I want to develop a system on solar that the load is 250 watt continous. the battery bank should support it to 2 day. can anyone guide me in it like the solar panel should i have it's ratting and about battery bank.
The math is pretty simple. Let's start with your total daily load. 250 watts * 24 hours = 6,000 watt/hours per day.
Now battery capacity. 6,000 w/h/day * 2 days = 12,000 w/h capacity. Actual capacity needs to be twice that, since you drastically shorten battery life if you draw them down more than 50%. Hence, 24k w/h capacity battery bank. Converted to amp/hrs = 2,000 a/h. In case you don't know, that is a HUGE battery bank. Most sailboats in the 35' or so range will have maybe a 500 a/h battery bank. On a 40' boat a 1,000 a/h battery bank would be considered fairly big.
How about solar panels? You need to replace 6,000 watt/hrs per day. Depends on where you are, but in the tropics you can typically take the wattage rating of your solar panels, multiply that by about 5, and get a good estimate of how much energy you will generate each day. So we have 6,000 / 5 = 1,200 watts worth of solar panels. On average, solar panels generate about 9 watts per square foot. Therefore you need 1,200 / 9 = 133 square feet worth of solar panels. That's assuming no shading at all on the panels. Any shading will drastically reduce the panel output.
Like noelex said, you need a VERY large catamaran (like, in the hundreds of feet range) to have that much unshaded space for solar panels. Methinks you might want to find a way to reduce the electrical requirements.