Can a solar PV system do this?

[fresnoboy]

Hi. We're in the process of building a large house and will have about 20KW of solar panels on top of the house and the detached garage. We live in PGE territory so energy prices are pretty high so I am trying to make sure we stay in Band 1 for usage. The utility power where the house is can also be somewhat flaky, and do I am planning on generator backup. I plan to use the new TOU rates that PGE has gone to, with more restricted times.

What I'd like to do is have a system that can operate in two modes:

1) When utility power is on, to charge a set of batteries (powercells or others) through the morning and afternoon, and then when peak hour kicks in, push the stored power back to the grid for maximum economic value through that window, and then go back to utility power through the evening.

2) When utility power fails, I'd like to have the solar charge the batteries, support the house demand as needed, and then when the batteries hit a low charge point, have the generator kick in to charge the batteries at roughly the solar panels normal rate, and then have the generator go back to standby. This avoid running the generator continuously and at low demand when it's inefficient, and still allows solar to charge the batteries and use solar power during an extended outage.

Is it possible to design a solar power system that can do both of these things?

Sorry if I'm missing something obvious, I have a PV system at our current house, but there are no batteries or gensets involved there.

Thx
mike

[Mike90250]

1) the Power Co does not like battery based "co-generation" and are unlikely to approve the scheme.
2) you will loose money in battery costs, daily cycles to "reduce" your peak loads, will quickly age your batteries and the $600 you save in a year, is more then lost in 5 years when you have to purchase another $20,000 battery.

Grid power is really cheap, and load shaving with PV, is the best return for your $$. Sadly, "peak hours " that used to be 11am - 5pm (when mfg plants were sucking up electrons) have now been "adjusted" to 2pm - 9pm, negating any solar benefit from 4-9pm.

[bcroe]

You can't save money that way. Batteries only have so many charge/discharge cycles, and then
you buy a new one. Just pick a battery and figure a depth of charge in KWH, and multiply the number
of cycles its good for. Divide the $ cost of the battery by the lifetime KWH ability, and you will see a cost
per KWH far above what the PoCo would charge you.

Use a grid tie, check what terms of your permit are possible. Then the PoCo becomes a free battery.

Backup, use a generator. You would have to have one anyway with batteries. Bruce Roe

[inetdog]

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Backup, use a generator. You would have to have one anyway with batteries. Bruce Roe

If you are concerned about minimizing generator run time during an extended (multiple day) outage, you can add a small battery/inverter system that simply charges from generator power when it is running. No need to go to the expense of equipment to charge the battery bank from solar PV.

[sensij]

If you are committed to the plan, Google "SGIP". California has an incentive plan right now for people who install battery systems and use them as you have described.

[fresnoboy]

1) the Power Co does not like battery based "co-generation" and are unlikely to approve the scheme.
2) you will loose money in battery costs, daily cycles to "reduce" your peak loads, will quickly age your batteries and the $600 you save in a year, is more then lost in 5 years when you have to purchase another $20,000 battery.

Grid power is really cheap, and load shaving with PV, is the best return for your $$. Sadly, "peak hours " that used to be 11am - 5pm (when mfg plants were sucking up electrons) have now been "adjusted" to 2pm - 9pm, negating any solar benefit from 4-9pm.

Yes, this shift in TOU demand hours is what I am trying to work around. It means making the array bigger than it should need to be to be able to generate more power in the 2-9PM window, and that's why the system is sized so large right now. If I can shift generation into when PGE will pay more for it, it changes the economics.

Why would the utility care? PGE never pays me more than I actually consume, and me shifting my power generation to when they need it would seem to be good for them, esp if they are not paying for the batteries. A couple utilities are working with Tesla to use Powerwalls and Vehicles for shifting power generation into their peak hours.

I understand the issue about cycles, but Tesla is offering a warranty supporting unlimited cycles for 10 years on their powerwall 2, so this kind of cycling would seem to be something they thinking about.

thx
mike

[fresnoboy]

If you are committed to the plan, Google "SGIP". California has an incentive plan right now for people who install battery systems and use them as you have described.

Ah, that's right - I had heard about that too. I am not necessarily committed to the plan, but this goes back to my question about how to design a system that can implement this strategy. How do the SGIP systems work?

thx
Mike

[sensij]

What you are suggesting will work, but is much more expensive than a simple grid tie system with generator backup for outages.

[fresnoboy]

What you are suggesting will work, but is much more expensive than a simple grid tie system with generator backup for outages.

I wouldn't push to optimize genset operation if I didn't have the batteries in the first place.

Part of why I am asking about what kind of design of a system would support this would allow me to do the cost tradeoff of the simpler system vs the more complicated one. I can't seem to find a reference to a system that can do the kind of thing I am asking about, which is prompting this thread.

It may not pay in the end, but I want to try and do an apples to apples comparison. It's a brand new house in an early stage, so I have the flexibility to design things in now that would be much harder later.

Can someone point me to an SGIP system design with the solar power feeds I can use to estimate costs?

Thx
mke

[inetdog]

Look for Hybrid solar systems in the Off Grid topic area. Instead of a grid tie system with batteries you get there by starting with an off grid system and paying lots more to get the additional option of operating tied to the grid.
The storage and time shifting part comes along more or less for free when you get a Hybrid system with flexible controls.
But the economically feasible (arguably) time shifting storage systems all use Lithium chemistry battery banks, and few pure off grid systems are designed that way. (Although most will work properly with Lithium if you are careful about the settings.)

[sensij]

Can someone point me to an SGIP system design with the solar power feeds I can use to estimate costs?

Since the system needs to be installed by a certified/licensed contractor to be eligible for SGIP, your best bet would be to get a quote.

Roughly, budget $3 / W for the PV system. Add another grand or so for a hybrid inverter if you go the battery route.
Maybe $11000 for a 14kWh Powerwall.

Energy storage is currently at the $0.40 / Wh SGIP incentive step, so that could potentially cover half of the powerwall cost.

I don't think you will find a reference system here because no one has figured out how to make the numbers work yet. But, please, have at it, I'd love to see sound financial justification for residential storage.

[fresnoboy]

Since the system needs to be installed by a certified/licensed contractor to be eligible for SGIP, your best bet would be to get a quote.

Roughly, budget $3 / W for the PV system. Add another grand or so for a hybrid inverter if you go the battery route.
Maybe $11000 for a 14kWh Powerwall.

Energy storage is currently at the $0.40 / Wh SGIP incentive step, so that could potentially cover half of the powerwall cost.

I don't think you will find a reference system here because no one has figured out how to make the numbers work yet. But, please, have at it, I'd love to see sound financial justification for residential storage.

Actually, the powerwall 2 (which has AC in and out) is 7K for 13.5KWH. I am unaware of battery systems with equivalent warranties, but will look at the offgrid forums. have people done the genset optimization piece?

thx
mike

[bcroe]

Tesla is offering a warranty supporting unlimited cycles for 10 years on their powerwall 2, so this kind of cycling would seem to be something they thinking about.

thx
mike

How many KWH capacity is your powerwall? How many do you want to shift? Most battery systems are far too small
to operate on a utility powered scale.

You can shift your peak generation time somewhat by orienting panels, but that implies a ground mount. Bruce Roe

[ButchDeal]

I understand the issue about cycles, but Tesla is offering a warranty supporting unlimited cycles for 10 years on their powerwall 2, so this kind of cycling would seem to be something they thinking about.

I think you need to look more closely at that cycle count on the tesla powerwall warranty! It does not cover degradation over time from cycles.

[fresnoboy]

I think you. Need to look more closely at that cycle count on the tesla powerwall warranty! It does not cover degradation over time from cycles.

Really, where does it say that in the warranty?: https://www.tesla.com/sites/default/...nty_us_1-4.pdf

thx
mike