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Having decided on a grid-tie system, it was now necessary to decide on what components (brands) of equipment to use to build the system. There are a dazzling array of manufactures and products out there.
There are now two fundamental types of inverters to go with for grid-tied systems. The most common grid-tie inverter configuration uses a single inverter for the whole system. Several companies make these large inverters intended for grid-tie systems. These inverters (sometimes called string inverters) are designed to take a string of PV panels connected in series. The voltage from a string of PV panels can be several hundred volts, and the string inverters for new grid-tie systems are designed for these voltages. There are several good manufactures of these inverters, including Fronius, SMA Sunny Boy, Xantrex, ...
These inverters are highly developed 2nd generation products. The scheme they use, in which multiple PV panels are connected together in a series string both simplifies the wiring and increases the efficiency by keeping voltage drops down without the need for very heavy wire gages.
The other fundamentally different choice is to use an individual "micro-inverter" for each PV panel. In this type of system, each PV panel gets its own grid-tie inverter to convert the DC output of one PV panel into 240 volt AC, grid compatible power. As far as I know the only company making such a system is Enphase.
I think both of these approaches can result in a good system, and we went back and forth for quite a while on which way to go. In the end, we decided on the Enphase micro-inverter system.
Some of the features of the micro-inverter system:
- Each PV panel gets its own inverter -- this means that each PV panel gets its own MPPT tracking, and the PV panels do not have to be the same size or brand. Wholesale Solar actually has a demonstration system in which each PV panel in the system is a different brand and power level.
- Expansion of the system is easy, in that you can add additional PV panels and micro inverters -- the PV panels do not have to be the same brand or size as the existing ones.
- If one inverter goes down, the rest of the inverters continue to produce.
- Enphase has a unique system in which each inverter reports how it is doing to an "EMU" unit which you buy along with the system. The EMU unit in turn sends this information on to the Enphase website. Each system gets a website set up by Enphase which reports power production and any problems the system may be having.
If you don't have a sufficiently good internet connection to support this, you can connect a computer directly to the EMU and still get some simple reports from it.
There is a $2 per year fee for this online reporting per inverter per year. Here are a couple examples:
Here is an example of the what it looks like on Guy's system...
- The inverters are designed to live in a very wide range of environmental conditions.
- The system is probably a bit more straight forward for a DIY home owner to install.
On the down side, multiple micro-inverters means multiple potential failure points, and the even though the company appears to be doing well, it is relatively new. When you look strictly at the dollars per watt of inverter capacity, the Enphase system appears to me to be more expensive, but it may earn that back with the way it tracks and optimizes each panel separately -- that is, it may get more out of the system.
In the end we decided on the micro inverter approach by a small margin. Part of this decision had to do with the ease of adding capacity to the the Enphase system. You don't have to worry about having to replace your entire expensive single inverter, you just buy as many more micro-inverters as you need, and if you can't get the same PV panels you used on the initial system (a common problem), you can use different PV panels. The other reason, which may be somewhat unique to our situation, is that if we were to go with a single inverter, by far the best place to install it would be on the outside wall next to my meter. But, this is Montana, and while these inverters are designed for outdoor use, the low end of the temperature range in the inverter specs does not go low enough for our very low winter temperatures. This may be something that the inverter can handle OK, as it would tend to warm up as it starts getting power from the PV panels, but I got mixed opinions on whether it was a good idea or not. If I were to mount the inverter inside (which some people recommended fairly strongly), the whole job becomes significantly more complex. On the other hand, the Enphase inverters are rated all the way down to -40F, and are designed for long term outdoor exposure.
We found the Enphase customer support people easy to get hold of, knowledgeable, and genuinely interested in resolving the couple small questions and problems that came up. They also provide a good library of downloadable docs on the system that cover the installation well...
|Update January 7, 2010:
One small concern I had about the Enphase micro inverters is that they are recommended and often packaged with PV panels that have a higher power output rating than the inverter itself has. For instance my system uses the 190 watt inverter with 215 watt PV panels. We have had a chance to see how this works out -- details here...
Having selected the Enphase micro-inverter system, the only other significant choice on components was to select the PV panels.
The most commonly used Enphase inverter (right now) is the 190 watt model. This inverter handles a wide range of PV panels up to 230 watts. Enphase publishes a list of modules that are compatible with each of their inverters. You need to make sure the PV panels you buy are on on this list. While there are many brands and models on the list, some well known brands (e.g. Evergreen) are not on the list because their panels are not electrically compatible with he Enphase inverter.
I ended up selecting PV panels made the REC Group. To be honest I had never heard of these panels until I saw them in the WholesaleSolar kit, but when I looked into them and the company, they appear to be a high quality choice. They seem well made, and have worked fine so far (all 3 days since I put them in :).
One good thing about the way the PV industry has matured is that there are lots of good choices out there, so have a look around, and see what seems to fit your situation.
About the only remaining components are:
- An AC disconnect switch near the meter. This is a very standard, off the shelf item. I just bought the one that the outfit I bought the system from used.
- A junction box at the array where the connection from the string of Enphase inverters is made to the underground wire that runs to the house and meter. I went with a large metal box that our local hardware carried.
We ended up buying most of the components for the system as a kit from WholesaleSolar.com -- this is the kit we got...
The kit approach (I think) turned out to be a good way to go.
Wholesale Solar was good to deal with. They always answered the phone and were willing to answer questions, the kit was pretty complete and the components supplied in the kit were all high quality. They also met all of their time and delivery commitments. To the extent that I looked into it, their prices were competitive. I'm sure there are lots of other good outfits out there, but there are also definitely some marginal ones as well -- I would take the time to at least talk to your prospective source over the phone -- ask them a few questions, and try to get a feel for whether or not they will be there if you have a problem.
If when you read "kit" you are expecting a Fisher-Price style insert slot A in tab B level of instructions, you will be disappointed. The kit basically includes the parts you will need and that's about it. It will be up to you to read the individual installation manuals from the manufacturers and work it all out.
Also, be aware that kits are offered by a lot of retailers, and content of these kits varies quite widely. Some are way short of what will be needed to do a system -- for example, some don't even include the PV panel mounting rails. Even the kits that are fairly complete will almost always require adding some parts. You really need to figure out what you will need right down to the last little clamp, nut and washer and then compare this to what's in the kit to see how much extra will be needed. As it turns out, the Wholesale Solar kit assumed that the 10 PV panels would be installed in two rows of 5 (probably the most common arrangement), but, since we were doing one row of 10, the kit came short one set of mid row PV panel clamps, and with an extra set of end of row PV panel clamps -- had I done a careful job of verifying what I wanted, I could have saved an extra order and shipping time.
I think that we could have done somewhat better on total cost by looking very hard for bargains, doing a roof mount system, and (possibly) with a single inverter system. But, we are all around pleased with the system and feel it was worthwhile to spend a few more bucks in some areas.
|Next -- Permts and Net Metering|
Gary November 22, 2009