Solar sizing

With reference to the typical load profile described of 3000 watt-hours, which represents a small, modern, ultra high efficiency home, ( project "Earth") we will now calculate the various equipments needed to satisfy these requirements . Battery capacity is shown for both stand alone, and grid switched scenarios using the mains as a backup. The latter being appropriate for use in cities and towns with erratic and substandard electricity supplies :-

• 1 The average daily power consumption is 3000 watt-hours ( 3.0 Kwh)
  
  

• 2 The sample site is Perth, Western Australia, latitude 32 degrees South. Average daily solar insolation ( radiation input) 5.9 peak sun hours at mid season equinox.
  
  

• 3 The combined system losses of the D.C cabling, battery and D.C to A.C inverter are 25%.
  
  

• 4 Thus, the maximum output wattage of the required solar array is 3000 watts + 33% divided by 5.9 peak sun hours = 672 watts. Note: This must be the true charging power of the solar array @ 50 degrees cell temperature, not peak output @ 25 degrees C !
  

• 5 The battery capacity in a stand alone solar power system without wind, genset, or mains backup must be a minimum of 10 days the average daily load. ( In cloudy climes 15 days) Therefore, 10 x 3000 watt-hours = 30,000 watt-hours battery storage capacity (30 Kwh) In a "grid switched" system with mains backup, 3 days storage is appropriate. 3000 watt-hours x 3 = 9000 watt-hours.
  

• 6 We have decided on a 48 volt battery bank for optimum efficiency. The capacity in ampere-hours for stand alone is thus 30,000 w/h divided by 48 v = 625 ampere-hours. For "grid switched" it becomes 9000 w/h divided by 48 v = 188 ampere-hours.

Choice of equipment

• Our ultimate choice of solar power array is 12 x UNI-SOLAR USN-65 Advanced, triple junction, thin film, photo-voltaic panels. Panels are wired as three "quads" in series / parallel, yielding 3 x 4.5 amperes @ nominal 50 volts charge = 13.5 amperes x 50 = 675 watts x 5.9 peak sun hours = 3982 watt-hours, minus 25% system losses = 2987 watt-hours. The UNI-SOLAR panel is the most technically advanced and sustainable P.V module on earth. It has minimal embodied energy, ( unlike crystalline) is cheap, temperature stable, very simple to install and practically indestructible as it is formed onto a steel sheet, without glass. It has a 20 year performance warranty, with a 30 year estimated lifespan according to accelerated aging tests. Don't even consider using anything else. Would you accept a 20 year warranty on a sheet of glass ??
  

• For the energy storage battery, as stand alone ( 10 to 15 days storage) we select 12 x UNI-SUN 12 volt, 250 ampere hour, maintenance free batteries, wired as three "quads" in series / parallel. Total capacity 750 ampere hours x 48 volts = 36,000 watt-hours @ C100 .( 100 hour discharge rate) This particular battery bank is shown in the price lists, complete with its mounting frame.

• In a grid switched system with mains backup, ( we use 4 of the above batteries, daisychained into a single series quad. Total capacity 220 ampere hours x 48 volts = 10560 watt-hours @ C20 ( 20 hour discharge rate)

• The UNI-SUN battery is a closed cycle, lead / calcium unit. Safe and clean to use, never needs maintenance and is eminently suitable for use in a domestic or office situation, unlike dirty, smelly, old fashioned types that regularly need water, and that can explode in your face if you arc them out whilst they are gassing hydrogen .
  
  For the battery regulator and control system we use the PLASMATRONICS PL-60 "Smart controller". The optional PLS2 adaptor is used, together with the SH200 current shunt, installed in the D.C battery input to the inverter. This enables you to monitor all solar energy generated and consumed on a daily basis. The PL60 also incorporates a built in 30 day data logger . Critical information for effective management of your power system. A single phase, kilowatt hour meter is also installed, so one can easily ascertain total solar power production over time.
  

• Finally we need a "true sine wave" D.C to A.C inverter to convert the direct current, generated by the P.V panels, and stored in the battery, into 240 volts A.C, to power up your carefully selected suite of high efficiency appliances. Choice of inverter is determined by the average, short term and peak loads presented by the appliances. In this case, a Latronic LS2548 or similar, would be a appropriate, with a continuous rating of 2500 watts, half hour rating of 3000 watts, and a surge rating of 7500 watts.