PV Gap

Leslie Said:

What is the optimum tilt angle for a solar panel in our area? What would the tilt?

We Answered:

That's actually a more complicated question than it might seem at first.

The simple answer for anyone above 23 degrees latitude is to tilt at latitude, so if your house is at 35 degrees latitude, you would tilt at 35 degrees.

However, the best financial payback may be to optimize for summer (latitude + 15 in the northern hemisphere), if you have air conditioning bills that kick up your electric rates into the expensive tier. On the other hand, if you live in an area that gets very hot, the panel efficiency will go down, so it might be better to go latitude + 5 or so.

To optimize for Winter, the simple answer is latitude - 15, but again, not so simple, if there's a chance you'll get snow on the panels.

I have our panels set up so that I could readjust the tilt twice a year if desired, but I have never done so, yet. Too much trouble for a rooftop installation. And I'm concerned about 100 mph wind gusts picking up the panels in winter if I were to tilt them at a high angle. As far as I know, the vast majority of residential installations just follow the slope of the south-facing roof and take what they can get.

If you're talking about water heating panels, I think the general practice is to just match the slope of the roof, and point them southwest (not south) in our area, because that's the optimum balance between the sun and the daytime temperature.

Keith Said:

What solar panel is needed to charge four 12 volt batteries?

We Answered:

It's impossible to answer accurately without knowing the capacity of the batteries, their chemistry, and how they're hooked up (series or parallel). If you have specs on the batteries, that will help someone to answer.

Yolanda Said:

How to select solar panel for this application?

We Answered:

First, you need to make sure that your battery can take a regular 34Ah discharge without damage. The usual wisdom when using deep cycle lead-acid batteries is to allow for no greater than 50% discharge. This is to prevent the battery from ageing prematurely. Note that automotive batteries are not usually considered suitable for deep-cycle applications. Best allow for at least 200Ah capacity.

Your solar panel capacity needs to take into account that whatever the nameplate output of the panel, that will be what it will produce under ideal conditions, actual output will always be less, often quite a bit less. You also need to allow capacity to cope with dull, overcast days when the light level is very low.

You can get solar maps that will give you the sunshine hours and irradiation levels at your location.

The solar panel capacity ends up being a juggling act between available funds/space and how critical it is that the light always works as desired.

Let's say you had a 400W panel, which sounds quite generous. In the middle of winter, it may produce only 75W for 5 hours on a dull day, not quite enough to run the light for 10 hours. Would you expect several such days in a row? How many days could a 100Ah of battery capacity cover under these conditions?