SF Bay Area solar system planning: help me avoid making dumb mistakes

[Blammar]

I'm about to get a new roof installed and am going to coordinate with a solar panel installer, so that the conduits and supports for the panels can be put in place during the reroofing. (There's also an apparent additional tax credit that applies for the new roofing under the panels, I am told.) (The installer is Provoltz, in Campbell, CA.)

The current plan is to use Sunpower E19/240AC panels. Each panel contains a microinverter and generates +/-240VAC directly. The reason I suggested that is we have only one south-facing roof exposure, and the smaller size of these panels will allow the maximum amount of roof surface to be covered. Additionally, I like the multiple redundancy of the design -- multiple panel failures merely drop the power delivered, and we don't need to install an inverter in the garage.

I would also like to install a battery backup, preferably a Li-Ion system, similar to Corvus's AT6500.

Sadly, I consume about 6KW during the day. I'm aware I should reduce that, but I do have multiple computers active >16 hours/day and so it won't be easy to reduce power draw.

It's not clear how much power I will be able to get from the panels, due to lack of south-facing roof area as well as tree shade we are not going to cut away. Let's assume, all things considered, I can actually get 6kw peak.

The functionality I want from the battery backup is a whole-house UPS. Grid power fails, backup (+ solar if sunny) kicks in, gives us time for a graduated shutdown. Ideally, I'd like most of the AC circuits to be shut down, and only the essential ones powered until the UPS runs out. I'd be delighted if the UPS capacity were about 6KWh (that'd give me maybe 4 hours for the essential stuff, at a guess.)

Basically, I just want to install a non-lead-acid UPS on say half of my panel and do a "normal" connection from the solar system to the grid/house. (I'm going to have to go to 2 panels anyway since the current one at 200A really has too many doubled slots for my liking. So my thought was to assign one panel as nonessential and the other as essential.)

Does that make sense? Is that something I need to have engineered, or can a competent electrical contractor just make it work? Note that, due to the AC solar panels, I am running 240VAC only everywhere, which hopefully will drastically simplify things. (Though the panels appear to be 240VAC single phase...)

If I need it engineered, how do I find an engineer to do this?

Has anybody done something like this already? Are there any good Li-Ion based UPS systems? (The Corvus ones apparently do not have a U.S. dealer...)

[Naptown]

What you are describing is a hybrid system.
Your panel choice may preclude you from doing this as the micro inverters will only allow an AC coupled system
Consider going to a panel without the micro inverter like an X21 345W panel
This will allow you to use a hybrid inverter such as the Xantrex XW6048 which will provide the 6Kw you need and will also grid tie
This will also give you the advantage of being able to charge the batteries during an outage and is simpler than an AC coupled which will shut down if the batteries get too low and will not restart until either a generator is connected or the grid comes back up

[Blammar]

Understood.

Upon further thought, using AC solar panels with a UPS as the backup storage is less efficient for one, and has a potential issue when the grid power is out. At that point, the UPS is getting AC power from solar, but the UPS is likely to be confused when the power draw on the house side is more than the AC power coming in from the solar side.

I'll go learn about hybrid converters now, thanks!

[Naptown]

Actually no
The grid tie inverters shut down when the grid goes down so no charging at that point till the grid comes back up.
They can be used in an AC coupled system but the problem comes in managing the charge put on the batteries and loads on the system.
Really too complex to discuss here but the most reasonable approach to what you want is the Xantrex or outback systems. Xantrex is preferred as you can use a 600 V charge controller
( XW 80-600) and this solves a number of issues in a hybrid.

[Blammar]

I'm finding that grid-tied systems will shut down when the grid goes off, even if more than enough power is coming in from the solar panels to energize my house. Clearly, that's unacceptable.

It looks like SolarCity is the only vendor that has the Tesla Li-ion energy storage I want to use (I do not want to use lead acid batteries at all.) So I've put in for some of their free consultation. I'll post here once I get more information.

[Naptown]

Just curious why no lead acid and yet A more complicated and expensive battery. If maintenance is an issue use AGM batteries. They are maintenance free and do not require watering etc.
also you will not be able to get the very high efficiency panels you started with as solar city does not have access to them so you will be able to fit about 2/3 the power on the roof.
The Tesla / Solar city connection is not curious at all as both were founded by the same person.

[Sunking]

Sadly, I consume about 6KW during the day. I'm aware I should reduce that, but I do have multiple computers active >16 hours/day and so it won't be easy to reduce power draw.

Well I think you mean 6 Kwh/day which is almost nothing. National average for a small 1200/ft2 home in the USA is 30 Kwh/day so you are living like a hermit at 6 Kwh/day

[Sunking]

I'm finding that grid-tied systems will shut down when the grid goes off, even if more than enough power is coming in from the solar panels to energize my house. Clearly, that's unacceptable.

That is the way they are suppose to work.

It looks like SolarCity is the only vendor that has the Tesla Li-ion energy storage I want to use (I do not want to use lead acid batteries at all.)

That would be pretty foolish. The lithium batteries you refer too will cost you roughly 10 times more then FLA and not last as near as long as quality FLA. Not to mention Lithium batteries are far more toxic than lead acid.

[Blammar]

Well I think you mean 6 Kwh/day which is almost nothing. National average for a small 1200/ft2 home in the USA is 30 Kwh/day so you are living like a hermit at 6 Kwh/day

No, my monthly consumption is between 1300 and 2000 KWh per the PGE bill. The 6KW was the peak I believe I measured correctly during a hot summer day.

I'm surprised the national average is 30KWh/day, as PGE is telling me my energy consumption is 4x their average (i.e., PGE is saying their average is closer to 15-20Kwh/day.) You're saying I'm only 2X the national average.

[Blammar]

That is the way they are suppose to work.

Understood, and I can come up with a hypothesis as to why they are designed that way (primarily, there's likely to be a mismatch between what your house requires in power and what the array can deliver.) The "unacceptable" part comes from my wife who says, if we're paying for solar we damn well better have some power to the house when it's sunny and the grid's down.

That would be pretty foolish. The lithium batteries you refer too will cost you roughly 10 times more then FLA and not last as near as long as quality FLA. Not to mention Lithium batteries are far more toxic than lead acid.

I was looking at AGM batteries instead, as we're sloppy when it comes to maintenance. A U.S. Battery bank of 8 AGM L16 batteries is about 18.7KWh and sells for $4.1k or $220/KWh. The Tesla Li-ion packs are expected to cost in the range of $200/KWh (though the current SolarCity prices appear to be around $2000/KWh, which does seem to be price gouging or perhaps just the early-adaptor premium.) The Model S battery is 60KWh with a ~10 year expected lifetime, current replacement cost is $30k, projected replacement cost is $12k.

Okay, I have to agree with you on the prices, though I'm not so sure about the lifetime. Don't AGM batteries have a ~4 year life if you're constantly deep-discharging them?

Regarding toxicity, used lead acid batteries are considered hazardous waste, whereas used li ion batteries are not.

[SunEagle]

No, my monthly consumption is between 1300 and 2000 KWh per the PGE bill. The 6KW was the peak I believe I measured correctly during a hot summer day.

I'm surprised the national average is 30KWh/day, as PGE is telling me my energy consumption is 4x their average (i.e., PGE is saying their average is closer to 15-20Kwh/day.) You're saying I'm only 2X the national average.

First off there is a difference between kWh and KW. The kWh is the number of kilowatts use per hour. If you used 30kWh/day that comes to an average of 1.25 kilowatts per hour. The 6KW is your peak measurement which is the number of kilowatts used within either a 15 to 30 minute time period depending on your utility. That refers to the highest amount of energy used within a short period of time and it is called your Demand.

Your 1300 to 2000 kWh per month comes to 43 - 66 kWh / day for a 30 day period. If PGE's avg is 15-20kWh/day then you are using at least 3 times that amount and almost 2x the National average.

I would say your first order is to lower your daily use which will save you a lot of money.

[Blammar]

Just curious why no lead acid and yet A more complicated and expensive battery. If maintenance is an issue use AGM batteries. They are maintenance free and do not require watering etc.
also you will not be able to get the very high efficiency panels you started with as solar city does not have access to them so you will be able to fit about 2/3 the power on the roof.
The Tesla / Solar city connection is not curious at all as both were founded by the same person.

Yeah, it does look like there's a premium on Li ion batteries still, sadly. I was hoping that the tech was advancing more rapidly. Sunpower's panels are ~20% efficient; I wasn't able to find any info on Solarcity's panels.

So Plan A is currently a hybrid inverter with a charge controller hooked up to an AGM battery bank. It's still not clear to me what happens when the house needs more power than the inverter can supply. (I'm aware you're not supposed to do that, but I can imagine a couple of years from now I put on a couple more circuits and forget to check the drain vs. the inverter's supply capacity.)

Oh, our house is surrounded by trees, so there'll be shading issues on individual panels during parts of the day. Should I be taking a micro-inverter approach here per panel, which as I understand allows me to get the maximum power that each panel is able to deliver?

[Naptown]

Understood, and I can come up with a hypothesis as to why they are designed that way (primarily, there's likely to be a mismatch between what your house requires in power and what the array can deliver.) The "unacceptable" part comes from my wife who says, if we're paying for solar we damn well better have some power to the house when it's sunny and the grid's down.



I was looking at AGM batteries instead, as we're sloppy when it comes to maintenance. A U.S. Battery bank of 8 AGM L16 batteries is about 18.7KWh and sells for $4.1k or $220/KWh. The Tesla Li-ion packs are expected to cost in the range of $200/KWh (though the current SolarCity prices appear to be around $2000/KWh, which does seem to be price gouging or perhaps just the early-adaptor premium.) The Model S battery is 60KWh with a ~10 year expected lifetime, current replacement cost is $30k, projected replacement cost is $12k.

Okay, I have to agree with you on the prices, though I'm not so sure about the lifetime. Don't AGM batteries have a ~4 year life if you're constantly deep-discharging them?

Regarding toxicity, used lead acid batteries are considered hazardous waste, whereas used li ion batteries are not.

A couple of things
You will not be deep discharging them on a regular basis as they will only be used during the odd outage unless this occurs every day.
Lead acid batteries are almost 100% recyclable and are recycled regularly ( Why do you think the auto parts store charges a core charge if you dont bring in your old battery when you buy a new one. Almost none of a battery ends up in the waste stream.
I do not know about lithium Ion and their ability to be recycled.
$2000 a KWH sounds about right for the Lithium batteries

[Blammar]

First off there is a difference between kWh and KW. The kWh is the number of kilowatts use per hour. If you used 30kWh/day that comes to an average of 1.25 kilowatts per hour. The 6KW is your peak measurement which is the number of kilowatts used within either a 15 to 30 minute time period depending on your utility. That refers to the highest amount of energy used within a short period of time and it is called your Demand.

Your 1300 to 2000 kWh per month comes to 43 - 66 kWh / day for a 30 day period. If PGE's avg is 15-20kWh/day then you are using at least 3 times that amount and almost 2x the National average.

I would say your first order is to lower your daily use which will save you a lot of money.

Understood. I've ordered a kill-o-watt meter which will arrive tomorrow, and we're going to go to the ac panel and shut off each circuit and check the power use delta (using a stopwatch to time how long it takes the meter to make 1 turn.)

At some point though we do still want to live in the 21st century, so we are still going to fire up our A/C when it gets too hot...

Thanks to everyone so far; I'm learning a lot from this discussion.

[Naptown]

Yeah, it does look like there's a premium on Li ion batteries still, sadly. I was hoping that the tech was advancing more rapidly. Sunpower's panels are ~20% efficient; I wasn't able to find any info on Solarcity's panels.

So Plan A is currently a hybrid inverter with a charge controller hooked up to an AGM battery bank. It's still not clear to me what happens when the house needs more power than the inverter can supply. (I'm aware you're not supposed to do that, but I can imagine a couple of years from now I put on a couple more circuits and forget to check the drain vs. the inverter's supply capacity.)

Oh, our house is surrounded by trees, so there'll be shading issues on individual panels during parts of the day. Should I be taking a micro-inverter approach here per panel, which as I understand allows me to get the maximum power that each panel is able to deliver?

You could use microinverters however if you use Enphase and they catch on your warranty is void. This would have to be an AC coupled system at that point so if you run your batteries down past the low voltage cut off you will need a generator to get the system back up by charging your batteries. it gets complicated to do this. Outback has probably the best system for this and how to deal with voltage rise if you are not using all the power from the panels.