Overestimate and you might end up spending more than you need to. (It might be fun but most people don't buy solar water heater price in Chennai an expensive Ferrari for a quick solar water heaters in Chennai trip to the shops).
There are three main stages to a solar powered system.
Power generation (the solar panel)
Power storage (the battery)
Power use (your items you want to run off solar power)
To work out the wattage correctly, the panel needs to solar water heaters in Chennai be sized according to how much power you are going to solar water heater price in Chennai use. (Later on you're going to need a bit of info on each electrical item you need to be solar powered).
The battery, no matter how large or small, is a solar water heaters in Chennai storage container for solar power gathered in daylight for use immediately or for later on. Highly important. For example without a battery, a solar solar water heater price in Chennai powered torch would be rather pointless in the dark!
Just remember that to maintain the solar water heaters in Chennai power storage at a constant level the solar panel needs to put in to the battery the solar water heater price in Chennai same amount of power as is going out of the battery now (or will go out of the battery later on).