I just finished up an off grid project on our classroom. My advice would be to always oversize what the calculators say you need. You will lose efficiency over time on both the planels and the batteries. I have three 200 watt panels on the roof with 560 Ah of wet cell batteries. It runs everything I need for 3 days of classes (lights, computer, big screen TV for power points, on demand water pump, etc. but bad weather and winter reduces its capability. I started with 400 watts of panels and that wasn't enough so I added another. I live in Alabama so that's an advantage over higher latitudes.