Photovoltaics or solar electricity
There are two primary factors that determine the size and cost of a solar
The daily power consumed by your electrical loads, as determined in the energy
The sunlight levels or insolation available at your particular location.
Insolation or sunlight intensity is measured in equivalent full sun hours.
A full sun hour is equal to the amount of sunlight striking the earth in one
hour when the sun is directly overhead in a clear sky.
Equivalent Full Sun Hours
The lower the sun is in the sky the more atmosphere the light must pass
through. Water molecules and other gases in the atmosphere reflect and absorb
some of the light passing through it reducing the insolation. Most of the sun's
energy is delivered between nine am and three pm when the sun is highest in the
sky. Shorter days, cloud cover and the sun's position in the sky reduce the
amount of energy available in the winter.
To calculate the size of a photovoltaic array you must first determine the
amount of sunlight at your location. The full sun hours table gives average
monthly sunshine hours. Sunlight varies considerably throughout the year. If
you are designing a PV system for a summer cottage then you should use the
figures given for the summer months. If you are designing for a year round
residence with a backup source such as a generator then you should use the
yearly average or the lowest value for the months when you do not want to
operate an auxiliary source.
As a rule of thumb for BC, the spring to fall minimum is about four hours of
full sun per day. A system designed around this value will require generator
back up for about five months of the year.
1. From your energy budget find the total daily load in amp / hours (a/h).
2. Determine the insolation in full sun hours from the chart. Divide the total
daily load in a/h (from the energy budget) by the full sun hours to give the
array current in amps.
3. Select an appropriate number of modules to supply the array current.
1. From your energy budget you calculate a total daily load in a/h for a 12
volt system, of 96 a/h per day.
2. Divide this figure by the number of sunshine hours to arrive at the size of
the PV array required. If we use four hours of sunshine then 96 / 4 = 24 amps.
3. Therefore you need a PV array that produces 24 amps at 12 volts in full
sunshine. Four Solec SQ75 PV modules (4.6 amps each x 4 = 18.4 amps) would be a
suitable array to install for this system.
Once you have selected solar power to be the best option for your electric
needs it is necessary to choose a mounting structure. The first step is to
select the location of your PV array. The ideal location is an area receiving
full exposure to sunlight as close as possible to the location that electricity
will be used or stored. There are several options for mounting PV modules.
There are two main types: roof and pole mounts.