How do I tilt my panels when roof faces east?

[Bugout1]

My roof faces east, gets plenty of sun in the morning, but not direct sun in the afternoon. Is there an auto tilt mechanism to keep my panels facing the sun?

 

[Hedges]

While there are some tracker mechanisms, they probably cost more (per kWh harvested) than more panels.

If you populate your entire roof with panels (or as much as allowed if walkways for fireman are required), that will get full sun in the morning and the panels will never shade each other.

If you use either a motorized tracker or a fixed tilt to get more sun later in the day, such tilted panels will cast a shadow on other panels further down the roof. You would have to put less panels up to avoid that until somewhat later in the day when a single row of tilted panels at he ridge would shade the entire roof. Only a panel casting a shadow on the ground to the East of your house is capturing light that a flat array would not have captured.

To get more light later in the day you need either panels on a West face, or panels that cast a shadow on the ground.
Sometimes people make an awning that shades the yard. Could you make an awning tilted up rather than down? It would catch lots of afternoon sun. If not to steep, it wouldn't shad the roof which could catch morning sun.

Or, you could make a rack that starts at the ridge and slopes up not down (a West facing PV panel "roof" above your existing roof.) It would be quite tall.

Any particular reason you want Western exposure and aren't happy with just Eastern?

If anything, I would consider a Southern tilt of panels, and spacing between columns of panels, to catch the same amount of sun in Winter with half as many panels as required to cover the entire roof.

 

[svetz]

If you're in an area with high winds (e.g., hurricanes) you'll want them flat against your roof, even a little tilt is a huge amount of force.

If you want to see how different factors (e.g., tilt, number of east panels or west panels, orientation) effects the economics based on your local weather (e.g., if you get frequent afternoon storms/cloudiness east is better!), check out SAM.

 

[Bugout1]

While there are some tracker mechanisms, they probably cost more (per kWh harvested) than more panels.

If you populate your entire roof with panels (or as much as allowed if walkways for fireman are required), that will get full sun in the morning and the panels will never shade each other.

If you use either a motorized tracker or a fixed tilt to get more sun later in the day, such tilted panels will cast a shadow on other panels further down the roof. You would have to put less panels up to avoid that until somewhat later in the day when a single row of tilted panels at he ridge would shade the entire roof. Only a panel casting a shadow on the ground to the East of your house is capturing light that a flat array would not have captured.

To get more light later in the day you need either panels on a West face, or panels that cast a shadow on the ground.
Sometimes people make an awning that shades the yard. Could you make an awning tilted up rather than down? It would catch lots of afternoon sun. If not to steep, it wouldn't shad the roof which could catch morning sun.

Or, you could make a rack that starts at the ridge and slopes up not down (a West facing PV panel "roof" above your existing roof.) It would be quite tall.

Any particular reason you want Western exposure and aren't happy with just Eastern?

If anything, I would consider a Southern tilt of panels, and spacing between columns of panels, to catch the same amount of sun in Winter with half as many panels as required to cover the entire roof.

Thanks for your reply
I have always read that panels should always face south, so wasn't sure if it would work at all with facing east. You're telling me it will work, at least in the morning. That's good. As far as west facing, I can do that on my garage roof, but then would need a way to switch the output at noon, from the ones on the back of my house roof, to those on my garage roof.

I get what you're saying about south tilted panels casting a shadow. Hadn't considered that. Thought about the walkway idea. I have a sunroom that would block sun from where a walkway cover could go, so it would have to be so high as to require too much infrastructure to support and look out of proportion for my little back yard. The Mothership (mother in law cottage in my back yard) has a great south facing roof, but it only has 2x4 roof joists and no attic. I wouldn't trust it to support the panels (and it's not entirely mine to mess with). Since I only need 1.6 kw to charge my battery in 5 hours, and I'm planning on 2kw worth of panels, maybe it will be enough with just morning sun.
Thanks a bunch for your feedback on this.

 

[Hedges]

By "Walkway" mean one required on top of the roof for firemen to access, to hack a hole in the roof for venting smoke - depends on building codes. Originally no requirement. Then 3' on two sides and top edge of roof face. Then 3' walkway on whichever edge fireman would access plus 18" both sides along ridge (to hack vent holes). You need to see what PV codes are for your city/county/state and follow those.

If panels are flat on a flat roof, or on an East or West facing slope, then in winter the oblique angle of light from the sun produces significantly less power. Light passing through a small rectangular area (representing panels aimed directly at the sun) is spread over a large area of panels. The small area is how much light you capture, power you can produce.

With East and West, you will get a lot of power during summer but little in winter. Not bad for net metering, where power is "stored" as a credit on your bill, but for actually powering the house during grid outages or offgrid, not very good.

No need to switch anything. Just have a series string (however many panels add up to proper voltage) on the East roof, and connect in parallel same number of panels on the West roof. (Or, some inverters have multiple MPPT inputs, in which case connect one string to each.) So long as two strings of PV panels are same length, they can be connected in parallel. Peak output is determined by area exposed to sunlight, considering the angle between them means less area than if all oriented the same. You will get to install more panels than if all one one face of the roof, produce more kWh/day.

2x4 roof joists are plenty to hold PV panels. Our "no mechanical permit required" rules apply if PV mounts are no more than 40 pounds per attach point and some number of pounds per square foot (I forget the number.) The weight of PV panels isn't much; mine are around 2 pounds per square foot. Probably they figure at 40 pounds, screws into sheathing are sufficient. If you lag screw standoffs into the joists, it will be very secure.

"Since I only need 1.6 kw to charge my battery in 5 hours, and I'm planning on 2kw worth of panels"

Check a solar calculator for your location and roof slope/orientation. It will be fine in the summer, but in winter some areas get as low as 0.8 hours effective sun, and that for fixed orientation facing South. That would only deliver 1.6 kWh/day, not 1.6 kW x 5 effective hours = 8.0 kWh/day. 5 effective hours is a reasonable average over the year, but you need to plan for shortest day of the year which might be only 2 hours effective sun.

You can get 2kW of panels for as little as $400, so how about at least 4kW, half facing East, half facing West? Probably a bit more, since 4kW STC of panels is more like 3.5 kW actual output.

 

[JoeHam]

As far as west facing, I can do that on my garage roof, but then would need a way to switch the output at noon, from the ones on the back of my house roof, to those on my garage roof.

Put each array on it’s own MPPT controller, connect both to the battery.

No switching needed.

 

[Hedges]

Put each array on it’s own MPPT controller, connect both to the battery.

No switching needed.

But you can get by with less total wattage of MPPT controller by connecting both arrays to a single controller.
Penalty is about 2% power loss, less than the cost of extra controller (assuming neither array gets major shading from trees.)

 

[Bugout1]

By "Walkway" mean one required on top of the roof for firemen to access, to hack a hole in the roof for venting smoke - depends on building codes. Originally no requirement. Then 3' on two sides and top edge of roof face. Then 3' walkway on whichever edge fireman would access plus 18" both sides along ridge (to hack vent holes). You need to see what PV codes are for your city/county/state and follow those.

If panels are flat on a flat roof, or on an East or West facing slope, then in winter the oblique angle of light from the sun produces significantly less power. Light passing through a small rectangular area (representing panels aimed directly at the sun) is spread over a large area of panels. The small area is how much light you capture, power you can produce.

With East and West, you will get a lot of power during summer but little in winter. Not bad for net metering, where power is "stored" as a credit on your bill, but for actually powering the house during grid outages or offgrid, not very good.

No need to switch anything. Just have a series string (however many panels add up to proper voltage) on the East roof, and connect in parallel same number of panels on the West roof. (Or, some inverters have multiple MPPT inputs, in which case connect one string to each.) So long as two strings of PV panels are same length, they can be connected in parallel. Peak output is determined by area exposed to sunlight, considering the angle between them means less area than if all oriented the same. You will get to install more panels than if all one one face of the roof, produce more kWh/day.

2x4 roof joists are plenty to hold PV panels. Our "no mechanical permit required" rules apply if PV mounts are no more than 40 pounds per attach point and some number of pounds per square foot (I forget the number.) The weight of PV panels isn't much; mine are around 2 pounds per square foot. Probably they figure at 40 pounds, screws into sheathing are sufficient. If you lag screw standoffs into the joists, it will be very secure.

"Since I only need 1.6 kw to charge my battery in 5 hours, and I'm planning on 2kw worth of panels"

Check a solar calculator for your location and roof slope/orientation. It will be fine in the summer, but in winter some areas get as low as 0.8 hours effective sun, and that for fixed orientation facing South. That would only deliver 1.6 kWh/day, not 1.6 kW x 5 effective hours = 8.0 kWh/day. 5 effective hours is a reasonable average over the year, but you need to plan for shortest day of the year which might be only 2 hours effective sun.

You can get 2kW of panels for as little as $400, so how about at least 4kW, half facing East, half facing West? Probably a bit more, since 4kW STC of panels is more like 3.5 kW actual output.

I don't know where you find 2kw of panels for 400.00. I was looking to save roof space with a panasonic over 350 watt panel so I wouldn't need to series them, just 6 in parallel . It runs around 345.00 per panel. I can get cheaper panels with lower output, but don't have that much roof space. I think I will get the rack and mount on my garage roof first. That will take the longer wires to the power shack, and is an older roof so I can afford to learn on it. Then when budget permits, do the east facing house roof using the same length wires. If you're saying that I won't be losing amps by having both sets of panels connected, then the 2 roof option would be the best one. Thanks, and appreciate any input you may have on where to buy, low cost, high wattage panels.

 

[snoobler]

My roof faces east, gets plenty of sun in the morning, but not direct sun in the afternoon. Is there an auto tilt mechanism to keep my panels facing the sun?

You can use link #6 in my signature to identify your location, array size, panel orientation and tilt. This will report annual kWh production and kWh by month. It will also return your solar radiation hours per day by month. It also factors in weather's impact on your production based on averages.

You can play "what if" by changing orientation and tilt.

Unfortunately, it doesn't permit arrays in multiple orientations, but you can get results for them separately.

 

[Hedges]

I don't know where you find 2kw of panels for 400.00. I was looking to save roof space with a panasonic over 350 watt panel so I wouldn't need to series them, just 6 in parallel . It runs around 345.00 per panel. I can get cheaper panels with lower output, but don't have that much roof space.

Check out SanTan solar.
New/used/damaged panels ranging from $0.13 to $0.50/W, a few at higher prices.
For mounting on the house, skip over the ones that had UL stickers stripped off. Skip the ones with cracked backsheets. Maybe "snail trails" are OK.
Plenty of choices with fairly high watts/meter.
A lot of what he sells are from commercial ground or rooftop arrays that were taken down for some reason. Some are new with factory warranty.

Here's $0.28/watt, no factory warranty but seller gives 1 year warranty. 147W/m^2 so about 14.7% efficient, would occupy 33% more area to match high efficiency panels.

SunTech 285W | SanTan Solar

store.santansolar.com

Here's $0.43/watt, factory warranty, 191W/m^2 or 19.1% efficient

Seraphim 370W - SanTan Solar Store

These are new Seraphim 370W 72 Cell mono solar panels. They come with the manufacturer's warranty: standard 15/25 year warranty through Seraphim.

store.santansolar.com