ORCHID ISLE SOLAR  ​808-345-0660  719-330-2597  or John@orchidislesolar.com

Continued from previous page (Battery sales):

On the previous page we gave an example of a small system whose loads need 9.820kwh a day to operate. Based upon that data we have sized a 24 vdc battery bank (3 strings of 4 batteries each for a total of 12 batteries) for a system capacity of 1170 AH and a 48vdc battery bank (2 strings of 8 batteries each for a total of 16 batteries) and a capacity of 780 AH.

       At first glance, the 780 AH 48vdc bank seems smaller than the 1170 AH 24vdc bank since the 24vdc battery bank's AMP-Hour capacity of 1170 AH is larger than the 48vdc's 780 Amp-Hour capacity. However, remember that for a given wattage, the 24vdc battery bank will require twice the current as the 48vdc battery bank. In reality the 48vdc battery bank has a larger capacity due to the smaller currents that will be taken out of the battery bank for the same load. The 48vdc battery bank is actually (780/1170)*2= 1.33 = 33% larger in actual ability to deliver current-- its AMP-HOUR capacity versus current required for a given load-- than the 24vdc battery bank.

Solar Array sizing:

In order to size the solar array, we go back to the loads which are figured at 9.82kwh. In order to determine the size of the solar array we need to know the Peak Sun Hour data for the locations of the solar array. A Peak Sun Hour (PSH) is a measure of the available sun insolation-- available energy from the sun-- for an area measured over the typical time period of a month up to a year. 1 PSH is equivalent to 1000watts of power striking a square meter of ground for 1 hour.

      In our example, lets assume that the annual PSH for our area is 4.8 and the lowest PSH is for December is 4.4. For an off grid system we should ideally design the system for the shortest month of the year which is December. Many systems are designed for annual PSH; the trade off is that the system designed for minimum winter sun will be able to run longer on batteries without relying on another source of energy such as a backup generator, but it will cost more in terms of a larger solar array for the system. If the solar array is designed for the annual PSH figure it will necessarily have a smaller solar array (less expensive initially) but the homeowner will need to run a back up energy supply such as a backup generator more often-- especially in winter when the days are shorter.  This is similar to what we found with the battery bank sizing; there is always a trade off between initial cost and cost over time. If the budget will tolerate it it is always better to spend a little more up front for a larger solar array and reap lower back up operating costs over time.

9820 wh/ 4.4 (winter PSH) = 2232 kw of solar required for the array. We also need to account for system losses which in this type of system usually account for about 35% to 40%. 2232 watts/.65 = 3434 watts.

In summary;

        9.820kwh of loads would require a battery bank of either 1170 AH (24vdc) or 780 AH (48vdc)  and a solar array of approximately 3434 watts. If a 280 watt panel was selected one would need 3434w/280w = 12.26 or about 12 panels.

​This is a simplified example since there are many other factors that go into the design process including roof slope issues, shading, azimuth of the slope. However, it does give an idea of the design process.


John McDonald

808-345-0660  719-330-2597