Off Grid and Bi Directional Solar Calculator

FAQ Notes:

• Where do I find my daily kilowatt hours?
  • There are three ways of obtaining this information. The first is based on your electrical bill. Generally if you are billed directly by Eskom they will reflect your monthly consumption under the title ‘Difference’ and additionally will reflect the number of billed days. Once obtained you can just divide the ‘Difference’ amount by the number of billed days.
  • If you are billed by Municipality, they generally do provide you with a daily average reflected at the top of your bill.
  • If however you are on a prepaid meter (the evil things) then you will need put a power monitor on your board. You can buy them fairly cheaply (R1000 – R1400), they are easy to install and will log your power consumption accurately.
  • Alternatively you can also just calculate how much you are spending per month, divide that figure by your price per kilowatt and divide that figure by the number of days. Not as accurate but still works.
• Why are the solar day hours so low?
  • The solar day is based on a fixed pitch in a perfectly northern direction. The angle of the pitch varies from place to place but is set at the best median for both summer and winter generation. We are completely aware that in summer you will generate more than what is stated here but we need to have a single hour for calculation and the listed hours are reliable as they include loss efficiency adjustments.
• What Voltage should I choose?
  • As a rule of thumb work like this:
  • 0-3000wh (total generation per day) – 12v
  • 3000 – 5000wh (total generation per day) – 24v
  • +5000wh (total generation per day) – 48v
  • Important to note that these figures are rule of thumb. You can squeeze or alter slightly on either side, but bear in mind that higher power(storage and draw capacity) at lower voltage causes system wide inefficiency and increases cable costs etc.
• What DOD (Depth of Depletion) is right for me?
  • This would depend on your battery. In lead acid (read: Gel/ AGM/ Lead Acid) you need to always preserve 50% of your battery as most inverters will cut out as you approach that point. In lithium systems you are still trying to preserve battery but for different reasons. In lithium systems you have more cycle capacity but if you over deplete a lithium battery it can become problematic to restart it. In addition to this, lithium warranties are commonly set around number of cycles, so using as much of your battery as you safely can while avoiding non essential or top up charging keeps the cycle count low and your battery healthy.
• Days of store?
  • Batteries are crazy expensive. They are the achilles heel of a solar system. Wherever possible, use only exactly what you need. Having an extra day or two storage is nice but costly so bear that in mind. In a nutshell should you have days of poor generation (solar) and do not have Eskom as a backup for battery charging then adding additional days of store is a worthwhile consideration.
• Solar panel requirements and batteries?
  • In as far as possible, use as few panels or batteries as possible to make up the required output/storage.

Other useful links:

Grid Tied (No Storage) Solar Calculator

Correct Configurations for Battery Banks

If you have any questions, or would like some additions to this calculator please drop me a mail info@gwstore.co.za