building heat load with heat loss per day=420000 Btu
I have calculated the area of collector required to heat the building is 75m² for water storage capacity  of 3000 liters. Solar insolation of the region is 4Kw,hr per day per m² with collector efficiency of 40%.
Is my calculation appropriate?

I took a quick look at this First off I work in BTU’s so I calculated it that way I would assume that you have at most a 40 degree usable range of water temperatures so the heat storage capacity is3000*2.2 lbs/L*40°F=264 kBtu potential storage capacity. If solar insolation is 4 Kwh/day/m2 *3412 btu/Kw *0.40 = 5459 btu’s per day per m^2. How did you pick 40% efficiency? 264/5.459= 48 m^2 of panel will generate enough to store 264 kBtu. If you account for the amount you are using while you are producing heat,it would be 420kBtus/5.459 = 77m^2 of panel to reach 100% My first question is whether the 42kBTU/day is the coldest day of the year, or an average winter day? I would not design a system to produce 100% of the heat on the coldest day of the year with solar because it would be grossly oversized most of the year Is the 4 Kwh/day/m2 consistent or are sunny days 12 kwh/day/m2 but 2 out of 3 days are cloudy? Does it snow a lot? Secondly, Why not use passive solar or solar air heaters to generate a moderate fraction of the solar gain Generally a lot cheaper then all active solar hot water heating. Third, figuring out where to put 75M^2 of solar panels on the roof is not trivial on most houses, since most panels are only ~ 4m^2, that would be 19 panels ouch. At ~200 lbs each that is 3800 lbs extra weight on the roof

The efficiency of water collector should be higher than 40%. If I am not wrong, the water collector efficiency should be greater than 70%. Well it depends on the manufacturer and the absorption technology but 40% will oversize the collector.

Efficiency of solar hot water when used for heating is 50% if you are lucky. Naresh, what country are you in?

We are from Nepal and we will be importing all of these collectors and radiators most probably it'll be from India.