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My wife and I have been talking for a few years now about getting some solar panels installed on our roof. We even went so far as to have a sales rep for an installer come out and talk through our options. He told us that we could:

  1. Have a system that ties into the grid and sells electricity back to the power company. This sounds like a great idea, and seems cost-effective as well.
  2. Have batteries that store surplus power to provide our home with backup power in case of a blackout. Again this sounds like a great idea, but it also sounds like the cost would be prohibitively expensive for us.
  3. Have a battery-less system that provides power to our house when the power goes out, but only when the sun is shining.

.... but that we could not have both (1) and (3) without (2). He insisted that if we are connected to the grid, there is no way to switch our system to directly powering our house in the event of a blackout; if we wanted to be connected to the grid and have emergency power, a battery backup system is the only way.

This makes no sense to me, and he was unable to explain why (other than "it's just a different kind of system"). I suspect he was just trying to upsell me to a battery backup system. But since I don't really understand the technology I thought I would ask here. Can someone explain to me what, if anything, are the issues here? Please be gentle, as this stuff is well beyond my zone of competence.

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  • It's my understanding (hence comment, not answer) that the "sell to the grid" option pushes power to the grid all the time and only does that. Nothing from your panels goes into your house. You pay the PoCo for what you use, and they credit you for what you produce. With batteries, the panels charge them then push back to the grid once they're full up. Not sure why they can't do "feed the grid" with an "oops, power's out, feed the house" switch, but I just don't think any system offers it. Probably because it's useless on cloudy days and at night. – FreeMan Jun 12 '20 at 18:57
  • Not quite. I have 12kw solar array which is a battery-less system. We call it "net metering". Sometimes I'm using more power than I'm producing and the PoCo makes up the difference, other times, I'm producing more power than I'm using and selling it back to the PoCo. It's not an all or nothing thing. – George Anderson Jun 12 '20 at 19:06
  • The system we have will provide power to the house and any excess goes to production - so the less you use on a sunny day the better. We get paid 5* the price to sell compared to purchase... but that is an old contract that they no longer do (about 12 years left on a 25 year contract)... – Solar Mike Jun 12 '20 at 19:08
  • How can you expect power at night if the grid fails and you don’t have any storage - moonlight won’t cut it... – Solar Mike Jun 12 '20 at 19:10
  • One more comment. I also have a backup generator for when the power goes out. It's frustrating to have to run it when I have a large solar array that doesn't work if not connected to the grid. The inverters disconnect when the power goes out so as not to backfeed into the grid, potentially electrocuting lineman. We don't want to do that! I've done a lot of reasearch into options. The biggest issue is switching and loading. An inverter with battery backup isn't going to give you anywhere near 200 amps. So then you have to decide what loads are supplied by battery backup and asso wiring. – George Anderson Jun 12 '20 at 19:12
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Would it be accurate to re-state the option 3 differently: Have a battery-less system that provides power to our house when the power goes out, but only at all times when the sun is shining, and sells excess to the grid?

Probably the biggest reason why your solar installer can't do #1 and #3, ie have panels that sell to the grid normally, and work islanded when the grid is down, all without a battery bank, is this: His company is trained for and sells only a few packaged PV systems. They simply don't carry one that functions this way.

It's possible to do, of course, but accomplishing it requires a custom design using lower-level components. Many solar installers prefer to stick to plugging together pre-designed commodity systems comprising off-the-shelf modules.

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  • Sure, I'll take that paraphrase of Option 3. Does it change the answer? – mweiss Jun 12 '20 at 19:42
  • The issue is a fair bit deeper-seated than simply "the installer doesn't have a system that works the way the OP wants it to" -- the battery in a multimode system serves as a buffer of sorts, not only between day and night, but against fluctuating solar outputs during the day.... – ThreePhaseEel Jun 12 '20 at 19:44
  • @ThreePhaseEel the grid tie is a better buffer available than the battery... – Solar Mike Jun 12 '20 at 19:45
  • @SolarMike -- but only when it's available – ThreePhaseEel Jun 12 '20 at 19:49
  • @ThreePhaseEel well we have a buy/sell contract and very few power cuts. Switzerland has a very stable power infrastructure. – Solar Mike Jun 12 '20 at 19:51
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The easy part

A Great Big Switch is simplicity itself. You need any Siemens subpanel of 12 spaces or more, and the ECSBPK01 interlock ($25) and two 2-pole breakers. A 60A from your main panel and a whatever-A from your off-grid source.

You leave 2 rows empty for style points, then below that you fit the breakers for the circuits you wish to be capable of functioning on alternate power. You can have 36 of those (32 if you want to keep those style points) and if that isn't enough, feed a sub-sub-panel.

Throw the 2 top breakers to the right, you're on utility. Throw the 2 top breakers to the left, you're on ???.

Part 1: The above.
Part 2: ???
Part 3: Profit

Trying it batteryless

Can we do this thing totally batteryless? I seriously doubt it. Here's why. Your freezer starts up, and for a few milliseconds it's going to pull LRA of about, say, 3000 watts. And it expects this in AC power, and so you have an inverter behind an MPPT controller. The inverter goes "OK, that's 25A, hey solar system, I need 125A" and the solar system goes "Unable".

Voltage plunges toward zero, which means inverter output voltage plunges toward zero, and the freezer sits there at LRA unable to start. And this continues indefinitely until ??? some safety system in the inverter kicks in and shuts off the inverter.

Why didn't this work? You have 5000W of solar panel. Yes, if it's aimed directly square on to the sun. At the angle it's at, at the angle of the sun, 3000 is the best you could do, but it's also cloudy. In fact your panel has been passing in and out of clouds all day, bouncing practical output between 500 and 3000.

So you need at least an ultra-capacitor to get loads started, and really it should be enough of a battery so the freezer can run a normal cycle without emptying the ultracaps in 5 seconds because a cloud rolled over. These are not good failure modes for either the inverter or the freezer.

Size Matters

Obviously what you are afraid of is a $5000 battery bill, and then, the recurring $5000 battery bill if PV batteries are anything like car batteries. Some are, some aren't.

But the size of the battery is variable. It's all about sizing, really. You need the battery to carry you through the stuff you don't expect - like clouds. So it can be quite a small battery pack, as small as $100 on the low side.

If you don't want to gird up for full fridge + lights + Roku + router + TV for 3 days straight with little usable sun, you don't have to.

If you ever change your mind you can always add battery capacity.

Don't overestimate your solar panel's capacity

You may imagine solar is a steady-state source, but that's not really true. See if you can get a second-by-second readout of someone's solar system. They're all over the map on days with any clouds at all. Again, batteries make that moot for in-and-out-of-clouds days.

But don't overestimate their generating ability during deep clouds. Storm clouds can be quite dark - yesterday I was working by daylight and suddenly it simply became too dark to work - a big storm rolled through. And of course snow obscures the panel totally. (except Solyndra which had a very slick answer to that, but nobody cared.) You could wake up and find the panel covered in ice from an ice storm that also knocked out your power. (It's too cold; the ice doesn't chisel off without risk of panel damage).

So I think you are laying an assumption that solar power will always be ample during a power outage. I'm not sure of that at all, but I'm from the rust belt - power outages usually happened in winter storms, where you would not have the solarization to make a panel very productive, first because of clouds/snow/ice, and second because it's winter in the first place.

You can also top up battery capacity with a generator if you have long sunless runs.

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  • Putting the solar panels at an angle of 75 degrees (ie nearly vertical) makes them self shedding for snow as well as optimising the winter output - summer output is easy. – Solar Mike Jun 12 '20 at 21:53
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    "You may imagine solar is a steady-state source" I think too many people think this. – FreeMan Jun 13 '20 at 13:19

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