Guestimating Solar production

Yoshi_TAB

Hi, 

First, I know this is a bit geeky and please let me know if anything does not seem correct.  I thought it might be helpful to some...

For those who use solar, I found a neat government website that provides monthly solar energy data in kilowatts for any city you plug in and for every month.  This can allow you to predict how much solar power you will get when you travel to different places at different times of the year.   This is important because the amount of sun in the US SW can be different then the US NE.   The web site is: PVWatts Calculator (nrel.gov)

At the top where it says "get started", you plug in any location, then hit go.  Then hit "Go to system info" on right.  On the top bar you put in your solar panel size in KW.  If you have a 200W panel, you would put in 0.2 KW.  You can play with the other variables.  Then click on the right "Go to PV watts results".

A chart then comes up with the amount of KW produced each month.

You then divide that  number by the number of days per month to get a daily rate in KW.   Then multiply by 1000 to get  the number in watts/day.

To convert watts to amp hours (for battery usage), you divide watts by your battery voltage, usually 12V.

Here's an example; 

Lets say you put in Washington DC ( not that I live there and a 200 watt panel (0.2KW).  For October, 17 KW comes up in the chart.

October has 31 days.  So to convert to a daily amount, you divide 17 KW / 31 days=0.548 KW/day produced
To change to watts, you multipe by 1000.   0.548 KW/day x 1000= 548 watts/day produced

To convert to amp hours you divide by your battery voltage, usually 12 V
548 watts / 12V = 45 amp/hrs per day your solar panel will produce to charge your batteries.

Of course, there are other factors such as panel efficiency, line losses, etc.  But this gives you a good starting point.

Thoughts?

klenger

I've used that site for both the solar on our house and the RV.  Great site.  It seems that the information is gathered from real world data as the predictions seem to vary day to day, probably from cloud cover.

Horigan

Cool site.  The difficulty with T@bs is that the panels are mounted on a curved surface, one in front of the fans and one behind, at least in my case.  This makes it difficult to input a correct tilt angle, which is critical to getting a somewhat accurate estimate