# how to calculate panel position with only half-day sun?

Solar Expert Posts: 630 ✭✭
I want to add an area light to a medium sized hay barn. Due to limitations of wiring, the 10 watt panel has to be located on the East-facing side. It will feed two series-connected 6 volt, 3.4Ah batteries, on a light switch, to power a 12 V, 4W LED in an all-weather housing. As an aside, I have a cheap 10A CC, but I didn't think I would need it as the panel has a bypass diode on it.

Should I just point the panel at the rising sun, angle it to 45 degrees, and call it good, or is there a formula to figure a better alignment? Thank you.

Re: how to calculate panel position with only half-day sun?

Depends on how far you want to go...

PV Watts will allow you to "test" different panel orientations--And/or, you can download the hourly data into a spreadsheet and (for example), zero out any solar values past 12 noon for the rest of the day--Then sum up what is left.

Note that PV Watts will only allow a minimum of 1kW of solar array... If you load into a spread sheet, you just need to multiply the Watt*Hour numbers by 0.01 (for 0.01 kW or 10 Watts).

Also, you may have to make choices... If you need the power during the summer or winter (such as lighting during winter), then tilting the panel more towards vertical will help the winter production numbers (short days). If you are only out there doing projects in the summer, then tilting the panel towards horizontal would be better...

Or, perhaps, you will want to adjust the panel tilt a couple times a year to optimize your solar collection.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: how to calculate panel position with only half-day sun?

a bypass diode (or even a blocking diode for that matter) will not do anything as far as regulating the charge to the batteries as that is not its function. use the 10a controller.
• Solar Expert Posts: 630 ✭✭
Re: how to calculate panel position with only half-day sun?
niel wrote: »
a bypass diode (or even a blocking diode for that matter) will not do anything as far as regulating the charge to the batteries as that is not its function. use the 10a controller.

There have been several threads I've read here, which disregard CC in very small applications. This seemed to be one of them. Yes, I mis-spoke by using the term 'bypass'. I should have written Blocking Diode, so that the panel isn't back-fed at night. The paper that came with the 10 W panel writes that it contains a blocking diode. Thanks for the correction.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: how to calculate panel position with only half-day sun?

bmet,
like i said use the controller as the current is a large % in terms of charge to the battery capacity so regulation is important here. 10w/12v=.83a. .83/3.4ah=24.5%.

on a sidenote, i noticed what you list in your sig line as 3 ventilation fans and as a diversion load. first of all, solar does not need a diversion load. secondly, if those fans are running all of the time or even often that it could drain off much of your daily charge to your battery depending on the current ratings on the fans and the time they are used.
• Solar Expert Posts: 630 ✭✭
Re: how to calculate panel position with only half-day sun?
niel wrote: »
on a sidenote, i noticed what you list in your sig line as 3 ventilation fans and as a diversion load. first of all, solar does not need a diversion load. secondly, if those fans are running all of the time or even often that it could drain off much of your daily charge to your battery depending on the current ratings on the fans and the time they are used.

Without the fans my battery reaches float before noon, so there is extra power available. I use it to circulate air in my shed, and even with them running 24/7 my battery still reaches float by afternoon.