How to size a Solar Panel & Battery

Having developed small led lights I wanted to set up my own home lighting with some of these. From that experience I want to put this information up here for someone who wishes to use slar pv for his home.
  1. Start with the load - How many lights will you use - In my case each Light is either 1.5 watts or 2.5 watts. Let us take the total load in watts as X.
  2. You may want to use this load for Y hours a day.
  3. Calculate the Solar Panel required with a simple formula: Panel required = 2*(X*Y/6) for regions between the two tropics and = 2*(X*Y/5) for regions away from the two tropics. Notice that the difference is the figure 5 in place of 6 - you may take anything in between for regions closer to the tropics.
  4. The Panel size that you arrive at in step 3 is to be rounded off to the nearest available Panel. Remember we are talking here of 12 Volts Panels which usually have 36 cells in series. Let us say you arrive at Panel size of A watts. This is the size you will use.
  5. Now about the Battery: Your Panel size is A watts. Minimum Battery capacity required will be = A*12/17. The unit for this is AH. It is always preferable to use a higher battery capacity; hence round this figure to the nearest HIGHER capacity commercially available.
  6. One important point - I sometimes prefer to work back ward starting from battery capacity, since some capacities are very common and I prefer to use these - They turn out to be the most cost effective & they are likely to be fresh from factory (rather than ones that have remained on the shelf of the dealer - which is not good for the battery).