Grid-tie system questions

[filippomasoni]

Hello, I have a pretty good understanding of off-grid systems as I'm building one for a camper and have been researching for a long time.
Now my parents are considering getting a PV system and I wanted to explore a few options. I'm based in Italy, so many things could be different from the US but should be pretty much like most EU 230V 50hz.

Do you have any resources where I can get some more information? I've searched the forum but didn't really get many answers.

In my mind, a vehicle-mounted off-grid system is more straightforward because it always works in DC at the battery voltage, and everything ties into it on the busbars, you basically have loads and chargers, and the inverter is just one of the many loads.

A grid-tied system is different and now that I'm researching I'm learning things I didn't expect. Here the inverter is the main component and there are different types, here's what I learned, correct me if I'm wrong or point me to some resources to learn more.
String inverters work without a battery and only when the grid is providing power, in case of a power outage the whole system becomes useless. Cost-effective solution that has its place but not for me.
If I put energy into the grid they currently don't pay me for it or they pay very little, which is not worth it. I'd like to have a battery to use the power at night and for backup, power outages are very rare where I live, but with the current situation in Europe, if natural gas is not enough for next winter, there could be more power outages then we are used to.
Off-grid inverters also have a grid input, but looks like they are not very well suited for an always grid-tie solution. I'm a fan of Victron and the EasySolar-II looks like a very simple solution, even if it's expensive and hard to find here. But there are different ways of connecting it, like grid in-line and grid-parallel and I'm not sure which is best for what. From what I understand off-grid inverters need to have a battery to function, which is planned for the system but considering shipping time from china it will take a long time, so I was thinking to start using the system without a battery and adding it later on, but I'm realizing it's probably not possible.
Hybrid inverters are another beast altogether and I'm not very familiar with how they work, will these be able to work with a PV and grid connection during the day, and then I can add a battery in the future? Which brands are recommended in Europe?

Thanks in advance

 

[svetz]

The Frequently Asked Questions thread has links to the most common questions and answers, after that the wiki topics, and finally of course just asking questions in a thread. The best place to start is the DIY Checklist.

The "marketing" terms for inverter types are often confusing, best is to ignore it and read the actual specs. String Inverters just means the inverter operates at a high voltage which is achieved by putting panels in series to increase the voltage, and using parallel strings of panels to increase the current. See What does it mean to have solar panels in parallel and series? A subset of string inverters are hybrid inverters that can invert DC power from the solar array or from a battery - so they can power the home in the event of a grid outage.

All inverters fall into one of a few categories, the two main ones are:
Off-grid - This is what you'd use when no grid is available, it generates its own sine wave.
Grid-tied - To be connected to the grid, the phase of the power from the inverter must be synchronized to the grid's phase. These inverters do that, and it allows them to push excess power into the grid. They may have a net-zero or no-export feature that keeps them in phase with grid power, but doesn't allow export. These inverters must shut down when the grid is down; the exception to that is hybrid inverters, which when the grid shuts down go into off-grid mode and provide battery power.

An alternative to string inverters are "Microinverters". Microinverters are either grid-tied or hybrid inverters and IMO superior in most ways to string inverters:

• More reliable with longer warranties (25 years) than inverters (~5-10 years)
• If one fails you're only out a panel's input until you can get it replaced. Replacement shipping costs are less because they're each small and light.
• Typically they have the highest conversion efficiencies
• They are silent
• They have very low self-consumption (e.g., not running cooling fans)
• Per panel MPPTs will always outperform or equal a String system
• Shade on one panel doesn't cause loss from the rest
• Unlike string inverters, they can handle odd roof geometries where the solar angle is different for multiple panels (solar optimizers (e.g., SolarEdge) can also do this).
• They sit underneath the panel and don't take up additional space (this was important for me as I couldn't fit an inverter under the house due to the building codes, but the rooftop was well above the flood elevation).
• Lightweight, quick, and easy for one person to install, but one per panel means more time overall spent on it
• Typically you can get per-panel diagnostics which makes it much easier to diagnose problems
• Lower voltage off the roof.
• Don't need to do string designs or calculate string voltages/amperages (see Figuring out how many panels in series and parallel based on your MPPT and Microinverter Voltage Rise Design Issue)
• Allows for different panels of different ages to work together without loss, excellent for growth over time
• The RSD and arc-fault is generally built-in
• Enphase is the best known with the best support and monitoring (at this time), but also the most expensive. Other companies have lower cost microinverters that are on par cost-wise with string inverters.

I believe they would have replaced string inverters long ago except for one major problem: until recently they were only grid-tied, so couldn't provide power when the grid went out.

Enphase now has a "sunlight" solution using "hybrid" microniverters (IQ8s) that provide power while the sun is up without an expensive battery (as battery prices are expected to fall it's pretty attractive).

But, for night consumption Enphase's Enlighten solution solves that as does AC coupling, but both increase costs giving string inverters an edge over Enphase. When V2H comes of age or 3rd party AC batteries start coming out (see the thread incrementally adding AC batteries) then microinverters will probably become the best and cheapest way to go.

But it's sort of a religion, for everyone that likes microinverters you'll find someone ready to say string inverters are better.

Hope that's of some help!

 

[filippomasoni]

Thank you very much for the thorough explanation. I have some material to go throw now.

Microinverters are definitely the future as that's going to be the most convenient solution for all of those reasons. When you talk about AC batteries, do you mean batteries with a built-in inverter? Or is there actually development of a chemistry that delivers AC, how the hell does that work?!

As of right now, there is no way to have a battery backup with microinverters, right? The hybrid ones can provide power when the sun is up, but when it's not you would need a battery with another inverter/charger, which would make using microinverter pointless.

 

[BentleyJ]

When you talk about AC batteries, do you mean batteries with a built-in inverter?

As of right now, there is no way to have a battery backup with microinverters, right? The hybrid ones can provide power when the sun is up, but when it's not you would need a battery with another inverter/charger, which would make using microinverter pointless.

Microinverters can have battery back up but only indirectly. For Example Enphase does offer a proprietary battery/inverter solution that works with their microinverters. It is also possible to use add generic battery back up with AC Coupling. This would be accomplished with a Tesla PowerWall or Franklin Whole House system for example. Both of the aforementioned products are battery/inverter systems packaged in one cabinet.

 

[davgraham]

I too am looking for a system now. I almost bought an Enphase system with a loan that was cheaper than pg&e monthly costs today and it's only going up. That was until I found that Enphase is cloud tied in a big way, it will apparently operate if it looses internet, but you loose all monitoring which is phone or PC based connected to their servers in the cloud. If you lose the internet connection you have no way to control or monitor AND you can have the battery deauthorize if it can't phone home. How's that for the next emergency, you have all the power on your roof and garage wall but you can't get to it because Comcast, etc... has gone down!

I'm back to looking at 48v hybird grid-tied with cheap EA4 batteries. I have to have a licensed person do install because of HOA, if I can find install only guy to put stuff on roof I can do electrical hookup.

 

[svetz]

... the battery deauthorize if it can't phone home....

Do you have a link for that? Haven't heard that before and couldn't find anything while googling for it other than "the warranty may be affected".

Fortunately, Enphase is not the only microinverter company and there are other battery solutions for microinverters.

... you loose all monitoring which is phone or PC based connected to their servers in the cloud...

That was true for a couple of months at the beginning of the year, but isn't any longer.

The Enlighten cellphone app can work with the system totally offline, I've tested it and had no problems. At the time you had to switch the Envoy into AP (access point) mode which wasn't super convenient, but they were going to change that. Possibly they already did and I've forgotten about it.

Update: Supposedly AP mode isn't required. Here's what it says:

• if your Wi-Fi router is on and the gateway and cell phone are connected to same router [does not matter router is off internet], even in that case local path is established. You don’t need to go via AP mode. So, there are two ways for local connectivity
  • Both cell phone and gateway on same network [Same Wi-FI SSID].
  • Or if above does not work, AP mode network. This is most reliable and it will work always. We are working on automating this process of pressing the AP mode button and navigating to phone wifi etc.

For local monitoring via their API when the network is up, you can get a token from their internet service that allows you to locally connect to it for up to six months at a time. My local monitoring system gets a new six-month token every day, and reuses the prior token if the system isn't responding for any reason. It was annoying when they changed it, but wasn't hard to update.

 

[filippomasoni]

Thanks for the advice. After looking up legal stuff I learned that here in Italy it's still not possible to do a DIY grid-tied system, only an off-grid one. Recently new legislation came in with some simplification and I thought it was finally possible, but turns out it's only made to shorten approval times by a few months. A grid-tied system has to be installed by a certified company with all of the paperwork included. There's a 50% discount/incentive but since then and all of the eco-incentives they've been doing for the past year, prices have gone up by more than 150% so in the end, it will be more expensive than a few years ago without incentives... It's a mess!
I have someone come in next week for a site inspection and we'll see how it goes.

 

[svetz]

Can you DIY it and then just pay someone a low amount to do the "certification"?

If you don't have incentive-based net metering in Italy it might be more worthwhile to install an off-grid system for critical circuits (or zero-export system if allowed).