Will Tesla install solar + storage without NEM? i.e., one-way grid connection

[wattsup]

I am researching adding solar and battery storage to my home.

Question: If I simply want to utilize only my own self-generated power, yet still be connected to the grid (basically, as a rarely-used "backup") will Tesla Solar install a system that is still connected to grid, but only to consume from it (when needed) and never send power back... allowing me to avoid an NEM contract (and its limitations). Everything on the Tesla Solar website only talks about NEM. Also, at the PG&E website, it never mentions not switching to NEM when adding solar.

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More backstory...

I live on the central California coast, firmly in PG&E territory, with a mild climate and an average of 5+ hours of sunshine per day. Ideally, I would like to power my home completely from solar, and be able to power-through (pun intended) outages in comfort and without compromise.

That is, I want to power everything.... all appliances, HVAC, and EV charging, from solar and battery.

Currently, my water heater, furnace, stove, and fireplace are all gas-powered. I plan to switch the water heater and furnace to heat-pump-based units in the future, and perhaps also move from a gas stove to electric. (So, maybe someday it will be only the fireplace that uses any gas.)

I am still trying to figure out how much electricity a heat-pump furance will consume, but my current estimate for the worst-case (winter) energy needs to power the entire house in the future (with everything electric, as mentioned) is something like 90 kWh per day. I the summer, it might be more like half that (where I live, in summer, there is not much need for air conditioning).

Anyway, this plan puts me way past 150% of my current historic energy usage, and beyond what is allowed by NEM 3.0 (with attestation of planned future use). This is also pushing me into the realm of Tesla's largest residential offerings, as it would require (according to my calculations) an approximately 20 kW solar array made up of 50-60 panels (I believe Tesla's maximum is 60 panels).

As far as I know, California places no limits on the size solar system one can install, but PG&E does if you want to interconnect under a NEM contract and send power back to the grid.

So, again, can you just opt-out of NEM? -- and will Telsa install a one-way grid-connected system that does not require or use net-metering?

 

[miimura]

The only way you have any chance of avoiding a PG&E Interconnection agreement is if you wire the system so that it is impossible for the solar to go back into the grid. Tesla doesn't design their systems that way.

The most straight forward way to accomplish this is to basically wire your house as if it is off-grid, but put a battery charger for your off-grid battery on the grid connection. That way you are autonomous, your solar is not interconnected to the grid, but when your battery gets low, it is maintained or intermittently recharged by the battery charger.

 

[wattsup]

The only way you have any chance of avoiding a PG&E Interconnection agreement is if you wire the system so that it is impossible for the solar to go back into the grid. Tesla doesn't design their systems that way.

The most straight forward way to accomplish this is to basically wire your house as if it is off-grid, but put a battery charger for your off-grid battery on the grid connection. That way you are autonomous, your solar is not interconnected to the grid, but when your battery gets low, it is maintained or intermittently recharged by the battery charger.

Thanks... and bummer.

I assume a system like this would also be capable of powering the battery from solar? I assume there is some "right piece of equipment" out there than can intelligently charge the battery from excess solar and/or grid power (basically, select from some number of available power sources, which could be separate solar arrays, generators, or a grid connection).

As I understand it, the key would be that grid power can never be directly connected to the home's circuits, or at least not in conjunction with other power sources.

I mean, could I not have a big switch that either connects my home to the grid (as normal) or to an alternate source of power (such as a generator, or battery, or solar-powered battery).

Isn't this what people already do/did to use a home generator? Do those folks have to jump through all kinds of PG&E hoops to install such a switch and generator?

 

[miimura]

There are several basic components that are common to this kind of installation.

Switching:
- Double throw Disconnect switch - this is basically an A/B switch for AC power. This would allow you to do things like manually select whether a subpanel is powered from the grid or an inverter or inverter bank.
- Automatic Transfer switch - this is what is typically used with generators. It is also a kind of A/B switch where it will disconnect the grid when it goes down and connect the generator to the loads. Sometimes these have an integrated subpanel that has breakers for your critical loads.

Inverters:
- grid tied solar inverter - this is the typical kind of inverter that is used in grid tied systems. The inverter takes the DC power from the solar panel, finds the optimum balance between DC voltage and current (MPPT - max power point tracking) and creates an AC waveform to push the power into the grid. If there is no AC waveform to follow, like during a grid outage, it will shut down and wait for the AC power to return before it resumes generation. Grid tied solar inverters can be string inverters that operate at a higher voltage from a series wired string of solar panels, or micro-inverters that take lower voltages from one or two solar panels. String inverters can also have optimizers that compensate better for shaded panels on a string and also MCI (mid-circuit interrupter) devices that provide rapid shutdown for improved safety. Micro-inverters don't need optimizers or MCIs.
- off-grid inverter - this is a basic battery inverter that can provide AC power from a battery or battery bank. It is a free-running AC device and cannot be connected to a grid or any other off-grid inverter unless it is designed for "stacking" where the multiple inverters will synchronize with each other to provide more aggregate output power.
- hybrid inverter - this is a more advanced battery inverter that is designed to work with a grid connection and/or a generator. Some are also referred to as "inverter-chargers" because they include functions of both an inverter and a battery charger that can recharge the battery bank from a generator or grid connection. This type of inverter has many more features like "UPS mode", "Grid Zero mode", or "Boost mode". Some of these units can optionally export surplus energy to a grid connection. Solar is usually connected to the battery directly through a solar charge controller that provides MPPT and detects when the battery is full and curtails the solar generation so that it is not overcharged. Many hybrid inverters can also "AC couple" solar by connecting a normal grid-tied solar inverter just like it was a load on the system. The wattage of solar you can connect in this way usually has to be less than the wattage of the hybrid inverter itself, so that it can feed the AC power back into the battery bank. With solar charge controllers, you don't have that limitation.
- all-in-one inverter - this is usually a hybrid inverter that also integrates a solar charge controller. The quality and features of these units vary. Some are limited to off-grid use while others can be grid interactive.

So, to implement a system that would avoid the need for a utility interconnection agreement for your solar, you could do any of the following, in order of increasing sophistication and functionality.

1. Create an off-grid system with battery and solar that normally powers your house. When you run out of battery energy, you go and throw the red handle on your disconnect switch to connect your loads to the grid instead of your off-grid system.
2. Create an off-grid system with battery and solar that normally powers your house. Connect the grid through an automatic transfer switch, but reverse the normal connections so that "grid" input is powered by your off-grid system and the "generator" input is powered by the grid. That way, when you run out of battery and the system shuts down, you automatically fall back to the grid.

Both of the above options have the significant disadvantage that you have an interruption when you switch to or from the grid. The options below do not have that problem.

3. Create an off-grid system with battery and solar to power your household loads. Use an external battery charger with a grid power switch that is triggered by the generator start signal from the inverter.
4. Use a hybrid inverter to power all your household loads. Connect the grid as if it was a generator. That way, the grid will only be used when the batteries are low and the power can only flow through a battery charger circuit. Surplus solar and battery energy cannot flow back into the grid. Some hybrid inverters can vary the energy taken from the grid to maintain the low SOC point on the battery bank while others are simply on and off with configurable low SOC ON point and high SOC OFF point.

 

[GregBallantyne]

This thread has me wondering about Powerwall operation. Can Powerwall and solar operate in island mode indefinitely? We'll be getting our final electrical connection for our system on Monday, and will be operational soon. I guess what I am wondering about is more than just no local grid - what about no communication with Tesla servers?

 

[GregBallantyne]

I am researching adding solar and battery storage to my home.

Question: If I simply want to utilize only my own self-generated power, yet still be connected to the grid (basically, as a rarely-used "backup") will Tesla Solar install a system that is still connected to grid, but only to consume from it (when needed) and never send power back... allowing me to avoid an NEM contract (and its limitations). Everything on the Tesla Solar website only talks about NEM. Also, at the PG&E website, it never mentions not switching to NEM when adding solar.

------

More backstory...

I live on the central California coast, firmly in PG&E territory, with a mild climate and an average of 5+ hours of sunshine per day. Ideally, I would like to power my home completely from solar, and be able to power-through (pun intended) outages in comfort and without compromise.

That is, I want to power everything.... all appliances, HVAC, and EV charging, from solar and battery.

Currently, my water heater, furnace, stove, and fireplace are all gas-powered. I plan to switch the water heater and furnace to heat-pump-based units in the future, and perhaps also move from a gas stove to electric. (So, maybe someday it will be only the fireplace that uses any gas.)

I am still trying to figure out how much electricity a heat-pump furance will consume, but my current estimate for the worst-case (winter) energy needs to power the entire house in the future (with everything electric, as mentioned) is something like 90 kWh per day. I the summer, it might be more like half that (where I live, in summer, there is not much need for air conditioning).

Anyway, this plan puts me way past 150% of my current historic energy usage, and beyond what is allowed by NEM 3.0 (with attestation of planned future use). This is also pushing me into the realm of Tesla's largest residential offerings, as it would require (according to my calculations) an approximately 20 kW solar array made up of 50-60 panels (I believe Tesla's maximum is 60 panels).

As far as I know, California places no limits on the size solar system one can install, but PG&E does if you want to interconnect under a NEM contract and send power back to the grid.

So, again, can you just opt-out of NEM? -- and will Telsa install a one-way grid-connected system that does not require or use net-metering?

My thoughts on your original question.... when you include EV charging, you will need many Powerwalls to get by in an extended outage. Turns into $$. Actual install cost of Powerwalls may be higher than what some realize - 3 of them, installed less than 2 months ago at my house have a price of $45500, which I have yet to be invoiced for. Those 3 PW's hold 40.5 kwh. Compare that to my Model 3 LR battery - 200 miles on the battery takes nearly 50 kwh to replace. Not to mention roughly 80 kwh battery in it for a $60k purchase price..... in any extended outage, I'll either charge at a Tesla supercharger, or isolate the barn and run a generator to charge.
And if you have enough storage, will you have enough generation? It all starts getting a lot bigger fast. At my house in Maryland, I can see 120 - 130 kwh day's usage with the coldest weather plus some car charging, my house is all electric w/2 ton heat pump. My Solar Roof is 17.2 kw capacity, but the 2 inverters can handle 15.2 kw. I don't expect over generation, you never get max capacity. My final electrical hookup and inspection are scheduled for Monday, so I have not experienced operation yet. In my case, winter is when I will have the highest need and lowest generation rate. I would need a large number of Pw's, maybe 3000+ kwh worth, to store the summer's overproduction until winter. And if you can't store it, it's gotta go somewhere I believe, or shut off. That reduces your annual production number. With Solar Roof, more generation capacity should just be more inverters and perhaps additional gateways. The Solar Roof route is also more $$ - mine was about $90k, simple gabled roof covering 1200 ft2. The 17.2 kw system maxes out my available roof. My original goal was to be off grid capable, but that isn't feasible. I'm adding some propane heating to close the gap, but it's a lot easier with grid as battery available.
My understanding is that you can operate the way you want considering some input you already got above, assuming you have enough generation and storage, but I don't know about the PG&E details.

 

[wattsup]

Thanks for the thoughts Greg.

My thoughts on your original question.... when you include EV charging, you will need many Powerwalls to get by in an extended outage. Turns into $$. Actual install cost of Powerwalls may be higher than what some realize - 3 of them, installed less than 2 months ago at my house have a price of $45500, which I have yet to be invoiced for. Those 3 PW's hold 40.5 kwh. Compare that to my Model 3 LR battery - 200 miles on the battery takes nearly 50 kwh to replace. Not to mention roughly 80 kwh battery in it for a $60k purchase price..... in any extended outage, I'll either charge at a Tesla supercharger, or isolate the barn and run a generator to charge.

I know it... I am thinking about getting 4 PW (total 54 kWh) and I am going for the maximum solar that I can, somewhere near 24 kW. I have a great roof for it too, all the panels would be in a single plane, on the southern facing portion of my roof, at a 19 degree pitch, completely unshaded. Plus, it never snows here. I didn't buy the house with solar in mind, just lucked out!

I don't commute much anymore, so most of my vehicle charging is more sporadic, with plenty of opportunity to "manage" the timing of it. I imagine most of my charging will occur during (sunny or sunny-ish) days, feeding off the solar, as opposed to night using stored energy. The PWs would be mostly used to run the house at night and/or through outages, though with 54 kWh, I could probably do a pretty decent car "top off" overnight if ever needed, and still run the house just fine... certainly for most of the year. Deep in winter, I will have to manage the available energy a bit more closely.

And if you have enough storage, will you have enough generation? It all starts getting a lot bigger fast. At my house in Maryland, I can see 120 - 130 kwh day's usage with the coldest weather plus some car charging, my house is all electric w/2 ton heat pump. My Solar Roof is 17.2 kw capacity, but the 2 inverters can handle 15.2 kw. I don't expect over generation, you never get max capacity.

Yeah... I hope so. I'm trying to size my system so that, on average, the panels can produce enough even in winter. Fortunately for me, the coldest it gets is typically low 40s (occasionally 30s) on winter nights or very cold days. Most of the time, much milder.

My final electrical hookup and inspection are scheduled for Monday, so I have not experienced operation yet. In my case, winter is when I will have the highest need and lowest generation rate. I would need a large number of Pw's, maybe 3000+ kwh worth, to store the summer's overproduction until winter. And if you can't store it, it's gotta go somewhere I believe, or shut off. That reduces your annual production number. With Solar Roof, more generation capacity should just be more inverters and perhaps additional gateways. The Solar Roof route is also more $$ - mine was about $90k, simple gabled roof covering 1200 ft2. The 17.2 kw system maxes out my available roof. My original goal was to be off grid capable, but that isn't feasible. I'm adding some propane heating to close the gap, but it's a lot easier with grid as battery available.
My understanding is that you can operate the way you want considering some input you already got above, assuming you have enough generation and storage, but I don't know about the PG&E details.

When I overproduce in the summer (or any other time), it will go back to the grid and I'll get some compensation from the utility. Or, as I mentioned, when needed/available, I can always charge my cars during the day, to give the excess somewhere to go.

Best wishes for your install!

 

[GregBallantyne]

Thanks for the thoughts Greg.



I know it... I am thinking about getting 4 PW (total 54 kWh) and I am going for the maximum solar that I can, somewhere near 24 kW. I have a great roof for it too, all the panels would be in a single plane, on the southern facing portion of my roof, at a 19 degree pitch, completely unshaded. Plus, it never snows here. I didn't buy the house with solar in mind, just lucked out!

I don't commute much anymore, so most of my vehicle charging is more sporadic, with plenty of opportunity to "manage" the timing of it. I imagine most of my charging will occur during (sunny or sunny-ish) days, feeding off the solar, as opposed to night using stored energy. The PWs would be mostly used to run the house at night and/or through outages, though with 54 kWh, I could probably do a pretty decent car "top off" overnight if ever needed, and still run the house just fine... certainly for most of the year. Deep in winter, I will have to manage the available energy a bit more closely.



Yeah... I hope so. I'm trying to size my system so that, on average, the panels can produce enough even in winter. Fortunately for me, the coldest it gets is typically low 40s (occasionally 30s) on winter nights or very cold days. Most of the time, much milder.



When I overproduce in the summer (or any other time), it will go back to the grid and I'll get some compensation from the utility. Or, as I mentioned, when needed/available, I can always charge my cars during the day, to give the excess somewhere to go.

Best wishes for your install!

In your situation, EV charging from excess solar especially in summer sounds like a great strategy. I would love to acheive enough generation to not need utility power in the winter, but I'll never get there - or at least according to my estimates. I simply don't have enough roof with good enough exposure to get there. It sounds like you've got a more than credible chance to make it. I am turning to propane, but it's not a great spend - $$ for contingency I'd rather not turn to routinely, and only will when required. One of my goals is removing fossil fuel use, and I'll have to settle for a steep reduction on that reliance while serving another perhaps hogher priority goal of energy security. Of course the biggest part of fossil fuel reduction was accomplished with turning to EVs from ICE.... my utility power is emissions free already.