Typical DIY Question: I have a 35 ft x 16 ft. Roof

This is a typical DIY Question on a forum

"So i need help .. I need to know how much power I'll need to operate this boat... washer dryer. Ac unit. Lights 10 plugs water heater. Basically a whole house on water. If someone can help me get the right direction on how to start the power . It's a full time live on boat and don't want to use gen no more except for back up. roof is 35 ft x 16 ft."

To get a rough idea of how much power you can get out of a given area roof, let's just start with some rough back of the envelope calculations.

We know that the roof is 35ft X 16 ft.

Now a quick search on Google with the prompt of "what are the ranges of dimensions of modern solar panels and watt output" tells us that:

"Modern solar panels typically range from around 5.5 feet by 3 feet in size, with a power output between 250 and 450 watts for residential use; larger commercial panels can be significantly bigger and produce higher wattage, reaching up to 500-600 watts or more depending on the number of cells "

So let's do a simple calculation to see 1) how many panels will fit in that area, and 2) How much power will they produce.

Remember Area = Length * Width

And Number of panels that you can fit in a given dimentson is Distance Over All / Length of Panel along that Dimension

So in the above case we've got 35 ft / 5.5 feet = 6.36, which we have to round down to 6 whole panels which will fit in the length of the roof.

And we've got 16 feet wide / 3 feet of the width of each panel = 5.33 panels which rounds down to 5 whole panels to fit in the width of the roof.

So we can fit 6 panels by 5 panels = 6 * 5 or 30 panels total.

Now we have to figure out approximately how much power they will produce. You can get used panels cheap like $20/pop and at like 175w. And in the case of used panels, they are generally still delivering between 80%~95 of their rated power. While older panels are generally in the 185w-250w, you can get deals on newer ones of 300-485w. And brand new panels are as high as 675w-800w.

So let's compute the output for a couple of given outputs.

1) Old Used panels at 250w * 30 panels = 7.5 kw

2) or Old Used panels at 450w * 30 panels = 13.5 kw

3) Not so old panels at 500w or 600w * 30 panels = 15 kw or 18kw

4) New panels at 800w * 30 panels = 24 kw

So you can see there is a very wide rang of possible power outputs anywhere from 7.5kw to 24 kw. Of course at the top end of power with new panels will be larger than our assumption for the number of panels, so there won't be as many panels that will fit in the given area.

Hopefully, this will give a quick rough guide to figuring out 1) how many panels will fit on a given area according to its length and width, and 2) how much power we can get out of those panels given the individual power output from each panels times the power output. This could be expressed as PW(tot) = PW(indv)*Number of Panels.