Most output from a small roof

[Razinni]

I have a limited amount of space and want to get the most output out of fewest panels. Budget is not a problem and nothing is purchased yet. My project is a year round man cave that will serve as a backup generator for critical loads in main house. So 24v or 48v panels?

 

[uptopsolar]

You mention backup, so you plan to have energy storage as well? Do you have a battery in mind or are you asking for both?

 

[jimbob420]

I have a limited amount of space and want to get the most output out of fewest panels. Budget is not a problem and nothing is purchased yet. My project is a year round man cave that will serve as a backup generator for critical loads in main house. So 24v or 48v panels?

If you mean a 24v or 48v system, yes, this is a good idea. Your panel voltage will need to at least exceed this by a few volts to make it feasible to charge your set.

How much roof space do you have? And how much "critical load" do you need to be able to serve?

I wish I'd learnt when I started that bigger panels are both cheaper per w of output and give you more output per square meter, as there's less losses to framing etc, but the system obviously needs careful design and selection of appropriate components like chargers, inverters, batteries.. all need to be sized for loads, charging capacity, etc etc.

 

[Razinni]

Yes to battery but but basing all sizing decisions on how much I can produce.

 

[Razinni]

Assume I will have at least 100 square feet if that helps

 

[jimbob420]

Assume I will have at least 100 square feet if that helps

Depending on the exact shape, you could have (for example) 4 400w panels, 1600w peak. In smaller panels, it would be about 12 100w panels, so 1200w peak, but you'd have more options around stringing arrangements to suit.

 

[wattmatters]

Assume I will have at least 100 square feet if that helps

With such a limited area, the shape of the roof will matter, in particular the ratio of length to width. If the roof has the same aspect ratio of PV panels then you'll be able to get more up there.

At best 4 large wattage panels is about the most you can hope for, up to 1.8kW, but if roof shape isn't ideal then you might be confined to half that.

Any chance of a ground mount array instead? That way you can size it however large you need.

 

[Hedges]

Some panels are 20% efficient, produce 200W per square meter.
There are bargain priced panels that only put out 2/3 as much power per unit area.

Angle matters, especially if you want more power in winter.
Two angles, East and West with also a tilt to the South, can produce more Wh/day.
Panels extending beyond the eves will give more area.

Plan your design, including a charge controller or hybrid together with a number of some model of panels in series and/or parallel before buying anything. Make sure the voltages and current are all compatible.

100 sf ~ 10 m^2, 2000W PV is possible. At 12V, 165A which is two large charge controllers.
At 24V, one large charge controller. At 48V, one smaller charge controller.

Obviously 48V is 4x as many batteries/cells as 12V, so determine how many kWh of storage desired. That can also drive voltage.

How many watts peak of AC? suggestion is about 2000W at 12V, 3500W at 24V, 5000W at 48V, but it can vary.

 

[David Hamand]

I have 6x270w panels on the bunky roof.
16'x6' available roof space. Still have the other side if necessary, but I'm facing SW, so this is what works best for me.

 

[wattmatters]

Panels extending beyond the eves will give more area.

I do wonder if there are structural issue to consider, such as wind loading? All the design requirements where I am have strict no go areas around the edges of the roof.

 

[Hedges]

I do wonder if there are structural issue to consider, such as wind loading? All the design requirements where I am have strict no go areas around the edges of the roof.

It can be wind, I've read there is reduced lift by keeping the panels back.
There have been rules for 36" walk way on two sides and the ridge for firemen. That has since been reduced a bit.
In my area, less than 40 pounds per attach point and less than 18" high, no structural permit. That could be just screws into 1/2" sheathing.
If you put several inches of lag screw into rafters, it will be much stronger.
My ground mounts (2" pipe and aluminum rails) are of course exposed to whatever wind there might be.

 

[Razinni]

Thanks to you all I did a little drawing which I hope helps. There is a bit of a breezeway to the right. I think I can fit five panels turned placed horizontal along the bottom edge of the room. The will extend to the covered breezeway which is angled with the roof at 19 degrees. I also think I can put a couple panels vertical in the center of the roof so to be safe six panels? Pair them up. What output do you think that would give me with six panels?

 

[Razinni]

I do wonder if there are structural issue to consider, such as wind loading? All the design requirements where I am have strict no go areas around the edges of the roof.

I will attach these panels into the roof joists so I think they will be secure and I do not plan on any overhang.

 

[Hedges]

Does that 17' sloped roof face South West?
If so, the opposite face North East isn't very useful. But the flat area would get good summer sun.

"vertical in the center of the roof"?
A sail, to catch the wind?

Pair them up. What output do you think that would give me with six panels?

Depends on wattage of the panels

"pair them up" ... meaning?
We use terms like "3s2p" to mean two sets of (3 panels in series), connected in parallel

 

[Razinni]

So back to my original question. If I want to take the most advantage of my limited space any suggestions? Feels like I should be able to get 250 watts each correct?

 

[Razinni]

Some panels are 20% efficient, produce 200W per square meter.
There are bargain priced panels that only put out 2/3 as much power per unit area.

Angle matters, especially if you want more power in winter.
Two angles, East and West with also a tilt to the South, can produce more Wh/day.
Panels extending beyond the eves will give more area.

Plan your design, including a charge controller or hybrid together with a number of some model of panels in series and/or parallel before buying anything. Make sure the voltages and current are all compatible.

100 sf ~ 10 m^2, 2000W PV is possible. At 12V, 165A which is two large charge controllers.
At 24V, one large charge controller. At 48V, one smaller charge controller.

Obviously 48V is 4x as many batteries/cells as 12V, so determine how many kWh of storage desired. That can also drive voltage.

How many watts peak of AC? suggestion is about 2000W at 12V, 3500W at 24V, 5000W at 48V, but it can vary.

Thank you so much

 

[Razinni]

About to purchase https://watts247.com/product/lvx-6048-hybrid-solar-inverter-split-phase-120v-240v/ . i figure i can keep it fed with 8-10 panels

 

[Supervstech]

You say man cave... and 100sqft...
Any chance this is a detached building? There is no fireman clearance unless it is the roof of your residence...