micro inverter

Re: micro inverter

This may help. It's a list of common first timer mistakes; it might save you a little trouble.
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Mistake #1 - "I just got a bunch of solar panels! How do I hook then up? I spent most of my money on the panels so the rest of the stuff can't break the bank."

The Big Idea:
I came across a cheap site on the Net and I got as many panels as I could, because these deals can't last! Getting enough panels must be 99% of the battle; the rest is just nuts and bolts and wires.

Reason this is a bad idea:
The inverters, batteries (if off grid) and the other BOS (balance of system) hardware can cost as much - in fact often costs more - than the panels themselves. You need lots of wire, since solar panels are usually mounted at least vertically distant from the load they are driving. The wire you need will often be expensive; for battery wiring you could easily pay a dollar an INCH for terminated cables. You need fuses or circuit breakers, DC disconnects that can handle high voltages and currents, inverters to use the power, charge controllers/batteries if it's off grid. You need a mounting structure that will keep your roof on your house and prevent those panels from becoming spinning 40mph battering rams in a bad storm. And all those things are very important if your goal is installing a safe system. Wiring and circuit protection are two things you CANNOT skimp on - and this is even more important for people experimenting with solar, because you're going to make mistakes.

We see this question a lot because people see a good deal and buy a bunch of solar panels and want to make them work. Nothing wrong with that, but if you are money constrained, it's much better to buy half the number of panels and spend the rest on good inverters, batteries etc.



Mistake #2 - "I just want to feed a little power back to the grid. I saw this grid tie inverter on Ebay for $99 . . . ."

The Big Idea:
Getting grid tie approval sounds like it's a pain in the butt, and I just want to generate a LITTLE solar without all that paperwork.

Reason this is a bad idea:
It's not hard to design a grid tie inverter that feeds power back to the grid. In some ways it's easier than designing a standalone inverter, since the impedance of a grid tie connection is much more constant, and you don't need to provide a time base. But there's a reason that grid tie inverters tend to cost a fair amount - they meet NEC requirements for power factor, safety and anti-islanding. And again, these things are even MORE important for experimenters than they are for professional installers, because experimenters are going to make mistakes - and you'll want that extra level of protection when you accidentally island your loads.

Also, UL listed grid tied inverters are going to be part of any serious grid tie installation. You'll learn more about real installations by working with real hardware.



Mistake #3 - "I don't want to do grid tie, I just want to charge a small battery bank with solar, run off that, then use the grid only when they get low."

The Big Idea:
Getting grid tie approval sounds like it's a pain in the butt, and this way I can avoid all that hassle and still generate most of my power via solar; just add a few batteries to my system.

Reason this is a bad idea:
Batteries are the biggest cost in any off-grid power system. Not because they are the biggest cost up front - but because they don't last long. Golf cart batteries will give you around 500 cycles discharging to 50%, which is around a year and a half of solar off grid usage. People don't want to replace their batteries every year and a half; it's heavy lifting, they're full of acid and lead, you have to recycle them, the new ones cost money, it's the dirtiest part of the system and you ruin your pants etc etc. So they find ways to make them last longer. They use larger cells that a) last longer to begin with and b) only have to be discharged to 30% (or some other lower number.) This extends their life.

For people on the grid the answer is much easier - never cycle them. Use a hybrid inverter that keeps them float charged all the time while feeding excess power back to the grid. This allows batteries to last much longer. Telecom batteries in such applications (generally called UPS, or uninterruptible power system applications) can last twenty years because they are not cycled and kept at their "happy" state of charge. Only during the rare blackout do they see cycling.

The reason Mistake #3 is a mistake is that it attempts to do the exact opposite of all of the above - it attempts to use a small battery bank and cycle it hard every single day. This means you'll go about a year between battery changes if you're lucky, and you'll be assured of a lot of heavy lifting and buying of new batteries.



Mistake #4 - "I get like ten hours of sun a day so I shouldn't need a big system."

The Big Idea: Since it's nice and sunny ten hours a day up here I should be able to generate close to full power ten hours a day - so I don't need as many panels.

Reason this is a mistake:
Equivalent sun hours refers to the total full power sun the system sees. Most systems really see full power for only a few hours a day; the rest of the time the sun is at an angle to the panels and shining through too much atmosphere to get to full power. Even tracking arrays only help a little with this. Thus 12 hours of daylight might give you only 6 hours of _equivalent_ direct sun - and direct sun is what you have to base your total energy output on. The NREL website has good maps that show you how much equivalent direct sun you can expect in various locations, and it's almost always less than you think it will be.



Mistake #5 - "My neighbor has a two kilowatt system but he uses 600 kilowatts a month! That's not helping him at all."

No Big Idea here. This is a common misconception; it confuses power (kilowatts) with energy (kilowatt-hours.) A two kilowatt system, if exposed to four hours of equivalent direct sun a day, will generate around eight kilowatt-hours of energy. Actually somewhat less due to efficiency losses in the wiring and inverter.



Mistake #6 - "I don't want to run everything, just air conditioning and my efficient refrigerator."

The Big Idea - It might be tough to run a whole house, but just a few appliances should be that much easier.

Reason this is a mistake: Air conditioning, refrigeration, and pool pumps (if installed) are the biggest users of energy in a house. (Resistance heating can be an even bigger energy user, but it's madness to try to run resistance heating on solar electric systems.) A small standalone system is much better used to provide some LED lighting, backup power for laptops and radios etc. Even better, do a small grid tied system and offset some of the load from the bigger users.



Mistake #7 - "This system is real expensive if I get it installed. But I looked online and panels are really cheap, so I'll do it myself to save money."

The Big Idea - Greedy installers make solar way more expensive than it needs to be, so doing it myself will save me lots of money and make me energy independent from the bureaucratic power companies.

Reason this is a mistake: Much of the cost you see on an installer's estimate has to do with the BOS components he will have to supply - heavy gauge wire, DC disconnects, racking etc. These are the easy to overlook pieces that you will need to install the system. Once you add all those in you'll see the actual labor costs are fairly low compared to the costs of the parts. Thus you really aren't saving that much.

For people who really want to save money, AND have experience as an electrician, you may be able to find an installer who will let you do most of the work. They will take care of the parts that are dangerous (like wiring the DC disconnect) and annoying (like applying for permits and scheduling inspections) and leave you to drill holes, bend conduit and pull wires. In general you have to know someone very well to do this, because the installer won't want you to botch the rack installation then get sued when your roof leaks.



Mistake #8 - "Why can't you combine solar-PV and solar hot water panels?"

The Big Idea - If solar panels get hot anyway, why not cool them with water, then you can use the water for your hot water! That kills two birds with one stone.

This is a not a bad idea but does not work for domestic hot water. To get 120F water in your tank your outlet water will have to be close to 130F - and that means the panels themselves have to be 140-150F. It _sometimes_ works for pool heating since panel outlet temps on a well designed pool system rarely rise over 90F, due to low water temperatures and high flow rates. This can keep the panels cooler and give you some heat. The drawbacks of this are increased complexity (two connections per panel) and safety issues (high voltage DC and water don't mix well.)