Finally joined the photon-capturing club!

[green1]

PVWatts has a great calculator to figure out how much difference the angle makes, and it's much less than you'd expect. In general if your roof is pointing roughly the right direction it seems to be more effort than it's worth to try to angle the panels to be exactly "right" (not to mention that it changes drastically between summer and winter)

 

[nwdiver]

In general if your roof is pointing roughly the right direction it seems to be more effort than it's worth to try to angle the panels to be exactly "right" (not to mention that it changes drastically between summer and winter)

Agreed; Also, a lot of HOAs REQUIRE the panels to be parallel to the roof... the one time you SHOULD put some effort into deciding the angle is a ground mount...

 

[Zaxxon]

Agreed; Also, a lot of HOAs REQUIRE the panels to be perpendicular to the roof... the one time you SHOULD put some effort into deciding the angle is a ground mount...

I hope you mean parallel.

PVWatts has a great calculator to figure out how much difference the angle makes, and it's much less than you'd expect. In general if your roof is pointing roughly the right direction it seems to be more effort than it's worth to try to angle the panels to be exactly "right" (not to mention that it changes drastically between summer and winter)

I second the use of PVWatts. It's a great resource.

 

[Electricfan]

Continuing with the dumb questions, what if I installed a giant shiny panel, maybe aluminum, to reflect sunlight onto my panels? Would that increase their output?

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I second the use of PVWatts. It's a great resource.

I just ran an estimate using PVWatts - it showed a first year "savings" of almost exactly what the solar installer gave me in the proposal.

 

[green1]

The only way that would work is if they were angled significantly off where you want them to be, if they are aimed straight at the sun, the only way to install a reflector would be to put it in front of them and then they'd be in the shade...
If you look at the angles, there's probably nowhere useful to put a reflector, and if there was, you'd be better off putting a solar panel there instead.

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I just ran an estimate using PVWatts - it showed a first year "savings" of almost exactly what the solar installer gave me in the proposal.

There's a reason for that... most of the installers use PVWatts to do their estimating...

 

[Robert.Boston]

PVWatts has a great calculator to figure out how much difference the angle makes, and it's much less than you'd expect. In general if your roof is pointing roughly the right direction it seems to be more effort than it's worth to try to angle the panels to be exactly "right" (not to mention that it changes drastically between summer and winter)

This is a great tool, and you're right: the results aren't very sensitive to angles.

A general observation from playing around with the calculator in different locations: the angle of your panels should be less steep than your latitude by 3-10 degrees because of the higher insolation in the summer months. E.g., here in Portland ME the optimum angle is about 37.5° (instead of our 43.65° latitude), while in Galveston the optimum is about 22.8° (compared to 29.3° latitude). If you're on an electric tariff that varies by month, this is even more important, because power prices are generally higher in the summer.

 

[green1]

A general observation from playing around with the calculator in different locations: the angle of your panels should be less steep than your latitude by 3-10 degrees because of the higher insolation in the summer months. E.g., here in Portland ME the optimum angle is about 37.5° (instead of our 43.65° latitude), while in Galveston the optimum is about 22.8° (compared to 29.3° latitude). If you're on an electric tariff that varies by month, this is even more important, because power prices are generally higher in the summer.

This will depend on where you live. Around here I use about double the electricity in the winter vs the summer (even though my heating is natural gas), add to the fact that there's also significantly less sunlight in the summer, and I'd be well advised to have the panels steeper if practical.

 

[Zaxxon]

Yesterday we had our first net-zero electricity day. Considering our system was estimated to cover 45% of our usage overall, this is a win. Thus far it has covered 74%. I know this will drop in the winter months, but so far it is overproducing even against June estimates. I'm a happy camper.

 

[slipdrive]

Since our new 9.8kW system went live, I have observed one significant ancillary effect. All 30 panels are on the due west-facing roof, and while the theoretical power due to time and solar incidence is said to be about 17% less than due south, with the entire western roof shaded now, the attic is significantly cooler in the late day. Because the upstairs HVAC unit and ducting is all in this attic space, the system works much better in a cooler attic environment. The power attic vent was never able to keep up with the hot wood deck under black shingles. So, I think the 17% production "loss" is paid back in better cooling efficiency. Another electric-nerd point is that the PV production curve is much closer to utility demand curve. That is, with afternoon production the system is actually backing off real utility marginal generation resources. At least with a coal based utility, they have much less "need" for power before noon, and that leads to lots of whining to the PUC, etc. So anyway, I'm pretty pleased with the Sunpower 327 panels and SMA inverters.
:biggrin:

 

[Robert.Boston]

Since our new 9.8kW system went live, I have observed one significant ancillary effect. All 30 panels are on the due west-facing roof, and while the theoretical power due to time and solar incidence is said to be about 17% less than due south, with the entire western roof shaded now, the attic is significantly cooler in the late day. Because the upstairs HVAC unit and ducting is all in this attic space, the system works much better in a cooler attic environment. The power attic vent was never able to keep up with the hot wood deck under black shingles. So, I think the 17% production "loss" is paid back in better cooling efficiency. Another electric-nerd point is that the PV production curve is much closer to utility demand curve. That is, with afternoon production the system is actually backing off real utility marginal generation resources. At least with a coal based utility, they have much less "need" for power before noon, and that leads to lots of whining to the PUC, etc. So anyway, I'm pretty pleased with the Sunpower 327 panels and SMA inverters.
:biggrin:

These are great points. Re the second (timing your production to match utility demand), it would be *really* great if retail customers (esp. those with generation) faced the real-time system price. Nearly all retail tariffs have flat pricing, or at best a few blocks of time preset by the system operator/utility. In reality, the value of power on the system moves a lot during each day (e.g., here's a daily report for California). As @slipdrive notes, the peak price is almost always in the afternoon. If PV installs were trying to maximize the value of the power instead of the quantity, they might look different.

 

[slipdrive]

Yes, California is WAY out ahead on anything like a true market for power, on both demand and generation. Most other states and thier utilities are still in "ostrich" mode. The idea of especially battery storage and renewables has them waking up, though. The days of building and then sitting around coal plants in spinning reserve, waiting for demand, and charging for it all to the ratepayers are coming to an end.
The "western roof model" talked about above is really not that big of a deal in Colorado, where summer AC loads are fairly light. Sacramento and inland California is of course quite different! My solar contractor was asked by Sunpower to set up in San Luis Obispo becasue of all the demand, and the attractive rates out there. Things are slowing in Colorado, (and XCEL would like it to stop).

 

[green1]

I had wondered about the insulating effects of having solar on the roof, it does make sense that they'd keep some heat out of the roof. (I wonder in the winter if they'll also keep more heat in?)
As for pricing. I wish we could have time of use pricing for our solar generation, most of my use would be at cheap times, and most of my generation would be at expensive times... no wonder the utility won't go for it.

 

[ItsNotAboutTheMoney]

I had wondered about the insulating effects of having solar on the roof, it does make sense that they'd keep some heat out of the roof.
As for pricing. I wish we could have time of use pricing for our solar generation, most of my use would be at cheap times, and most of my generation would be at expensive times... no wonder the utility won't go for it.

Not only can they help cool but they can actually help insulate. It depends on tilt and spacing, apparently. And, naturally, since more efficient panels absorb more sunlight, they have a greater cooling effect.

 

[cschock]

I had wondered about the insulating effects of having solar on the roof, it does make sense that they'd keep some heat out of the roof. (I wonder in the winter if they'll also keep more heat in?)
As for pricing. I wish we could have time of use pricing for our solar generation, most of my use would be at cheap times, and most of my generation would be at expensive times... no wonder the utility won't go for it.

We have definitely seen a lowering in our A/C use in the summer with the solar panels installed. We have a large (3200 sq ft) one story home facing north/south on the roof line.

Our array is on a good chunk of the south facing roof with a 7.5 kWh installation using Suniva OPTXXX-60-4-100 panels and Enphase 215 Microinverters. The microinverters were used to help significantly with total output due to some large trees on the property south of us that shade various panels at different times of the year/day. Very happy with our results over the last year and a half.

Driving on sunlight is good!

 

[dhrivnak]

It truly is cool to be fusion powered. Remember to vote and show your system on the driving on Sunshine thread. It looks like over half of Tesla owners are solar powered.