Mounting question - how much does pitch matter

[Ecoeco]

Hi

I have a newbie question. I’m putting panels on the roof of my house. The panels will face due north (I’m in New Zealand) however my (flat) roof pitches slightly down to the south. All the solar installers I’ve spoken to are keen to mount the panels with tilt legs so that the pitch of the roof is corrected and the panels with face the sun straight on at midday.

I’m keen to install the panels flat on the roof and accept that the pitch does not directly face the sun. I know this is not optimal for generation, but:
- tilting legs are expensive, I can use that money to buy and install more panels
- using legs to tilt the panels up will be less attractive than our current streamlined roof

So my question is how much generation will I loose due to the sub-optimal pitch of panels? If they will generate 50% less power I’m wondering if I can just install twice the number of panels to address this.

 

[12VoltInstalls]

my question is how much generation will I loose due to the sub-optimal pitch of panels

There’s an online calculator (that I didn’t look up right now) which compares sun angle in two planes with the expected output.
It can be “a lot” but it’s probably less than you think.

 

[Ecoeco]

There’s an online calculator (that I didn’t look up right now) which compares sun angle in two planes with the expected output.
It can be “a lot” but it’s probably less than you think.

thanks - that was super helpful. I used this calculator:

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

this shows a less that 20% difference in output between a 20% pitch and 0% pitch. I’m going to just buy more panels.

 

[BryceFreeman]

Hi, this web may also be helpful to you:

SunCalc sun position- und sun phases calculator

Application for determining the course of the sun at a desired time and place with interactive map.

www.suncalc.org

 

[SamDeleted]

I’m going to just buy more panels.

You can overpanel your way out of any situation , at least that's what my nan always said

 

[JS43]

thanks - that was super helpful. I used this calculator:

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

this shows a less that 20% difference in output between a 20% pitch and 0% pitch. I’m going to just buy more panels.

And 20% on a 20% efficiency panel is a lot. In the end it's a question of how many holes do you want to put in the roof as that's another possible leak. Also, how much more will it cost to replace roofing. Each panel you add will cost more when the roof is replaced because you will have to remove the panels and then put them back up again. Also, the panels being tilted will cool better than they will flat. So a lot for him to consider here.

 

[Ecoeco]

And 20% on a 20% efficiency panel is a lot. In the end it's a question of how many holes do you want to put in the roof as that's another possible leak. Also, how much more will it cost to replace roofing. Each panel you add will cost more when the roof is replaced because you will have to remove the panels and then put them back up again. Also, the panels being tilted will cool better than they will flat. So a lot for him to consider here.

Thanks, those are good observations. I‘ll do some more thinking about airflow, although I’m still tempted to start with a smaller somewhat over panelled system and consider adjusting the tilt and/or number of panels if the production ends up too low.

My roof is 10 years old and will hopefully last another 15+ years with regular maintenance, so when it is time to re-roof there is a good chance the panels are replaced as part of that cost. I know panels have a 20+ year life span, but in 15 years I would expect gains in efficiency and further cost reductions which make it economic to just replace the system.

Depending on production I’m expecting the system to break even in 10yrs or less. We installed a smaller 2kw grid tied system that we put in when we built the house in 2014 and that will break even in the next few months.

Ten years later I’m adding 6 Kw more capacity for the same nominal cost as the 2kw system we installed.

 

[JS43]

Thanks, those are good observations. I‘ll do some more thinking about airflow, although I’m still tempted to start with a smaller somewhat over panelled system and consider adjusting the tilt and/or number of panels if the production ends up too low.

My roof is 10 years old and will hopefully last another 15+ years with regular maintenance, so when it is time to re-roof there is a good chance the panels are replaced as part of that cost. I know panels have a 20+ year life span, but in 15 years I would expect gains in efficiency and further cost reductions which make it economic to just replace the system.

Depending on production I’m expecting the system to break even in 10yrs or less. We installed a smaller 2kw grid tied system that we put in when we built the house in 2014 and that will break even in the next few months.

Ten years later I’m adding 6 Kw more capacity for the same nominal cost as the 2kw system we installed.

Yup, just thought I'd throw that in there though. I heard of a guy here who had to replace his roof 4 years after putting solar up and he had leaks. When they redid the roof it cost him around 10k extra because of the added work of removing and putting the panels back up.

 

[12VoltInstalls]

And 20% on a 20% efficiency panel is a lot.

Not really. Panel efficiency is what it is; the labeled wattage/specs are what they are. It’s ~20% of output potential.

In a practical sense, what works for a situation is what should be done- many installations are not 180*S oriented at a perfect tilt. In my circumstances I have vertically installed panels. I “lose” a lot in summer but I can’t use it anyway- I have surplus 8 mos. of the year. In winter iirc my ‘ideal’ would be ~58* or something. But when I’m not home/working/it’s snowing and cold out I don’t want to or am not available to constantly clear the snow or scrape ice; it’s worth it to me to accept the “loss” because I gain more by always getting some charge without having to be here clearing the panels.
As far as flat-installed panels or other ‘roof mount’ sometimes that’s either a requirement or is the only practical solution in a circumstance. And panels are really inexpensive imho compared to alternative parts/mounts/struts/wire in many circumstances if you’re trying to achieve a particular solar input goal.

installed a smaller 2kw grid tied system that we put in when we built the house in 2014 and that will break even in the next few months

Current products and DIY should shorten up the ROI crossover I’d think. With prices of specific items going down in spite of inflation, AND the factor of inflation actually considered - diy solar is really at record lows. Regardless of demand this trend should continue as ‘economy of scale’ and the fact that in-service manufacturing equipment will continue being used for years (much of it is already fully depreciated and “paid for”). Batteries are approaching ‘commodity’ status, panels have been commodities for years, and the equipment just keeps getting better (though equipment probably isn’t going to become ‘commodity’ for years because they can design and manufacture on the fly so readily these days). I don’t have the patience or mental tolerances for a ten-year ROI though.

Ten years later I’m adding 6 Kw more capacity for the same nominal cost as the 2kw system we installed.

And when you factor inflation that’s actually probably 2/3 or less the cost of the 2kW system ?

Final thought: if it’s even possible I would do a ground mount, myself. The typical panel degradation over 25 years isn’t probably going to be as significant as I think you expect fwiw

 

[OffGridForGood]

Since you have a "flat" roof we are not talking about shingles with short lifespan. Perhaps you have EPDM or TPO PVC sheet roofing?
Many of the flat roofing products are long lasting systems. The shade from the PV may extend the life of the roofing by reduced solar degredation, at least in the areas that benefit from shading.
Keep in mind your solar panel manufacturer may specify a minimum slope for the PV - I have seen "10%" as a listed minimum slope on my own panels. I am told this is mostly about keeping water from ponding in the middle of the panel, and to aid with self-cleaning dust leaves off of the panels using rain water.

 

[Shimmy]

A big part of the impact doesn't seem to be captured by pvwatts. I have a similar situation with a roof plane that is back-sloped 5 degrees away from the sun at 20° latitude. PV Watts only shows a 5-10% penalty, but the winter penalty is actually closer to 30% (a little worse than that when shading is added in).

(The difference appears to be in assumptions of diffuse light during the winter. There is significantly less at peak power windows.)

So, if you are grid-tied with annual net metering on favorable terms it might not be a huge issue, but if you want to handle grid outages in the winter the impact could be much bigger.

 

[Ecoeco]

Since you have a "flat" roof we are not talking about shingles with short lifespan. Perhaps you have EPDM or TPO PVC sheet roofing?
Many of the flat roofing products are long lasting systems. The shade from the PV may extend the life of the roofing by reduced solar degredation, at least in the areas that benefit from shading.
Keep in mind your solar panel manufacturer may specify a minimum slope for the PV - I have seen "10%" as a listed minimum slope on my own panels. I am told this is mostly about keeping water from ponding in the middle of the panel, and to aid with self-cleaning dust leaves off of the panels using rain water.

Thanks - it’s a monopitch steel roof. The roof is flat but angled away from the sun (to allow for big windows at the front of the house and little windows at the back).

So, if you are grid-tied with annual net metering on favorable terms it might not be a huge issue, but if you want to handle grid outages in the winter the impact could be much bigger.

Fortunately we are connected to the grid.

I’m self installing the system as DIY. By doing it myself rather than through an installer I save around 50% of the cost. And I will also have all the fun of building it!

The system will have battery storage and will supply power via a transfer switch. It will be a DC coupled system. Basically I’m going to have up to 4,500 kw w of loads from the house on a sub-panel which then has a 30a generator input socket. My DIY solar system will then power those loads. I’ll have a Victron Multiplus-2 which will be able to either take grid input from a new 30a circuit, or batteries charged by the solar.

All of this has the added benefit that if we move house in the 20 year lifespan of the system we can take it all with us.

 

[OffGridForGood]

4,500 kw

maybe you mean 4500W...4.5kW unless your house is a lot bigger than most!

 

[OffGridForGood]

it’s a monopitch steel roof. The roof is flat but angled away from the sun

Wellington NZ is 41 degrees South, then your roof slope away from the sun = not ideal, especially during your winter (May-Sept)
Lots of extra panels and clear skys may work out for you.

 

[Ecoeco]

maybe you mean 4500W...4.5kW unless your house is a lot bigger than most!

Whoops!

 

[OffGridForGood]

Whoops!

Ha Ha Ha - don't worry we all do it!
4500W is your peak current, but this doen't tell us anything about daily power use in kW-Hours.
If your system actually used 4.5kW steady load x 24hs/day = 108kWh/day.
But I am sure you don't need this, you likely have a steady load in the 0.5 to 1.0 kw range, with occational peaks up to 4kW, and daily use is somewhere in the 15-30kWh/day range. If you can give us an idea of the daily total use and if it varies between winter/ summer we can help you with sizing.

 

[Ecoeco]

Ha Ha Ha - don't worry we all do it!
4500W is your peak current, but this doen't tell us anything about daily power use in kW-Hours.
If your system actually used 4.5kW steady load x 24hs/day = 108kWh/day.
But I am sure you don't need this, you likely have a steady load in the 0.5 to 1.0 kw range, with occational peaks up to 4kW, and daily use is somewhere in the 15-30kWh/day range. If you can give us an idea of the daily total use and if it varies between winter/ summer we can help you with sizing.

Thanks - this got me thinking about how I’m sizing the system.

Here’s my plan for critical review:

a) In winter we use an average of 40-45 kWh per day from the grid. In summer our average is around 24kWh. I’m looking to have around 50% of our daily needs supplied by the new system. I’ll then scale it up as storage becomes more economical over the coming years and panels continue to drop in price.

b) Our existing 2kw system is angled very well for solar (it’s on a part of the roof we built to put the panels on). In winter it has generally averaged around 5kW per day. With a scaling factor my new 6 kWh system should generate an average of around 14 kW per day on the same winter days, less the lost production from the poor pitch of the roof. Applying a 30% discount factor the daily winter generation will be around 10kW per day or very roughly 20% of our consumption. We will easily use all the power produced by the system.

c) In summer our 2kw system produces around 10kW on average each day, but the range is high (from 1.5kW to 14 kW). The 6kW system should produce around 60 kW a day on average in summer, less a 20% discount factor for the roof, or around 48 kWh per day. This is way more than we ever need in the summer months.

My challenge is to size the storage correctly to make the best use of the power. In summer 22/23 roughly 15 days of production each month were super sunny days. The new 6.0 kW system would have produced more than 24kWh used in the house for the day. There are usually for around 7 to 10 days at a time of super high production before the weather packs in for a few days.

So based on all this I’m thinking around 12kW of usable battery storage will be the optimum for this system (15 kW of SOK 48v LifePo4 batteries). On the max production 48 kWh days this will ensure we use 24kW in the house, with the excess mostly being fed back into the grid. We will have battery storage for a day of rain, or possibly two depending on usage and how much sun there actually is.

We have discounted overnight power due to time of day metering, so in winter I’ll charge the batteries overnight. This will mean around 50% of our power needs will be met through a combination solar and lower cost grid power (10 kW of winter solar generation + 12 kW of overnight low cost charging = 22 kW of winter power).

Thank you to anyone who has read this wall of text .

 

[SamDeleted]

So based on all this I’m thinking around 12kW of usable battery storage will be the optimum for this system (15 kW of SOK 48v LifePo4 batteries). On the max production 48 kWh days this will ensure we use 24kW in the house, with the excess mostly being fed back into the grid. We will have battery storage for a day of rain, or possibly two depending on usage and how much sun there actually is.

So I think it's already been said , the standard arrangement is 3 days worth of battery back up , you're looking at only 50% of summer days consumption, and only 25% of a winters day....

obviously each to their own!.. I know lithium is extremely expensive up front

But there are some draw backs, of course it goes without saying, if there is a power cut in winter your batteries will empty in a matter of hours

Also you may well waste a LOT of power, if your maths are correct (looks good) , with 48kwh average daily summer solar production your battery will be full by mid morning , the rest of the power your solar makes that day is going to be wasted

Say you wake up on a summers the morning, battery at 0%soc (unlikely it'll be so low)


48kwh daily solar production
Minus 12kwh to recharge the battery,
Minus 24kwh house consumption

Leaves 12kwh wasted solar you aren't going to get your hands on, a bigger battery won't solve this fully but it does help mitigate. Or you could just run air conditioning all summer ?

 

[Danke]

How much weight can you add to your roof? A ballast system may be an option.

Example: https://dynoraxx.com/

 

[OffGridForGood]

a) In winter we use an average of 40-45 kWh per day from the grid. In summer our average is around 24kWh.

Since you are in the Southern Hemisphere, I have to ask: do you mean Dec-Mar = "summer" /Jun-Sept = Winter?)

I’m looking to have around 50% of our daily needs supplied by the new system.

22kWh/d in winter, 12kWh/d in summer - and I assume you mean 7-days per week, since you don't say if this is weekend use or full time.

the daily winter generation will be around 10kW per day or very roughly 20% of our consumption

you need 2.2x this to meet your target above of 22kWh/d avg. more to accound for days of poor solar.

before the weather packs in for a few days.

This is the key statement in the whole 'wall of text' - a few days 2-3 days I assume.

I expect you already realize, the system will not meet the target you set in the first paragraph. You would need more PV - or better positioning of the PV you are planning - to bring in close to "50% of use"
As Sam already noted, the rule of thumb ESS = 3d use and then size PV to be able to recharge this on average. Perhaps you can get away with ESS=2d use if those 'few days' are only two.
And when I say "use" I don't mean your full day's requirements, I only mean the solar portion target.
22kWh/d winter x 3days ESS = 66kWh (expensive, ask me how I know)

My thoughts are: from the outset you are not attempting to supply all the loads with solar, in fact you appear to be more interested in being able to use all the solar you are able to collect. Small battery makes sense in this case, as you know going in, the solar will not supply all loads and you have no expectation that you have solar (from battery) for days of poor weather. This tells me you only need enough battery for one night of use. The next day if sunny will recharge for the next night, if cloudy your on grid anyway. Also you noted ToU low rates at night for recharging, so again you don't need 2-3 days of ESS only 0.5days.
I would target 20kWh of ESS to have some room in the bottom end - avoid draining the batteries too low too often.