I am considering a home solar photovoltaic installation, probably 5 kW or so. The usual configuration around here is with a grid-tie inverter and net metering. But when the grid goes down, the inverter shuts off (to avoid back-feeding into the power grid and electrocuting linemen).
We are interested in being able to use the solar when the power goes down (not infrequent in this rural area). According to the solar installer, the only way it is possible to do this is to also install a battery system like Tesla PowerWall. The idea is that when the inverter loses the 60 Hz sinewave from the grid it shuts off; the inverter in the PowerWall supplies this sinewave, so the inverter doesn't shut off, and there's also a mechanism to disconnect from the grid.
That sounds reasonable. But the cost is not. Two PowerWalls would roughy double the cost of the installation. During outages, we would be satisfied to only have power when the sun shines; this would keep food safe, allow us to draw water, etc. Electrically, there is no reason this can't be done without the expensive battery system. All that is necessary is to generate the 60Hz reference for the solar system's inverter, and to disconnect from the grid.
Has anyone been able to manage such an installation? Would it be strictly DIY (perhaps feasible as I'm a electrical engineer)?
Best Answer
This is an XY problem at the industry level. Imagine some guy says "Design me a dimmer" and you design him an excellent triac dimmer, no big. And then he goes "Cool, can you just tack on a VFD onto this thing?" Facepalm, not in any elegant or cost efficient way.
So it is with solar panels. Everybody and their dog is selling 2007 tech, a solely grid-tied system because it's established, built at scale, and financial models are well established for sharing profits with the finance company, etc. etc. They can build, sell and finance that all day all night. It's the cheapest way to get you in the door and talking about solar. But they are selling system X.
Now, post-Sandy, there's the hue and cry to "bolt on" off-grid capability, in a manner compatible with dumb consumers. They did it, but it's inelegant and pricey, like bolting a VFD on a triac. The interlocking-for-linemen takes extra resources. I'd call it System XX.
For what you want, you want system Y.
And since you presumably don’t want to design every inch of it from scratch, we should grab out of the traditional "off-grid solar" parts bin. That means a few minor concessions.
The inverters are simply too different
A grid-tie inverter needs to follow grid frequencies, take all its input power, and force it all onto the grid by pushing voltage as hard as necessary.
An off-grid inverter needs to generate its own AC frequency, take only enough input power to do the job, and provide only as much power as the loads are drawing.
These are dramatically different roles and one inverter doing both will be a costly engineering challenge. They do exist; there are "hybrid inverters" on the market capable of both. Of course they are pricey and have their own system design requirements, not least batteries. You are probably better off getting cheap commodity COTS inverters for each application.
That said, your "No batteries whatsoever" plan is flawed. There will be cases where you have loads simply too big for the panels themselves to carry, e.g. motors starting up.
An off-grid system, "plus"
This is where I want to raid the parts bins of the off-grid solar people. The only thing that isn't out of their wheelhouse is the grid-tie inverter, but that's no big.
Their systems are violently battery-centric, for obvious reasons. Because of this, there is no viable way to avoid it, but nobody says your battery can't be tiny. Last I checked, they sell used car batteries for $20, there ya go.
Now in the house, we have a main panel with the main breaker. It has 2 breakers in it: The breaker to the grid-tie inverter, and the breaker to the subpanel.
The subpanel is where the magic happens. It has a $23 Siemens or Square D style "generator interlock" which backfeeds two breakers (one at a time). One side it's on utility, the other it's on "generator" (read: off-grid inverter). Every load you ever foresee running off solar goes in this panel. If you put every single load in your house in this subpanel, I won't tell :)
"Normal" operation involves the subpanel interlock in "Utility" mode, the off-grid inverter driving into nothing (shut it off), the charge controller instantly topping up the tiny battery, and diverting all solar power to "dump". Dump then goes into the grid-tie inverter and is sold back to the power company.
"Power out" operation, you throw the interlock to "Gen", power up the off-grid inverter. As the off-grid inverter draws to supply household loads, the charge controller diverts power to it as needed, blocking the rest of available current from the panels (since "dump" is a dead-end). If a motor starts and pulls more than panels can deliver, then we see how good or big your battery is.
And, if experience shows you it's advantageous to have a real battery, you can get one.
Note that in all cases, power flow is one direction only, and nothing ever reverses flow. That is a huge advantage over the "System XX" type setups, which flow power every which way, and need to "bolt on" elaborate, expensive interlocking systems to avoid backfeeding. Here, backfeed protection is a plain interlock, except for the grid-tie inverter which is UL 1741 compliant; you can't find a grid-tie inverter that isn't.
The whole system is cheap, scungy, simple, bottom-tier tech. That's not to say "don't use good stuff" - yes, of course use Morningstar or Midnight Solar charge controllers. But we're using their standard boilerplate gear, not special-purpose exotica.
Of course, if you want to phone up one of the finance-- I mean solar installation companies, they aren't going to be conversant in a simple system like this. They won't want to build a thing like I describe here; they've got a standard package they sell over and over that they know how to build.
As far as panels
Many panels sold today have built-in grid-tie "micro-inverters", so their output is 120/240VAC and they drop out if the grid fails. Cheap way to comply with NEC 690.12 Rapid Shutdown, but totally useless off-line. There are other panels with smarts on board, too. You want plain Jane run-of-the-mill DC output panels, again, the cheapest thing on the market.
There's also the law to contend with. Roofs have a super important job, and I'm no fan of compromising that with a bunch of holes. Many other places on your property would benefit from shade/snow sheltering, so I say "build freestanding racks over your parking spot etc." But if you do roof-mount, you have to comply with 690.12:
All in, or all out
Panels aren't just panels anymore. Many have embedded electronics (like microinverters) to "help out" the installer. As such, there's no halfway anymore. You can't just let these people install what they please and expect to hack it later: you might, but you're relying on luck. So you really are at a courageous crossroads; you have to jump all-in, one way or the other.