What are the best strategies for orienting your array to get the energy you need?
Rule of Thumb
There is a very general approximation based on the fact that the sun does not stay at its low point or high point (solstices) for very long. These approximations will change slightly due to local seasonal weather patterns.
Best Summer Angle : Latitude minus 15 degrees
Best Winter Angle : Latitude plus 15 degrees
This means that for a city located on the 50th parallel, you would want your array to be tilted close to 35-degrees from horizontal in the summer, and 65-degrees in winter.
Want to be precise? Use a tool like PVWatts to show you the effect of choosing different angles for your array. Enter your city, use an array size of 1kW to keep things simple, and then change the array tilt angle to see what happens to your total annual yield.
Strategies
- The Residential Roof Case
You don’t want things to get ugly, so stick with a flush-mount racking system which looks good on your home. But, this also means you get whatever energy your roof’s geometry facilitates.
Unless you are building a new home and plan to build your roof facing due South at something near an 8:12 pitch, there’s not much you can do but forgive the original planners of your neighbourhood who didn’t consider sun angles when planning streets and lots. - The Mobile Case
Flat on your RV or VAN roof is the least trouble for mounting and wind-loading, and is omni-directional from the sun’s perspective. But if you want to park in the shade, consider a portable panel that you can move every few hours to aim at the sun. - The Off-Grid, Cabin or Home
You will want to maximize your worst-case months, so use your winter angle (or latitude if working with a 3-season home). Ground mounts, pole mounts, and tilt-up roof mounts are all ways to tilt the array the the angle you want. The stepper angles will also ensure adequate snow-shedding.
What About Seasonal Adjustable Tilt Ground Mounts & Pole Mounts?
Seasonally adjustable ground mounts & pole mounts are available, but they cost more, and the increase in yield through the winter months may not be worth the added cost.
Let’s take an example… and we’ll pick on Winnipeg, but will leave snow out of the equation for now.
According to PVWatts, if you leave your array tilted at your optimum summer angle (38-degrees in this case), which maximizes your best production months, your winter yield (October through March) will be : 509kWh for a 1kW array.
If we change the tilt to 68-degrees for those same winter months, the yield increases to : 538kWh.
A difference of 29kWh.
Now, if you had a 10kW array instead of 1kW, this would be a 290kWh difference. At an arbitrary electricity feed-in tariff of $0.10/kWh from the utility, this means the additional energy is worth $29 per year. So the question is then: How much more is the adjustable array than a fixed-tilt array, and is it worth the extra cash?
More Off-Grid Considerations
If you size your array to meet your winter needs, won’t you be overproducing in the summer?
Yes, and meeting the needs of an off-grid home using solar in winter months can mean a very large solar array! So what can be done?
- Equinox Priority : 3-Season Yield + Generator Back-up
Size your array for meeting your needs at the equinox sun angle, and then use a generator to make-up the shortfall every few days through the darkest months. Yeah, recommending a generator is not fun, but it is a handy back-up to have. Many folks also supplement with wind or micro hydro if they can. - Tilt your array vertically
Not only will that naturally, geometrically, attenuate your summer yield, but it will also keep your array snow-free in winter.
Our 1kW Winnipeg array will yield 1333kWh annually if left at the summer angle (and snow-free through manual labour), but will still yield 1040kWh when installed vertically (with no manual snow removal). That’s 78% of your fixed-tilt maximum. Not bad! - Split your array South-East & South-West
What does this do? Instead of maximizing your midday yield, it flattens your daily yield curve & shifts part of the yield East to recover from overnight loads faster. Similarly, the Western half picks-up more energy later in the day to ensure a good charge before the evening hours. Yes, you give-up some of your maximum yield for the day, but the goal is better charging at important times of day.
In our Winnipeg example, splitting the array so that half is 45-degrees East and half is 45-degrees West of due South, the total annual yield of our vertical array falls to 996kWh, which is only about a 4% reduction. Add another panel or two in each direction to compensate… they’re cheaper than replacing batteries!
That’s all for this installment.
If you have any questions about any of the numbers or tactics illustrated here, or would like to discuss your project, don’t hesitate to reach out.
Cheers!
– Moe