Please review some of these informative sites:
It is true, the application of your PV panel and battery system, determines the overall expense. If you only want to run an Okapi System off the battery (and no other appliances), we suggest you look at 40Ah and above for your 12V battery. Batteries with less than 40Ah may not get you though a full semi-cloudy week where the Okapi fans may be running partially, giving you some heat-harvest, but the solar panel may not be charging the battery at all.
Here are the power draws to consider (The values are rounded-up from the actual draws to ensure you do not choose a battery insufficient for your needs):
* These values are rounded up from the actual measured draws to ensure you get a good battery, charge controller and panel system to run Okapi optimally.These values should allow you to do the necessary simple calculations to choose a 12V "Deep Cycle Battery" that will last for many years, require little to no maintainence and always be able to do the job for you.
Good luck getting your PV panel and battery system set up. Once you do, it will feel great to know you are free from the grid!
Do keep in mind that the Okapi fan control systems have a very low running cost, and will barely effect your power bill, if you do choose to simply plug it into the mains power supply.
Please see the following FAQ link to learn more about the basic PV panel size we suggest: