Is there such thing: "on-grid solar-assist" system?

[ivanz]

Please bear with me to understand what I am looking for.

I want to build an on-grid solar system that will "help" my AC (the AC compressor system cooling my home when it's hot), by supplementing the power from the grid with solar-generated power.

To make it DIY-friendly and cheap:
- It should not use any batteries or battery controllers.
- It should continue "helping" my AC as much as it can, even when sun conditions are not great (e.g. cloudy sky, or when the sun starts setting down).
- It should never give power back to the grid. It should die when the grid power dies (or when the house main breaker is flipped off), and should start assisting again when the grid power comes back up. It should have no high voltage anywhere in it (or in the AC compressor) when the power is off. This way it's safe, and does NOT require main panel upgades, power meter upgrades, city permits, contracts with the utility company, etc.
- It should obviously waste all solar-generated power when the load (my AC) is off. Note: a more-advanced version of this could be made, with batteries, so that it will continue "helping" when the sun sets; I don't want to use batteries, myself, to keep this cheap and simple.

Obviously, such device can be used for just any load (not just AC compressor), e.g. it could be installed at a sub-panel, and help all appliances and loads that are being powered from the sub panel.

Why I need this: I am on a "EV2-A" time-of-use plan from PG&E in California. During peak hours (4 to 9 pm) the cost of power is 4 times! higher than off-peak hours. I want to reduce my energy consumption during peak hours, as much as I can.

Is there such device?
If not: can it be built?

Thank you!

 

[sunshine_eggo]

You're looking for a grid-tie system with a zero export option.

It's essentially a grid tie inverter that also has a hall effect sensor on the mains. It ensures that there is always something coming in from the mains, but it will supplement mains with available PV.

https://www.amazon.com/ECO-WORTHY-1000W-Solar-Inverter-System/dp/B095C6FTP6/

 

[DThames]

When power

You're looking for a grid-tie system with a zero export option.

It's essentially a grid tie inverter that also has a hall effect sensor on the mains. It ensures that there is always something coming in from the mains, but it will supplement mains with available PV.

https://www.amazon.com/ECO-WORTHY-1000W-Solar-Inverter-System/dp/B095C6FTP6/

Such units may or may not be in compliance with local regulations.

 

[sunshine_eggo]

When power

Such cheap units may or may are likely not be in compliance with local regulations.

Fixed.

Good point for sure!

 

[ivanz]

Could you give me an example of "local regulations"? Do you mean safety regulations?
If the unit never gives power back to the grid, and shuts itself down when the utility power is off, it is as-safe as a phone charger.

 

[sed6]

Could you give me an example of "local regulations"? Do you mean safety regulations?
If the unit never gives power back to the grid, and shuts itself down when the utility power is off, it is as-safe as a phone charger.

If you plan to have your solar installation permitted or inspected by local authorities there are very specific requirements for types of equipment you can use. For example, a cheap Chinese inverter isn't UL listed so it won't be allowed. If you don't have it permitted or inspected you can do whatever you want.

 

[wattmatters]

Pretty much every solar PV inverter authorised for connection to the grid in Australia can do this. That's because export limiting is very common requirement for connection approval.

Don't know about your local regulations but in Australia in a number of areas there are zero export limits due to the local infrastructure being unable to cope with more solar PV, and in all areas there is at least some non zero export limit.

Where a grid tied PV array and solar inverter is installed and which has an output capacity greater than the approved grid export limit then the system also has a private consumption meter fitted in the main circuit board / point of supply which continually monitors the flow of energy in/out. This private meter is compatible with and connected to the solar PV inverter which then manages its output so as to remain within the approved export limit. In the case of a zero export limit then the system's output will adjust to meet household demand only, whenever there is sufficient insolation.

This is very common here. My grid tied system has it (a Fronius Symo 10kW inverter with a Fronius smart meter), as does just about every system in my local area since the typical system size is ~5-10kW but the per phase export limit is usually no higher than 3kW. In some cases the export limit is zero, although this is more common in parts of regional Victoria than NSW.

 

[DThames]

While the referenced unit has a current detector and controls to avoid "export", there are safety concerns for the people worked on the power grid if someone were to back feed power to the grid. So while designed to not back feed, could a failure of the device back feed the line? Concerns like this are addressed in the design documentation of devices that are UL1741 rated. So the UL rating is the rubber stamp that says, "Have no fear" to those concerned about the danger of back feeding. Everyone needs to be concerned about that as we are responsible to do no harm to those around us. While temping to use cheap equipment, I would encourage not to connect such devices that are not UL 1741 approved.

 

[wattmatters]

While the referenced unit has a current detector and controls to avoid "export", there are safety concerns for the people worked on the power grid if someone were to back feed power to the grid.

All the grid tied inverters I referred to above require AC input power to operate - that is their source of operational energy, not the PV nor a battery. IOW when grid supply is cut off, those inverters shut down immediately, just like any other appliance supplied by grid power would.

 

[fafrd]

While the referenced unit has a current detector and controls to avoid "export", there are safety concerns for the people worked on the power grid if someone were to back feed power to the grid. So while designed to not back feed, could a failure of the device back feed the line? Concerns like this are addressed in the design documentation of devices that are UL1741 rated. So the UL rating is the rubber stamp that says, "Have no fear" to those concerned about the danger of back feeding. Everyone needs to be concerned about that as we are responsible to do no harm to those around us. While temping to use cheap equipment, I would encourage not to connect such devices that are not UL 1741 approved.

You are obviously someone who has never used one of these little SUN GTIL2 inverters (I own 2).

They are grid-tied and need the grid signal to function. As soon as the grid signal goes down, they power off just like every other Microinverter out there.

Whether they have been fully tested / certified to cut off exactly when the grid signal gets distorted by ‘X’ (frequency too low or voltage too low) according to specs like UL 1741 is a valid concern, so backfeed during a ‘grid out of spec’ situation might be a possibility.

But I’m any situation where there are Linemen working in the grid, which by definition means the grid will be switched off first, there is no chance of these GTIL inverters back feeding to the grid (they’ll be powered off, just like every other appliance in your home).

 

[fester]

My Local power company AEP here does not have time of day pricing differences, but my goal was the same as yours...use as little as possible from the local grid. The answer is yes for what you want, but "it ain't cheap". Did a lot of researching for a newer "Hybrid" inverter that would do what I wanted. I considered two different makes for this, The Outback Skybox and the Sol-Ark. I ended up purchasing an 8K Sol-Ark, because IMHO the things they add to a base unit (Chinese made DeYe) are designed by folks with Aerospace and Military engineering backgrounds at Sol-Ark and having some experience there, Military equipment are usually designed with a eye towards unusual operating conditions, which hopefully I won't encounter, but you never know. Outback has been around for quite a while and the Skybox may be fine too, I just decided to get the Sol-Ark based on my own engineering experience.
I've had the Sol-Ark online for only a couple of months now and am still learning things about all around operation, but here's my experience so far. The Sol-Ark can be used in a "Battery-Less" setup using only a panel string and a Grid "connection" (but is not "grid-tied", it is configured to be in a "Zero-Export" mode (which uses the supplied Current Transformers to monitor the Mains in the SE box). It also has "a time of use" configuration which you can set depending on your needs. Without a battery, (like the Skybox, if the Grid goes down), it will offline along with any "Critical Loads" you have connected to it. With a battery (needs a 48vdc battery voltage input), it will go into a backup condition with only a 4 millisecond delay (that's Sol-Arks spec, but I only see a very short "blink"in my house lights (they are LED lights, may not see a "blink" with incandescent) when the Mains feed is cut or an outage occurs.
Probably more than you are looking for now, but that's been my experience so far...

 

[DThames]

You are obviously someone who has never used one of these little SUN GTIL2 inverters (I own 2).

They are grid-tied and need the grid signal to function. As soon as the grid signal goes down, they power off just like every other Microinverter out there.

Whether they have been fully tested / certified to cut off exactly when the grid signal gets distorted by ‘X’ (frequency too low or voltage too low) according to specs like UL 1741 is a valid concern, so backfeed during a ‘grid out of spec’ situation might be a possibility.

But I’m any situation where there are Linemen working in the grid, which by definition means the grid will be switched off first, there is no chance of these GTIL inverters back feeding to the grid (they’ll be powered off, just like every other appliance in your home).

Yes, I have one and played with it before getting a system that could pass inspection. I know they normally turn off when the power is off. I also know things can fail in ways that sometimes seem almost impossible. The chances of a UL approved device having been designed and tested for more failure modes than a cheap import device, I would expect to be greater.

 

[fester]

I looked them up along with the specs. Information on them and the limited reviews I've seen are sketchy at best. Reliability seems in considerable doubt, and looking the the manufacturers specs you're limited to 90vdc panel input, so if you have a larger series string of PV you would have to break them down into a series/parallel configuration to stay below that. I'd suggest taking a pass on these...

 

[sed6]

You guys are getting way off track. He said cheap and DIY friendly.

 

[onokai]

One thing the OP asked for is NO batteries also he needs power 4-9 pm. No sun at 9 pm most times of the year so panels will be moot then
You will need batteries in late evening hours with low sun angle