In application, DC electric heater maintain the water (approx 50 liters) temperature to 70 degree using battery bank and PV panels, which will operate 24×7.
How should I design the solar system for this application…
Don't know you environmental duties or solar insolation, but something like a thermostat connected to the heater and the heater connected to a battery bank and a set of batteries, I'd run a DC system and have the thermostat run a relay. Figure your maximum daily load and multiply that times the maximum days with out effective sun for your battery storage then multiply that inversely times the maximum you should take the battery banks SOC "State Of Charge". Then take your battery bank capacity and figure a solar array that can charge the batteries at a 13% charge rate.
Let me ask you, How do I get to Cleveland?
Well you've given a tiny bit more information, but I'm still trying to get to Cleveland...
Solar Insolation can be found for your are, it will give better than approx." numbers, it will give history and if in the US you can also find numbers that will reflect the effective numbers given the angle and direction of your array.
I now have the volume of water, and the desired temperature, Those are 2 of many factors in determining load. Is the water in an open bucket, such as an animal watering bucket? is it in an insolated water heater, how much insolation? What is the environment, if you are in Alaska where the mean temperature is 5 C or in a southwest desert where the temps are 30 C. How often will the water be used and fresh water be brought back up to 70 C? What will be the water's starting temperature?
8 liters or 2 1/2 gallons of water is not a lot, I suspect you can do this with a fairly small system, if the load is minimal, if it's cycling. 70 degrees is below purification temperature, so I suspect it won't be cycled too often? I think 70 C is also higher temps than is recommended in most water heaters, I think they shoot for 130-150 degrees Fahrenheit. It will likely be a temperature that will require a good bit of insulation and energy to maintain with in a reasonable load.
Wish I could give you a simple answer, but it's not a simple question.
Delhi rarely gets below freezing;
A Bread Box type heart is likely all you would ever need, if this is to provide hot water for home use. Cheaper and arguably more reliable than a solar electric system, try to have enough volume for a few days and to even out the heating (takes longer to heat up and cool down)
I've even seen large water containers 55-225 gallon on roofs in Puerto Rico, with no real reflectors or insulation.