100% Solar ShantyBoat/Houseboat

[Amy@altE]

Amy my protein computer says you made wrong assumptions and might be leading you to the wrong conclusions. He is using PWM controller, not MPPT which drops system efficiency to 50% rather than the 67%. I have not checked the table for 0 degree tilt, but 2.39 hours sounds a bit high in December.

I used http://solarelectricityhandbook.com/...rradiance.html . I don't know how accurate they are, but I've been using it lately. Screen shot attached. SC_flat.jpg

NREL uses .77 for grid-tied, and I can't remember the source I got .67 for off-grid, but I've been using that since I was taught it in 2007. One of the reasons I like being on this forum is it challenges me to question and verify what I've been taught, and to further my learning. So I hit the books looking for the source. Solar Energy International (SEI) has in their latest book (2013) .65, but they don't specify if they are using PWM or MPPT. The example was for a 1kW array, so it could easily be either. Although I always round up, I don't think I round up that much. Based on the feedback I've gotten from the hundreds (thousands?) of systems I've been involved in, and I'm still at the same company (but no longer in Sales), I'm pretty comfortable with the numbers I've been using. So my first systems are over 7 years old, I expect they'll have to start replacing the batteries soon. But stay tuned, I'm definitely going to research this a bit more.

[Sunking]

OK maybe this will help.

For PWM you are correct in assuming 66 to 67 efficiency or for every 100 watts of panel into PWM you get 66 watts out. That is pretty easy to follow. However battery charge efficiency and wiring losses have not been accounted for at this point. A good optimistic number to use for that is 80%. So when you combine the two losses a 100 watt input gives you 100 x .66 x .80 = 52%. Throw in Inverter losses and you are at 50% or less.

A good rule of thumb to use for bar napkin designs for off-grid is to lump all losses together including inverter is 50% for PWM systems and 67% for MPPT systems. That gets you pretty close. Once you have all the equipment specs, wire runs then you can zero in. After you do that a few hundred times you discover the Bar Napkin approach is real close to real application data within +/- 5%. Personally I always round up with a little over kill of 5% because I sleep better at night knowing I got a little extra to cover my butt.

My formula for PWM is: Panel Wattage = [Daily Watt Hours x 2] / Sun Hours. For MPPT: Panel Wattage = [Daily Watt Hours x 1.5] / Sun Hours

If you prefer that can be changed to:

PWM: Panel Wattage = Daily Wh / .5 / Sh
MPPT: Panel Wattage = Daily Wh / .67 / Sh

Either road traveled gets you to the same place. It is just easier for me to do in my head my way.