ‘short circuit
current’. Incorporating a safety factor of 1.5
ensures the charge controller is never pushed to
its maximum capability. So, if the solar panel’s
short circuit current is 5A, a charge controller/
regulator rated at 5 x 1.5 = 7.5A would be a
good choice. (Note that if you are using two or
more solar panels in parallel, you must add the
short circuit currents together to arrive at a total,
which the safety factor is then applied to.)
The regulator must be able to pass current
from the battery to the inverter without being
damaged from overheating. The system used in
the example above drew a total of 12.5A from
the battery – the regulator must be capable of
passing this current. Again, a safety factor of 1.5
is worth considering, suggesting a minimum
of 18.75A, which we’ll round up to 20A to suit
commercially available products. Likewise, the
inverter must be able to accept this current from
the regulator and deliver 136W of AC power to
the audio technology – a 150W inverter would
be the minimum choice for this purpose, but a
200W inverter would be wiser.
The wiring used to interconnect all the
components must also be considered. Solar
power systems are low voltage systems, and
that means they need much higher currents
to produce power. The 12V system described
here needs 12.5A of current to produce 150W
of power. In comparison, a 240V AC system
needs only 0.416A to produce the same power.
Passing 12.5A of direct current (DC) through a
wire safely and efficiently requires heavy-duty
wiring – even the power leads used for domestic
appliances are insufficient for this application.
This is why the wiring used for car batteries
is so thick; a larger diameter wire has less
resistance, and is therefore capable of passing
a higher current with greater efficiency and
safety. If the wire is too thin and has too much
resistance, it will create a voltage drop (resulting
in a loss of power) and may even overheat and
cause an electrical fire. The companies that
provide solar power equipment can recommend
suitable wiring.
The UPS power board must provide sufficient
back-up power to keep the audio technology
running for about 10 minutes; more than long
enough to save your work and either shutdown
or change over to mains power. For the example
given here, the UPS should be able to provide
136W of power for 10 minutes. Most UPS
power boards come with a chart showing how
long they can power different systems.
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The audio technology used for this example is
typical of the smaller systems used in project
studios. Based on the calculations above, the
minimum requirements to power this system
are:
1 x 125W solar panel
1 x 47A/h deep cycle battery
1 x 20A charge controller/regulator
1 x 150W inverter
1 x UPS power board
Taking advantage of on-line purchasing
discounts from local suppliers, a suitable system
can be purchased for approximately $2000.
This cost may seem high, but remember that
the calculations used very conservative figures,
erring on the side of caution, to propose a solar
power system that should reliably provide three
hours of power per day, every day – along with
UPS back-up for easy transfer to mains power
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As calculated in the
example, a 12V battery
must provide 12.5A
to the inverter. That’s
a significantly high
current and will require
heavy-duty wiring between
the battery, charge
controller/regulator and
the inverter. If a 24V
battery and inverter were
chosen, this current would
halve. Here’s the maths:
to produce 150W from
a 24V battery requires
a current of 150 / 24 =
6.25A. So, a 24V battery
that can supply 6.25A for
one hour will be able to
provide 150W for one hour.
Because the system will
be used for three hours
per day, every day, the
total A/h requirement for
a 24V battery is 6.25 x
3 = 18.75A/h. Factoring
in a maximum discharge
depth of 80% means the
total A/h requirement of
the 24V battery would
be 18.75 / 0.8 = 23.4A/h,
which we’ll round up to
25A/h.
For smaller systems, like
the one in this example,
12V technology is usually
the most cost-effective.
With larger systems, 24V
technology may prove to
be more cost-effective.