On Grid- Best Voltage for solar panels minimum 4kw system, no battery packs

skip21al · February 2016

I'm a newbie and appreciate any replies to be made like you're talking to a 3rd grader... ok maybe second grader.

I want to DIY a system for my home. Family of 5. 2800 sq ft. High desert climate (hot summers and snowy winters). We are not minimalist by any means, but want to take advantage of the sun's rays and hopefully save money in the long run. We live in city limits and hope to recoup money from any power surplus sent back on the grid ("net metering" I think). Roof mount, no crazy setup on the home. I'll have a certified electrician inspect things before going "hot". I'm a DIYer but not to proud to spend a couple hundred bucks for peace of mind.

That being said, which solar panel voltage should I shop for? 12v, 24v, or 48v seem to be most common. I want to make 2 strings with DC optimizers on each module and a string inverter (would I need two inverters since two strings?) I'm guessing the panels would be 250-300W each. Any advice on brands would be helpful. I'm leaning monocrystalline type.

Whats the rule of thumb for solar panel voltage, if any?

Thanks!

mike95490 · February 2016

whoa!
If you have Grid, go for simple grid tie. If you don't have shadows on the roof, skip the otptimizers - you don't need them.
If you are going grid tie, you don't have batteries to deal with, forget 12,24,48V gear.
GT panels can run 70V or so, the string inverters take 500VDC and convert that to AC to feed the grid with.
Don't forget a 60A Transfer switch, and backup generator. Forget batteries & chargers.

skip21al · February 2016

No optimizers? Is that common opinion? I thought they help "balance" things even if no shadows just in case some shade from clouds, etc. Snow can be a factor too. Thoughts?

What do you mean by "GT panels run 70V". There is a 70V panel? How many watts are those panels and how many would I need?

Thanks!

BB. · February 2016

If you are going Grid Tied (no battery banks)--You have to choose a "central inverter" or "micro inverter" confuguration.

Central inverters have 10-40+ solar panels wired in series+parallel back to a central inverter mounted on the side of the house or in the garage (DC side). Then AC wiring from the GT inverter to your main panel. You can use "optimizers" in this configuration (I believe, but not sure, that you have to match Optimizer to Central inverter--They need to communicate).

Or, micro inverters. Basically 1 Solar Panel to 1 Micro inverter. The micro inverters are connected to a 240 VAC harness on your roof--That then runs back to the main AC panel. You may have 1 or more parallel strings of 240 VAC cables back to the main panel.

There are advantages and disadvantages to each type of inverter/system. And you have to match the solar panels (Vmp/Imp ratings) to the inverter you are using.

You will have to do a lot of reading/research to figure out your configuration. Do not buy anything just yet.

You also will need building permits and approval from your utility.

Enphase is an example of micro inverters. And SMA is an example of central inverters (and they now have a micro inverter too):

http://www.solar-electric.com/inverters-controllers-accessories/gridtiesolar.html

-Bill

Raj174 · February 2016

Read up on SMA grid tie inverters. I know some of them can handle 2 strings. Grid tie panels are usually 60 cell panels from 220 to about 280 watts, although many are now using 72 cell panels about 285 watts and up. If your going DIY, take the time to learn the equipment, wiring, connections and roof mounting techniques. A lot of this information can be found in the sticky's on this and other solar power forums. Also, since you're connecting to the grid, the system will have to be permitted and inspected.

skip21al · February 2016

Bill, when you say central inverter is that same as a string inverter? And is a GT inverter different from those?

What's the opinion on optimizers that allow me to see each module performance? Is that kind of monitoring through a smartphone app, etc really that useful?

zoneblue · February 2016

Your utility and building permit guys will likely require a certified solar isntaller. Those that try to DIY usually run into lots of hurdles, and usually decide its best to find a friendly installer who will let you help out with some of the labor. Also from what you have said, youll benefit from the expertise. 600VDC is no place to learn the rudiments of electrics.

BB. · February 2016

A string inverter is a "central inverter" (one inverter connected to many solar panels).

A Grid Tied inverter is the type that can take Solar panel power and directly convert it to 240 VAC and connect to the mains (aka tie the inverter to the utility grid).

Here is a thread where I tried to explain the "basics" of inverters... It is getting more complex--It is a very complicated subject.

battery-less Outback Radian (GT/Off Grid/Hybdrid Inverter options explained)

Here is a nice thread on a GT (central) inverter installation:

Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

With some of the new NEC codes--Your local building department (and fire department) may have a say in what type of gt inverter you use (micro vs central). For various reasons, you may have to use micro inverters (for safety+code requirements).

-Bill

vtmaps · February 2016

skip21al said:
What's the opinion on optimizers that allow me to see each module performance? Is that kind of monitoring through a smartphone app, etc really that useful?

If you want to see each module's performance, you will need electronics on each panel. In which case... you don't want optimizers, you want microinverters. Both of them are electronic devices that connect to one panel. The optimizer takes DC from the panel and makes 'optimized' DC, so you still need a string inverter to make AC and tie to the grid. A microinverter takes DC from the panel and makes AC, no need for a string inverter.

Here's a good explanation:
https://www.energysage.com/solar/101/string-inverters-microinverters-power-optimizers

By the way, putting an electronic device on each panel adds to complexity and cost... In my opinion, the very marginal benefit is not worth the cost.... If you have shading on some panels, the benefit is NOT marginal, it can be dramatic.

The newer electrical wiring codes that are being adapted require the installation of a module-level disconnect switch that is accessible to the fire department from the ground. This requirement, if adapted in your jurisdiction, will make microinverters the cost effective solution, even if there are no shading issues.

--vtMaps

skip21al · February 2016

vtmaps said:

skip21al said:
What's the opinion on optimizers that allow me to see each module performance? Is that kind of monitoring through a smartphone app, etc really that useful?
"If you want to see each module's performance, you will need electronics on each panel. In which case... you don't want optimizers, you want microinverters. "

You sure about that? Solaredge offers a DC optimizer that gives web/app access to each module performance. I'd imagine there are others as well. Seems like I'd love to see how each module performs over time, and have the benefit of knowing optimal performance even if partial shading due to dirt, snow, etc. Thoughts?

I think some optimizers give the same emergency shutoff capability as well.

From what I see, the optimizers are half the cost of microinverters (or less)... and I like the idea of having a less complicated component on the roof for 20 years. Less that can go wrong... and access to inverters on the ground.

I'm open to suggestions...

mike95490 · February 2016

The shading of clouds on a portion of an array is usually not helped at all by otptimizers, the clouds move on, or they don't. Maybe if you have a very widespread array, and very slow moving clouds, you might see some benefit, but not in my opinion, it's not worth the expense. MicroInverters and Optimizers are best at managing max power under conditions where shade from fixed objects that can't be remedied with a chainsaw, travels across the array. (shadow from a street light pole, another facet of your roof, chimney).

And you have to weigh the risk/benefit of the long term reliability of an electronics package on the back of each PV panel, and how easy is it to get to one for repair/replacement. Of course panels and string inverters also can go bad too. The rule of thumb is about 10 years for power electronics and 20-25 for PV panels.

vtmaps · February 2016

skip21al said:

vtmaps said:

"If you want to see each module's performance, you will need electronics on each panel. In which case... you don't want optimizers, you want microinverters. "

You sure about that? Solaredge offers a DC optimizer that gives web/app access to each module performance. I'd imagine there are others as well. Seems like I'd love to see how each module performs over time, and have the benefit of knowing optimal performance even if partial shading due to dirt, snow, etc. Thoughts?

I think some optimizers give the same emergency shutoff capability as well.

From what I see, the optimizers are half the cost of microinverters (or less)... and I like the idea of having a less complicated component on the roof for 20 years. Less that can go wrong... and access to inverters on the ground.

I am sure that if you want to see each module's performance, you will need electronics on each panel.

Both optimizers and microinverters can do that. When I wrote "you don't want optimizers, you want microinverters", I was expressing my opinion. The downside to both optimizers and microinverters is that you are putting electronics in a harsh environment. The devices probably won't last as long as the panels themselves. In my opinion, the microinverters are simpler than the combination of optimizers and string inverter, and they eliminate the danger of transmitting DC.

--vtMaps

skip21al · February 2016

Thanks vtmaps! Why is DC so much more concerning than AC? Can't they both burn my house down or kill me in the process?

Photowhit · February 2016

With DC it's easier to maintain an arc. With AC the current switches directions 120 times per sec. Basically an AC arc is switched of and reverses directions each time the wave changes directions. An electrician shocked by AC can typically let go, but shocked by DC the electrical information to his muscles is continuously telling his muscle to clamp down...

vtmaps · February 2016

skip21al said:

Thanks vtmaps! Why is DC so much more concerning than AC? Can't they both burn my house down or kill me in the process?

For one thing, switches, relays, and circuit breakers are MUCH cheaper for AC than DC. As I mentioned earlier, if you need to switch off high voltage DC on the roof from the ground, you will pay dearly.

With microinverters each panel is effectively producing AC. All microinverters (and string inverters) have an anti-islanding function. That means if they don't see a grid, they shut down. All you need to do to de-energize the roof is switch off the power to the house... the microinverters shut down instantly. It's simple and foolproof.

--vtMaps

skip21al · February 2016

I've had some people tell me "string systems are easier to setup bc you dont have to put a microinverter on each panel". The microinverter seems to install pretty darn fast underneath the panel. Am I missing something?

Why would the install of a string system be significantly "easier" than with a microinverter? Would the wiring of a string system be any less complicated or dangerous than a microinverter system?

My local supplier says the vast majority of contractors are putting string systems on residential GT systems. Why not more microinverters if they offer module performance monitoring and easier to work with AC power coming off the panels?

NOTE: Let's assume a single string with a single central inverter. Residential roof setup.

vtmaps · February 2016

I think a lot of installers fear having to service microinverters. It's a harsh environment up on the roof, and putting dozens of electronic devices up there means that something might fail. Think about how horrible it would be to have to pull up the center solar panel on a large roof system. Many installers feel that it is a lot of work for what little the manufacturer will pay them under warranty to replace/repair the microinverter.

In terms of servicing a microinverter, that would be easier on a ground mount system. But (in my experience) most ground mount systems don't have shading issues. Also, in terms of diagnosing module faults on a string inverter, it's easy to get to individual modules in a ground mount system, so there is less need for module level monitoring.

As I mentioned earlier, if your jurisdiction requires that an array on a roof have module level disconnect from a ground location, then microinverters become much more cost effective even if there are no shading issues.

--vtMaps

On Grid- Best Voltage for solar panels minimum 4kw system, no battery packs

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