Selecting & Sizing Solar PV Panel Array for DC Air Conditioner

Selecting the right PV solar panels for a small solar installation like a solar powered air conditioning application can be confusing. Here is what you need to know:

POLY VS. MONO

• There are 2 main types of commonly available and affordable solar panels in the market, made from either polycrystalline (Poly) and monocrystalline (Mono) silicon cells. The difference is that Mono costs more per watt but has a higher power density. In plain English, a standard size Poly panel may provide 240W at a cost of $168 whereas the same size panel in Mono may provide 260W at $202 per panel. Note the panels are always priced by the Watt and Poly has a lower cost per Watt. The advantages of Mono are when you have a larger solar panel installation, you may see that incremental costs for installation, shipping, and mounting hardware are less per watt on Mono bringing the total cost per watt inline with that of a Poly installation. Sometimes this gives the Mono panels a slight total cost advantage on larger projects, but the advantage is not usually seen on smaller PV projects.

• Mono panels take about 5% less roof space, so if roof area for mounting panels on a large project is tight, Mono may allow a few extra Watts to be installed in the same footprint. This is ultimately the net difference between these types of panels.

• We recommend either type but since Poly is more widely sold and has lower cost per Watt it is generally better for small projects (under 4-5kW total size).

SIZING THE ARRAY

• Panels are rated on their peak watts, meaning that under direct DNI (Direct Normal solar Insolation, 1000 watts per meter) this is the amount of power that a panel can produce per hour of sun at this strength.

• Sun hours and daylight hours are not the same thing. Each area of the planet has a total number of “sun hours” also referred to as DNI, solar insolation, radiation, or irradiation. These are annual averages and consider the average rain or cloud cover for the area. Don’t let it confuse you, all of these solar terms mean the same thing. Solar power is measured at kWh (kilowatt hours) available per day per square meter. So for example an area with a rating of 5 kWh per day of solar irradiance or insolation would have 5 "solar hours” or “sun hours” or a DNI of 5 kWh.  You can click here to see various solar insolation maps and charts that show the solar radiation for areas of the USA and around the world.
  
  
• To know how many Watts are needed you must consider the total amount of solar energy needed per day. For off-grid applications like solar air conditioning this means the amount of power needed to run the system during the sun period PLUS the amount of power needed to be stored in batteries to provide after-sun operation. The amount of after-sun hours also determines the size of the battery plant.
  
  
• For example, if your solar air conditioner needs 5 kWh per day to provide the number of hours of operation needed, and your area has 5 sun hours per day, you would need to install approximately 1 kWh of solar PV panels.  This is based on 1 kWh of production capacity per hour multiplied by the 5 sun hours of your location.
  
  
• The basic formula is to take the kWh needed per day by the system being powered and divide it by the sun hours for the location, then divide that by the panel Watts rating (in kW), to see how many panels are needed. Note that a 200 W panel would be expressed as .2 KW for this calculation.
  
  
• Always round up to the next number of panels to create strings of 3, they must be grouped in strings of 3, so it will always be 3,6,9,12 etc.
  
  
• Temperature is a factor. All solar panels are rated at a STC (standard test condition) at the time they are manufactured. The actual power will be lower at high temperatures and higher at lower temperatures. A string sizing calculator using the values from the panel specifications label can give you the best indication of real power at various temperatures. See string sizing information in the section on charge controllers.
  
  
• You should allow a little extra for power loss across the batteries, charge controller, cables, etc. and you may also wish to allow for non-typical daily insolation values, called a cloud-margin or rain-margin. This means you could allow for some extra solar/battery capacity if the DC system may need to operate under cloudy or rain conditions that affect the normal ability of the panels to provide the rated power.
  
  
• DC air conditioners require 48v batteries. The cheapest way to do this will be with 8x 6v golf cart batteries. The best way to do it is to use the 20-year maintenance free Carbon AGM batteries here.

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