Flat mount panel vs tilt mount in winter

[Jacob Scoville]

So I finally decided to dive into solar this summer now that my power uses are so high (65L fridge/freezer, tons of camera/drone batteries, pressure cooker, 3D printer) and a 100W panel kept up with the 12v fridge uses while I was gone on extended work trips.
I decided to put my 2 200 amp 12v batteries in series instead of parallel for more efficiency and an easier time handling power upgrades everything downstream of the charging output.
That means I can’t use that 100w panel anymore without a boost MPPT. Space is limited and a Victron 75v/10amp MPPT is about all I’d want to devote space for so I would need two panels in series. With winter approaching I’m thinking about just scrapping solar again until spring but I’d really like every bit of charging I can come up with and I’m curious how a tilting panel setup in winter in the northern US may compare to a flat setup in summer.

On a perfect mid summer day I’d get about 700wh with my single rich solar 100w panel. (To note since that could affect output, temps were ranging approx. 60-95F). Based off of the angle of the sun and what wattage I’d get at that angle, I estimated probably 100 wh in winter on a sunny day, so probably averaging 30-50 watts when clouds and rain/snow are factored in. I’m curious if it would be a big improvement to tilt the panels in winter (for instance, 50-60w at midday when the the flat panel may be a theoretical 30w) or if it’s just going to be a mediocre 25-30% increase to tilt the panels like the test videos I see show on what looks like a sunny summer day.

 

[sunshine_eggo]

That means I can’t use that 100w panel anymore without a boost MPPT. Space is limited and a Victron 75v/10amp MPPT is about all I’d want to devote space for so I would need two panels in series. With winter approaching I’m thinking about just scrapping solar again until spring but I’d really like every bit of charging I can come up with and I’m curious how a tilting panel setup in winter in the northern US may compare to a flat setup in summer.

Weather notwithstanding, substantially better. Flat panels in Winter tend to only produce about 30-40% of the summer energy production.

On a perfect mid summer day I’d get about 700 watts with my single rich solar 100w panel. (To note since that could affect output, temps were ranging approx. 60-95F).

Assume you meant 700Wh from a 100W panel. That's extraordinary. My system is in AZ, and that's about all I can hope for (7hX PV power) with my fixed tilt close to my latitude.

If you left them flat in winter, you'd get 30-40% of 700Wh. Worse the higher your latitude.

Based off of the angle of the sun and what wattage I’d get at that angle, I estimated probably 100 watts in winter on a sunny day, so probably averaging 30-50 watts when clouds and rain/snow are factored in. I’m curious if it would be a big improvement to tilt the panels in winter (for instance, 50-60w at midday when the the flat panel may be a theoretical 30w) or if it’s just going to be a mediocre 25-30% increase to tilt the panels like the test videos I see show on what looks like a sunny summer day.

100W on a winter day isn't likely to happen unless conditions are extremely favorable, temps are very cold and tilt is steep. Did you mean Wh?

30-50W out of a 100W panel in clouds/rain/snow is extremely optimistic. 10-20W is more realistic.

In indirect conditions flat panels tend to perform slightly better than tilted to to a slightly greater exposure to the diffuse source.

clear sky: winter tilt performs better.
cloudy sky: Flat or more shallow tilt (like year round) performs a little better.

 

[Zwy]

So I finally decided to dive into solar this summer now that my power uses are so high (65L fridge/freezer, tons of camera/drone batteries, pressure cooker, 3D printer) and a 100W panel kept up with the 12v fridge uses while I was gone on extended work trips.
I decided to put my 2 200 amp 12v batteries in series instead of parallel for more efficiency and an easier time handling power upgrades everything downstream of the charging output.
That means I can’t use that 100w panel anymore without a boost MPPT. Space is limited and a Victron 75v/10amp MPPT is about all I’d want to devote space for so I would need two panels in series. With winter approaching I’m thinking about just scrapping solar again until spring but I’d really like every bit of charging I can come up with and I’m curious how a tilting panel setup in winter in the northern US may compare to a flat setup in summer.

On a perfect mid summer day I’d get about 700 watts with my single rich solar 100w panel. (To note since that could affect output, temps were ranging approx. 60-95F). Based off of the angle of the sun and what wattage I’d get at that angle, I estimated probably 100 watts in winter on a sunny day, so probably averaging 30-50 watts when clouds and rain/snow are factored in. I’m curious if it would be a big improvement to tilt the panels in winter (for instance, 50-60w at midday when the the flat panel may be a theoretical 30w) or if it’s just going to be a mediocre 25-30% increase to tilt the panels like the test videos I see show on what looks like a sunny summer day.

Back a few years ago when everyone was learning about solar and installing panels on roofs of RV's, it was quickly learned that in the summer the flat mounted panels on the RV roof would easily keep the battery bank full, but in winter the yield would not keep up.

People would head down to the lower latitudes for the winter with the RV, park out on LTVA's and they kept running out of power. The solution is tilting panels on the roof. It does make a difference with a low sun horizon. The added bonus is that snow will not accumulate on the panels if the tilt is steep enough. My array is tilted to 65 degrees off vertical in winter.

You can always test this yourself by tilting the single 100W panel. You have to remember a 25-30% increase over several hours of peak sun adds up.

 

[Jacob Scoville]

Sorry, I did mean WH a few times.

Weather notwithstanding, substantially better. Flat panels in Winter tend to only produce about 30-40% of the summer energy production.



Assume you meant 700Wh from a 100W panel. That's extraordinary. My system is in AZ, and that's about all I can hope for (7hX PV power) with my fixed tilt close to my latitude.

If you left them flat in winter, you'd get 30-40% of 700Wh. Worse the higher your latitude.



100W on a winter day isn't likely to happen unless conditions are extremely favorable, temps are very cold and tilt is steep. Did you mean Wh?

30-50W out of a 100W panel in clouds/rain/snow is extremely optimistic. 10-20W is more realistic.

In indirect conditions flat panels tend to perform slightly better than tilted to to a slightly greater exposure to the diffuse source.

clear sky: winter tilt performs better.
cloudy sky: Flat or more shallow tilt (like year round) performs a little better.

Yes, I did mean watt hours. Edited my initial post. And I meant 100 watt hours on a completely sunny winter day. Judging from the wattage I’d get when the sun was at a similar angle when I installed the panels. How much would temps play into the output? I saw lower temps might output a higher voltage from the panel (all other factors being the same). Is there a “too low”?

 

[sunshine_eggo]

Sorry, I did mean WH a few times.

Yes, I did mean watt hours. Edited my initial post. And I meant 100 watt hours on a completely sunny winter day. Judging from the wattage I’d get when the sun was at a similar angle when I installed the panels. How much would temps play into the output? I saw lower temps might output a higher voltage from the panel (all other factors being the same). Is there a “too low”?

Most panels have a Power temperature coefficient that tracks pretty closely with Voltage increase due to lower temps. I made my panels mounts 3 position adjustable, but I have never set them anywhere besides year round. At 29.5° tilt at 34° latitude, my 2930W array produces ~2400W in clear winter conditions. The shallower tilt might help a little with cloudy skies, but output is typically no more than 10-20% of rated. What really kills PV power is snow. 0W

"Too low" would be below the operational temperature of the attached equipment OR if the panel voltage exceeds MPPT Voc limit.

 

[Jacob Scoville]

Back a few years ago when everyone was learning about solar and installing panels on roofs of RV's, it was quickly learned that in the summer the flat mounted panels on the RV roof would easily keep the battery bank full, but in winter the yield would not keep up.

People would head down to the lower latitudes for the winter with the RV, park out on LTVA's and they kept running out of power. The solution is tilting panels on the roof. It does make a difference with a low sun horizon. The added bonus is that snow will not accumulate on the panels if the tilt is steep enough. My array is tilted to 65 degrees off vertical in winter.

You can always test this yourself by tilting the single 100W panel. You have to remember a 25-30% increase over several hours of peak sun adds up.

I just found an app to estimate angle of the panels and where I’m located and it looks like ideal December angle for me is just under 65 degrees and if I drive further north in the US it could hit 70 degrees for optimal angle. Pretty wild. Summer it’s 17 degrees where I’m at.

But 25-30% of 100WH is still the equivalent of like 2 minutes with the alternator running.

 

[Zwy]

I just found an app to estimate angle of the panels and where I’m located and it looks like ideal December angle for me is just under 65 degrees and if I drive further north in the US it could hit 70 degrees for optimal angle. Pretty wild. Summer it’s 17 degrees where I’m at.

You must be close to my latitude.
I tilt back to 30 degrees in summer, I'm at about 45 degrees right now

But 25-30% of 100WH is still the equivalent of like 2 minutes with the alternator running.

That takes fuel.

Yes, for a 100W panel it doesn't seem like much. But if you have 10Kw, how long will the alternator need to run? And not only does it add up at the end of the day, it also means during the week it adds up. Rule of 72 applies. At a 30% increase, the amount harvested is double about every 2.5 years compared to the lower yield. Let's just throw some numbers in for fun.

700Wh/day x 365 days=255,500Wh/year

Now, about a 30% decrease, granted, summer you still get the 700Kwh but this is just for fun and an example. We are just putting some numbers out for comparison.

500Wh/day x 365 days= 182,500Wh/year

255,500 - 182,500= 73,000

73,000 x 2.5 years = 182,500

If you figure $0.20Kwh, that comes to $365 extra in 2.5 years for basically the same amount invested, one panel.

If you had a 10Kw array, how much is that in 2.5 years at $0.20Kwh?

 

[justinm001]

That takes fuel.

Yes, for a 100W panel it doesn't seem like much. But if you have 10Kw, how long will the alternator need to run? And not only does it add up at the end of the day, it also means during the week it adds up. Rule of 72 applies. At a 30% increase, the amount harvested is double about every 2.5 years compared to the lower yield. Let's just throw some numbers in for fun.

My 270a 24v alternator rated for continuous duty pushes out 108w per minute... if I could trust it. So 10kW is 92.59 minutes probably more like 126w per minute if 28V actually.

 

[Jacob Scoville]

My 270a 24v alternator rated for continuous duty pushes out 108w per minute... if I could trust it. So 10kW is 92.59 minutes probably more like 126w per minute if 28V actually.

In a SUV so the best I can get (that has high amps at low RPMs anyway) is 270a 12v. I wish there was room somehow for a second alt. It can do 200 amps at idle RPMs and I’m planning to work up my charge to 150 amps during idle and 200 amps while driving.

You must be close to my latitude.
I tilt back to 30 degrees in summer, I'm at about 45 degrees right now

That takes fuel.

Yes, for a 100W panel it doesn't seem like much. But if you have 10Kw, how long will the alternator need to run? And not only does it add up at the end of the day, it also means during the week it adds up. Rule of 72 applies. At a 30% increase, the amount harvested is double about every 2.5 years compared to the lower yield. Let's just throw some numbers in for fun.

700Wh/day x 365 days=255,500Wh/year

Now, about a 30% decrease, granted, summer you still get the 700Kwh but this is just for fun and an example. We are just putting some numbers out for comparison.

500Wh/day x 365 days= 182,500Wh/year

255,500 - 182,500= 73,000

73,000 x 2.5 years = 182,500

If you figure $0.20Kwh, that comes to $365 extra in 2.5 years for basically the same amount invested, one panel.

If you had a 10Kw array, how much is that in 2.5 years at $0.20Kwh?

This is in the vehicle mounted forum and that’s not factoring in the space and MPG loss though. I have rooftop boxes in front of the panel and my panel is countersunk as low as can be on my roof rack without anything shading it, but I it’s probably slightly more aerodynamic if the boxes were back an inch and partially shading the panel. Eventually I’ll have multiple panels and have them slide out when stationary, but during driving I’d probably be screwed unless I had multiple MPPT’s—is that how setups with slide outs work while driving?


On the flip side, alternators are way less efficient compared to other means of generating electricity and if I’m idling just to charge it’s even more inefficient. If I’m idling it’s 50-80 cents at the absolute best once I upgrade my charging capacity and currently as much as $2/kw if I’m idling and not driving.
But if I can 3D print a $50 part for a few dollars in filament and $2 in electricity it’s worth it.

 

[justinm001]

On the flip side, alternators are way less efficient compared to other means of generating electricity and if I’m idling just to charge it’s even more inefficient. If I’m idling it’s 50-80 cents at the absolute best once I upgrade my charging capacity and currently as much as $2/kw if I’m idling and not driving.
But if I can 3D print a $50 part for a few dollars in filament and $2 in electricity it’s worth it.

I don't think it's nearly as bad as you make it out. My huge RV with a detroit series 60 12.7 diesel switches from mpg to gph when stopped and it idles under 1 gph.... like .7gph I think. So that's $4 for an hour and i was running that alt above at half power at the time. I also have a 24v 90a chassis alternator running along with whatever else is connected to the engine. I'd think a much smaller engine would use a lot less fuel at idle.

 

[Jacob Scoville]

I don't think it's nearly as bad as you make it out. My huge RV with a detroit series 60 12.7 diesel switches from mpg to gph when stopped and it idles under 1 gph.... like .7gph I think. So that's $4 for an hour and i was running that alt above at half power at the time. I also have a 24v 90a chassis alternator running along with whatever else is connected to the engine. I'd think a much smaller engine would use a lot less fuel at idle.

But then the alternator is increasing the fuel and for maximum efficiency I’m ramping up the RPM’s of the car a little bit to get close to the 270 amps of the alternator. it’s at least 1/4 to 1/3 a gallon/hour at the very lowest RPMs and that’s without a big alternator load, currently only used up to 80 amps at once.

 

[justinm001]

But then the alternator is increasing the fuel and for maximum efficiency I’m ramping up the RPM’s of the car a little bit to get close to the 270 amps of the alternator. it’s at least 1/4 to 1/3 a gallon/hour at the very lowest RPMs and that’s without a big alternator load, currently only used up to 80 amps at once.

If you adjusted your alternator pulley size it'll work at a lower rpm but that's not always a good thing depending on the engine. My coach has a high idle switch so 1000rpm instead of like 700 and it'll pump full power that high

 

[Zwy]

In a SUV so the best I can get (that has high amps at low RPMs anyway) is 270a 12v. I wish there was room somehow for a second alt. It can do 200 amps at idle RPMs and I’m planning to work up my charge to 150 amps during idle and 200 amps while driving.

This is in the vehicle mounted forum and that’s not factoring in the space and MPG loss though. I have rooftop boxes in front of the panel and my panel is countersunk as low as can be on my roof rack without anything shading it, but I it’s probably slightly more aerodynamic if the boxes were back an inch and partially shading the panel. Eventually I’ll have multiple panels and have them slide out when stationary, but during driving I’d probably be screwed unless I had multiple MPPT’s—is that how setups with slide outs work while driving?

Either multiple MPPT's or a switch.

On the flip side, alternators are way less efficient compared to other means of generating electricity and if I’m idling just to charge it’s even more inefficient. If I’m idling it’s 50-80 cents at the absolute best once I upgrade my charging capacity and currently as much as $2/kw if I’m idling and not driving.
But if I can 3D print a $50 part for a few dollars in filament and $2 in electricity it’s worth it.

I have 2 alternators on my Duramax with the truck camper for a total of 355A @12V. I have a 2400W inverter in the back seat of the truck and hardwired a 30A cord to the rear bumper where there is a 30A RV outlet mounted. I can plug the 24V Growatt 3000LVM in and charge easily at 60A @24V without really working anything. I never need it in summer unless the battery was low and I want to run air conditioning in the camper. Late fall or winter I'll probably need it to supplement PV. I don't carry a generator.

 

[Jacob Scoville]

I don't think it's nearly as bad as you make it out. My huge RV with a detroit series 60 12.7 diesel switches from mpg to gph when stopped and it idles under 1 gph.... like .7gph I think. So that's $4 for an hour and i was running that alt above at half power at the time. I also have a 24v 90a chassis alternator running along with whatever else is connected to the engine. I'd think a much smaller engine would use a lot less fuel at idle.

Yeah at warm idle in park it’s like .4 gallons per hour. V8 4.7L toyota. Not the most fuel efficient but it will last 500k miles

 

[Jacob Scoville]

If you adjusted your alternator pulley size it'll work at a lower rpm but that's not always a good thing depending on the engine. My coach has a high idle switch so 1000rpm instead of like 700 and it'll pump full power that high

Yeah I’m emailing the manufacturer back and forth and will probably get a slightly smaller pulley. I think other vehicles they put these in must have a higher idle. But in park once warm it’s 550-600 RPM’s and 500 RPMs in drive and that seems to be completely fine and normal for these 4.7L Sequoias and Tundras. A second alternator would be a dream though for charging my LTO bank