Solar panel angles.
 

I'm sure panel angles have been beaten to death but maybe one more time? Right now I am tilted latitude plus 15 degrees. Pretty good for winter but I am thinking about locking them down permanently and adding another string. I read recently that a good full time angle would be lat * .83 plus three. What do you guys think? And on that 0.83 is from faulty memory, but is close. It seems that would more favor a summer angle. My latitude is 37 degrees give or take a decimay.

Thanks
Dennis

Dennis,

How much does your kWhr usage differ between winter and summer? If you use a LOT more electricity in winter, then adapt an angle that maximizes collection at those months.

With that same steep angle, you will still get a lot of power in summer months simply due to longer day.

The key is to know your winter/summer consumption ratio. Then you can set a rational collection angle.

Steve

You should be able to use the current version of PVWatts to predict any scenario you want, just keep changing up the tilt value.

For me, (lat*.83)+3° comes up 1.55° less than lat.

Another point to consider is the cost of each kWhr in summer and winter. The difference is often small, with the summer to winter difference being far greater.

Steve

And up north where I live, I would seriously consider installing solar PV panels vertical:

Snow won't cover the panels in winter.
Reflection off snow in winter will help.
No worries about hail damage.
Summer power generation will still be greater than winter.

Thoughts?

These posts are naive, in that they seem to be assuming perpetual blue sky.

Every locale has seasonal trends regarding cloud cover. It may be unadvised to maximize your angle for a season when the cloud cover is greatest, even if your power demand was also greatest during that period.

You need to dig deeper and find out what the statistical likelihood is of cloud cover during various parts of the the year, and include this in your considerations.

If you are able to sell energy to the grid, then 'sail when the wind blows'.

Best,

-AC_Hacker

I'll put my 2 cents in for PVwatts as well. It knows all the info for your area and can accurately estimate power output for a given array size. It knows cloud cover, average hours of light per day for each month, etc. That way there isn't much guessing at all.

PVWatts Calculator

JR, your perspective is an easy one - "just mount them vertical", but there are many factors to consider.

Summer/winter kWhr use ratios dominant consumption; PVwatts tells you the monthly supply (adjusted for parahelion, clouds/rain/snow, etc).

The local utility determines how much you can carry over month to month (aka "bank") when you have more production than consumption. Pinball (I believe) has a utility that averages over the entire year. Thus, he is able to "bank" Kwhrs that he supplies to the grid in the months of high production (April, May, June, July), but low usage. Then he can draw from that stored energy bank in times of high use or little PV supply.

Others of us have utilities that only bank one month. If you have excess production, this is often paid back to the producer as a net revenue adjusted Kwhr rate (typically about 1/2 the retail rate).

Knowing these determinants can be a BIG in terms of engineering a system. For example, in my planning, I saw low Kwhr consumption in the months of March, April and May. But this is the time that south facing panels (regardless of tilt) are producing the most energy. Contrast the months of December, January and March when the opposite is true. My geothermal heat pump is busy consuming electricity just at the time when the panels are at their production nadir.

To provide 100% of electricity for those cold months would take a HUGE panel system and my overproduction in March - May would be vast. To my dismay, my coop only reimburses excess on a monthly basis as the net revenue. If you don't use the excess "banked" Kwhrs in the next monthly, you essentially loose 1/2 the value. Bottom line, I would be loosing money to have 100% production in the months of Dec-Feb.

This calculus on a balance point is not easy, nor naive. I requires an individual to know their month to month consumption, the location specific PVwatts derived supply and then the utility banking protocol and reimbursement. Then, and only then can the "tilt" be rationally derived from an engineering perspective.

Your easy way (vertical orientation) does account for a significant reduction in hail damage and is one that I had not fully thought of before (pretty stupid considering my location in a hail area)!! One of these days, I will use some geometry to calculate impact loads from vertically falling hail onto a flat surface going through a sweep to 90 degrees (vertical). Of course that does not account for hail coming down at an angle from the south!

Some time ago, I did ask a Texas friend, also in the hail zone, what his PV lease says about hail. They replace the panels at no charge, but have had surprisingly low numbers of panels breaking due to hail over the entire system. In my 30 years here in Oklahoma, we have only had one bad hailstorm where the stones were bigger than golfballs. So I self insure and hope for the best.

With the exception of statistical hail, all the other factors are available to you. I just set up a spreadsheet and did the month to month production (from PVwatts), my five year averages monthly Kwhr consumption and then found out where proposed PV production was more than 100% of consumption. That determined my total kW needs from a PV array. I then tweaked the tilt until I got a yearly adjustment that gave me more Kwhrs in maximum consumption months. I could have gotten out my old diffyQ books and perhaps done it quicker, but there quite the education doing it manually and plotting out the results on a spreadsheet.

The results were surprising. Tilt was not the major determinant I thought it would be. So I am just going to mount them flat on my 3/12 pitch shop roof (14 degrees) and go from there. I did orient the shop so that the southern roof was perfectly orthogonal to due solar south.

I advise all to do their own similar calculations BEFORE installation. Just like "measure twice, cut once".


Steve

I am grid connected. The utility only banks for the following month and make adjustments kilowatt hour for kilowatt hour. I take care of my dad and with a 3500 watt system have never gotten close to producing a surplus kwh.

His oxygen concentrator, de-humidifier, and window air conditioner add up. The system does however have an effect on the electric bill.

I am in Kentucky and we do get a good dose of cloudy weather. I am planning to add another 600 watts of PV to compensate.

Thanks for all the replies. I guess the answer is (It depends).

Dennis

Not much difference. It's an all electric home. The inverter is set up to sell at float voltage selling everything above that. Just doing simple snapshot math right now on a good sunny day with temps around 60* F, the system operates at 80% efficient. That number is more like 60% when its hot outside.