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Solar Photovoltaic System Design Info Sheet - Feasibility Study
.Use this worksheet to help you design your own off-grid solar power system
Solar PV technology
Mostly solar cells are made from silicon. The cells are arranged into solar modules with different voltage, current and power outputs. There is a rating label
on the back of all solar modules. The main figure to note is the Watts - Peak (Wp) value - this is the output under standard test conditions. Also check the
voltage is OK for your system. There are three main types:
Approx 15% efficient
Long lifetime
Approx 12% efficient
Long lifetime
Around 7% efficient
Slightly better in diffuse sunlight
Load analysis
System design is an iterative process.
Why? Higher loads = larger system = higher cost
Generally the initial system design will be
Accurate load analysis is very important and is the first step in system
very high cost.
design. We need to know the power ratings of all appliances and also the
Can you reduce the load? Use more
time they are used.
efficient devices?
Power x time = Energy, which must be supplied by the system. To do
Use a smaller battery bank?
this write a list of all loads and times:
Why? Economic cost is usually
the determining factor
Add up full cost of all parts including:
cables, connectors, mounting structures,
battery racks, charge controllers, labour,
transport etc.
Power Time
Quantity Energy
20W X4hr/day X 3 =
5W X10hr/day X 1 =
Create full design
(See other side of this worksheet)
This is best done as a spreadsheet.
Always include 20% additional costs for
TOTAL: 255900WWh h
Why? Solar energy is variable, hence we need storage
Resource assessment
Why? Must know how much solar energy is available at the site
To do this we use online resource databases:
NASA Meteorological Data (
Meteonorm (
Solar data is accurate and reliable. Need to check seasonal variation. Usually design for
worst month. Need to include 80% PV charging
Inefficiency factor.
Daily solar energy, London, UK
Example: At our site we have 4hours sunlight
per day during the worst month.
Must supply:
Storage is usually lead-acid batteries. Many different types. Must use
‘deep cycle’ type (NOT car batteries).
Rules of thumb:
1- Storage should be sized for around 3-5 days with no solar input.
2 - Must also never go below 50% depth of discharge.
3 - Must include 80% battery efficiency factor.
Batteries are rated in Ah, therefore need to choose the voltage to find AH
storage required.
Example: 290Wh required per day by loads.
Battery must supply: 290Wh x 100/80 = 362Wh (rule 3)
Battery bank must supply: 362 x 4days = 1450Wh (rule 1)
Battery bank must be sized at: 1450Wh x 50/100 = 2900Wh (rule 2)
Assume 12V battery: Battery bank size is 2900Wh/12V = 241Ah
290Wh (from load analysis) x 100/80 = 362Wh
Therefore need PV array: 362Wh / 4h = 90.5Wp
We usually round up, so use a 100Wp PV module
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