All stand alone solar electric systems require battery storage. Solar Panels charge the batteries during daylight hours and the batteries supply the power when it is needed.
The two most common types of rechargeable batteries in use are lead-acid and alkaline lead acid batteries have plates made of lead, mixed with other materials, sub merged in a sulfuric acid solution.
Alkaline batteries can be either nickel akadmium or nickel-iron batteries. They have plates made of nickel submerged in a solution of potassium hydroxide.
Lead Acid Batteries:
Lead acid batteries are the most common in PV systems because their initial cost is lower and readily available nearly everywhere in the world. There are many different sizes and designs of lead-acid batteries. But the most important designation is whether they are deep cycle batteries or shallow cycle batteries. Shallow cycle batteries are not good choice for a PV systems. Deep cycle batteries are designed to be repeatedly discharged by as much as 80 percent of their capacity so they are good for power systems. Even though they are designed to with stnd deep cycling these batteries will have a longer life if the cycles are shallower letting a lead paid battery stay in discharged condition for many days at a time will cause sulfation of the positive plate and permanent lost of capacity sealed deep cycle lead-acid batteries are maintenance free. They never need watering or an equalization charge they can not freeze or spill so they can be mounted in any position. Sealed batteries require very accurate regulation to prevent overcharge and over discharged. Either of these condition will drastically shorten their lives.
The batteries store the solar power generated and discharge the power as needed. The battery bank consist of one or more solar deep-cycle type batteries depending on the current and voltages for certain applications the batteries are wired in series and parallel.
How to determine the size of your battery bank needed:
1.The first thing you have to decide on is, if you are going to use a 12V, 24V, 48V,96VDc systems. Systems with higher DC voltages lose less energy due to the resistances in wiring that those of lower DC voltage.
2.Determine the total watt hours you will be using per day.
3.Determine how many rainy days you might have in a row these are the days you will need to rely solely on your batteries for electricity during this time typically 3 to 7 days is used for homes, 7 to 14 for industrial purposes.
4.Multiple the watt hours by the number of storage days.
5.Determine how deeply you want to discharge batteries. %80 is considered the max amount you can discharge your lead acid battery bank where as %50 is an optimal amount for batter longevity then divide the amount calculated in step 4 by 0.80 our 0.50
6.Compansate your calculations for low battery temperatures as your batteries get colder, they are capable of producing less current using the table below multiple the value calculated in step 5.
7.Find the watt/hour capacity of the battery you selected. For example the 6V/220 A/H battery so watt/hour capacity is 6Vx220A=1320WH
8.Determine the number of batteries you need: Divide the value you calculated for step 6 by the value you found for step.
9.Round the number of batteries to fit the system voltage you selected for instance 12V system will require 6V batteries insets of 2,or a 24 volt system will required.
Irradiance is a measure of the suns power available at surface of the earth and it averages about 1000 wats per square meter(m2) with typical crystalline solar cell efficiencies around 14-17% that means we can only expect to generate about 140-170w per square meters of solar cells placed in full sun.