Residential Payback for Off-Grid Solar Q1 2023 (grid connected)

[wattmatters]

We have central air, gas furnace for heat and tankless gas for hot water. I did look at a heat pump hot water heater but once you add up the cost of the unit, the permitting and installation cost, the ROI was not there when compared to the tankless gas heater so until this unit dies I won't be looking at an alternatives just yet.

Like you I investigated a heat pump hot water system but it was not financially rational in our case. They are more effective for anyone with high-ish hot water consumption. Powered mostly via solar PV they use so much less energy (about 1/4) and also operating them in the daytime improves their operating effectiveness (higher coefficient of performance).

Hot water storage provides a very effective thermal battery where you store energy when it is cheap/abundant to be used later when it is convenient.

We have a resistive element hot water storage tank powered via a dedicated smart solar PV diverter which constantly monitors how much of our solar PV output is being exported to the grid and adjusts the amount of power delivered to the tank depending on the available excess solar PV. Here's a chart from last five days, which has had a combination of nice sunny days and crummy rainy/cloudy days.

It operates on a phase with just 3.6 kW of solar PV. 315 litre tank, 3.6 kW heating element. Two person household.



Yellow = PV output
Purple = Exports to grid
Orange = Energy diverted to hot water storage tank

Can see how on a nice sunny day the main heating cycle finishes during the morning, and the occasional short top up happens in the afternoon.

On crummy solar days the system just keeps operating most of the day, diverting as much energy as it can while always avoiding any grid imports. Because it is a large(ish) tank then if the system does not complete a full charge on a given day it's no big deal, it will catch up the next day. I have the automated off-peak grid energy boost function switched off as I do not want it to boost from off-peak overnight and instead just use available solar PV. But that option is available if I want it to top up from off-peak energy in case a full reheat was not completed. It also has a manual over ride to "heat now" if required.

I tested it earlier this year during a really bad six day stretch in the middle of Winter when it was cloudy the whole time. It still managed to get through with ample hot water supply, and enough heat to ensure no legionella risk.

Energy diverted to hot water on each of those past 5 days:

Thu: 3.8 kWh
Fri: 3.6 kWh
Sat: 4.6 kWh
Sun: 4.7 kWh
Mon: 3.7 kWh

Tank heat losses account for ~ 2 kWh of the daily demand.

By way of comparison, an empty tank would require in the vicinity of 16-18 kWh to heat up to 60°C or so.

Our off-peak is not free, it's about to move to ~18-20 c/kWh while our solar feed-in tariff is 7c/kWh, so each kWh diverted saves ~ 11-13c/kWh.

In our case that made it financially rational with diverter cost recovery inside of five years, although our primary motivation was to reduce our import of fossil heavy energy (our grid is dominated by coal fired power).

There are also smart EV chargers which can do similar, adjusting power delivered to the car based on available excess solar PV output. When the time comes for us to change to a BEV then I'll likely install a smart charger for it.

Treating the home like a thermal battery, pre-heating or cooling, is also a good strategy if you have time of use energy charging, provided of course your home is suitable for applying such a strategy. Ours unfortunately has terrible thermal performance and it's not something which works for us. I am working on improving things bit by bit but there's only so much one can do with an older home not constructed with energy efficiency in mind.

We are very used to operating main appliances during solar PV hours. Dishwasher, washing machine, clothes dryer if required (hanging clothes on a line is best) are mostly run during the day, use of timers but mostly just habitual. It wasn't hard to bring the wife along that journey. The pool pump is automated and runs during the day.

 

[GregTR]

There are also smart EV chargers which can do similar, adjusting power delivered to the car based on available excess solar PV output. When the time comes for us to change to a BEV then I'll likely install a smart charger for it.

I do have one, only caveat is that it can't tell whether energy is coming from PV or battery which has the unintended consequence of possibly draining my home battery to charge my car. It's kind of a race condition scenario where depending on which device throttles/adjusts quicker I can have the PV battery behave like panels in the equation.

Since I'm getting free electricity at night (2100-0700) right now and for the next 1.7 years, I have very little incentive to expand on my system as it covers my daytime consumption easily.

 

[Bob B]

I do have one, only caveat is that it can't tell whether energy is coming from PV or battery which has the unintended consequence of possibly draining my home battery to charge my car. It's kind of a race condition scenario where depending on which device throttles/adjusts quicker I can have the PV battery behave like panels in the equation.

Since I'm getting free electricity at night (2100-0700) right now and for the next 1.7 years, I have very little incentive to expand on my system as it covers my daytime consumption easily.

I'm curious if you can use your car as a backup battery for your home for emergency situations?

 

[wattmatters]

I do have one, only caveat is that it can't tell whether energy is coming from PV or battery which has the unintended consequence of possibly draining my home battery to charge my car.

Yeah that can be a tricky for some set ups to resolve and highlights the increasing need for integrated smart control. I'm pretty sure appropriate monitoring of the respective power supplies can overcome such issues.

We are starting to see more integrated solutions tying together solar PV, home battery, hot water and EV chargers so that they all work in concert.

Since I'm getting free electricity at night (2100-0700) right now and for the next 1.7 years, I have very little incentive to expand on my system as it covers my daytime consumption easily.

Yes that is indeed a fortunate (and unusual) position to be in. If it were a longer term option then storing energy in water tank by heating it overnight would also make a lot of sense.

 

[GregTR]

I'm curious if you can use your car as a backup battery for your home for emergency situations?

Nope. I don't have a Ford Lightning. Emporia Energy is working on a V2X charger, once this is available I'll be buying one for sure: https://www.emporiaenergy.com/how-the-emporia-v2x-charger-works

 

[Quattrohead]

I did look at a heat pump hot water heater but once you add up the cost of the unit, the permitting and installation cost

Who has ever pulled a permit to DIY replace a tank ????
Just do it quick discrete and don't put the old one on the side of the road !!!!

 

[GregTR]

Who has ever pulled a permit to DIY replace a tank ????
Just do it quick discrete and don't put the old one on the side of the road !!!!

My plan was more involved.... I have a tankless system so adding 900 lbs weight to the attic in series with the tankless setup is not just a simple remove and replace so plumbers wanted to permit it, make sure the attic floor can bear the load and thew two heaters an work together. Sure, if it was simply replacing one heater with another heater, I would have just done it myself but since it was more than that, so was the quoted work.

 

[Symbioquine]

The cheapest KWhr you’ll ever pay for is the one you never use!!!

Depends what the opportunity cost is. Saving power could be very expensive if it results in getting heat stroke, having insufficient water pumped for a dry spell, impacting quality of service (and thus profits) at a business, etc...

 

[philatio]

It's no net metering when you have to pay delivery charge on anything used. Sure you can get "some" money for your solar export but it's never dollar for dollar, at least not in Texas.

The only one I know of that still offers real kWh for kWh NET metering with annual rollover with no delivery charge is Green Mountain Energy but the caveat is that the system has to be installed by them which is an instant no go for most.

Up until Feb 1, 2023 Coserv (Coop with no other options, im in the far north Dallas burbs) had 1 to 1 net metering on a billing cycle plus regular $10 meter fee (applies to everyone, even those without solar) plus a $10 distributed generation fee. For the most part, since Nov 21, my bills have only been the two meter fees. Thankfully I am grandfathered until Feb 2028, becuase the changes are very unfavorable. Meter fee is still $10. DG fee increased to $15 and they will now pay a fraction for electricity fed back to the grid.:

The Customer shall be compensated for Received Electricity at the
Cooperative’s avoided cost, which shall be determined monthly by the
Cooperative and is calculated based on an average of the wholesale
energy costs and related demand charges avoided by the Cooperative as
a result of having distributed generation facilities on the Distribution
System during the 12-calendar month period immediately prior to such
determination.

Of course there in no way to get this info upfront or calculate it your self. March 2023 buy back at $0.044/kwh & April 2023 at $0.0684/kwh. I have modeled my data since November 2021 based on the average of these first 2 buy backs at $0.0562/kwh and even is I over produce big time I'm getting FU**** to say the least. At my lowest over production day (-14kwh net)) my bill would have been 258.74% higher than it actually was. At my lowest under production (2kwh net from grid) my bill would have 195.4% more than it actually was. My best case scenario was with an over production of 462kwh where the bill would have only been 77.78% higher than it actually was.

This is my long winded way of saying I found this thread in a search to find a solution and a path forward. I have just under 5years to figure it out.

 

[Hedges]

They compute avoided cost as average of wholesale including at night.
You provide power in the daytime when there is more demand due to A/C.
But there is so much solar that for some hours power is worth about zero.
That is only value of last kWh delivered. If we shut off all solar, utility would discover what market price is for the last kWh they have to buy.

Sounds like you'll be on the equivalent of NEM 3.0, so having a battery to smooth out one day's production/consumption may be the way to go.
What are your rates to buy power? Server rack batteries may approach $0.05/kWh of cycle life if they do last 16 years of daily cycling.

Is your primary use of electricity winter heating or summer cooling?
For cooling, what you want is A/C continuously operating at a rate which keeps import/export at zero.
Any other loads like pool pump can be controlled similarly.
Load control is far more cost effective than storage.

Do you have gas heat? Gas service? Can you get it?
Presently gas is cheaper than electric. That may change, however.

 

[philatio]

They compute avoided cost as average of wholesale including at night.
You provide power in the daytime when there is more demand due to A/C.
But there is so much solar that for some hours power is worth about zero.
That is only value of last kWh delivered. If we shut off all solar, utility would discover what market price is for the last kWh they have to buy.

Sounds like you'll be on the equivalent of NEM 3.0, so having a battery to smooth out one day's production/consumption may be the way to go.
What are your rates to buy power? Server rack batteries may approach $0.05/kWh of cycle life if they do last 16 years of daily cycling.

Is your primary use of electricity winter heating or summer cooling?
For cooling, what you want is A/C continuously operating at a rate which keeps import/export at zero.
Any other loads like pool pump can be controlled similarly.
Load control is far more cost effective than storage.

Do you have gas heat? Gas service? Can you get it?
Presently gas is cheaper than electric. That may change, however.

Thanks for the response @Hedges.

I actually started another thread to get feedback on my specific path forward.

My rates are cheap in comparison to most (11 to 13 cents/kwh - Coserv Coop), and I wasn't even really a candidate for solar, but as I mention in the linked thread above, cost is/was not necessarily the end goal. Autonomy & Greening up were the main factors.

Thanks again.

 

[Hedges]

"March 2023 buy back at $0.044/kwh"

By generating more power than you consume, you are being green.
With the reduced credits, you don't get much financial benefit.
Thank you for your altruism and saving the planet.

"Autonomy", for that a system which runs when grid down would be useful. Possibly minimal battery, only power loads during the day.

 

[wattmatters]

1:1 net metering is insanely generous and it's a surprise it's still offered in some regions.

 

[philatio]

1:1 net metering is insanely generous and it's a surprise it's still offered in some regions.

It just changed with my provider but I'm grandfathered for 5 years.

Perhaps I have just been spoiled in the last 22 months.

Am I making a mountain out of a mole hill?

 

[wattmatters]

Am I making a mountain out of a mole hill?

Don't get me wrong, it's nothing personal. I can certainly understand how much worse it is to go from 1:1 to asymmetric net metering. And you didn't make the rules, you just took advantage of them as any sane person would.

But the reality is the energy exported to the grid from a home PV system should be valued at about the same as the wholesale value of grid energy generated at the same time, plus perhaps any value associated with low carbon energy in that region, rather than at the value of the entire retail tariff.

If anything it should be valued at a little less than wholesale given rooftop PV is often not subjected to the same rules of curtailment as grid energy generators are. The grid just takes whatever you throw at it whether or not it wants/needs it. Meanwhile a power station has to follow grid operator instructions on how much energy to supply and when.

Retail energy tariffs are typically made up of more than just the energy generation costs. They also include a component to cover costs of the network and grid and transmission infrastructure required to deliver the power to/from your home.

A 1:1 net metering arrangement therefore results in a greatly unbalanced cost recovery equation. In effect it means the grid is paying you for their infrastructure. Fair enough that you are paid for the wholesale value of the energy you export, but why should you also be paid for using the grid's infrastructure? That makes no sense. We don't pay to drive one way on a toll road only to be credited that toll back when we drive on the same toll road going in the other direction.

When solar PV exports are tiny relative to the total energy supply of the grid, then it's no big deal. Sure, have 1:1 net metering, give those early adopters a little boost. But as exported rooftop solar PV energy starts to become a larger component of the total grid energy mix, then what happens is the network costs end up being spread over the homes without solar PV, even though homes with grid-tied solar PV still need and use the grid.

 

[Hedges]

But the reality is the energy exported to the grid from a home PV system should be valued at about the same as the wholesale value of grid energy generated at the same time, plus perhaps any value associated with low carbon energy in that region, rather than at the value of the entire retail tariff.

I think rooftop PV power exported to the grid should be valued the same as what wholesale value would be, if the rooftop PV wasn't being exported.
That is, not the value of one additional kWh above current consumption, but the value of kWh the utility would have to purchase if we didn't deliver to the grid.

How to find that value? All of us turn off our production (a solar embargo), and let PG&E buy the power elsewhere.
Very different price from when we export a bit more than is consumed, and PG&E pays another state to take our surplus.

And one more thing, an adjustment, a multiplier.
You come up with a price for power delivered by wholesaler to the grid.
Then you determine how many kW you have to buy from the wholesaler to deliver 1.0 kW to consumers (transmission efficiency.)
So long as rooftop PV in a city is less than consumption in the city, we are saving PG&E long distance transmission losses as well.

 

[wattmatters]

I think rooftop PV power exported to the grid should be valued the same as what wholesale value would be, if the rooftop PV wasn't being exported.
That is, not the value of one additional kWh above current consumption, but the value of kWh the utility would have to purchase if we didn't deliver to the grid.

Given the amount of grid-scale solar PV and wind farm curtailment we have, it would be 3/5ths of bugger all.

How to find that value? All of us turn off our production (a solar embargo), and let PG&E buy the power elsewhere.

Occasionally a disgruntled consumer here makes that same suggestion and they rouse up about 3 or 4 mates. It's just not going to happen.

Primarily because PV power which is self consumed is still valuable since it is offsetting what otherwise would have been imported from the grid. No one is going to shut off their PV as a protest - it will cost them money to do so.

Just shift as much discretionary load as is feasible to the time of day when solar PV is generating. In many cases it's relatively cheap and easy to do. I shifted my water heating from overnight grid power to the daytime instead and it only uses my solar PV.

If the import - export tariff differential is large enough, then install battery storage to enable even more load shifting.

 

[wattmatters]

Then you determine how many kW you have to buy from the wholesaler to deliver 1.0 kW to consumers (transmission efficiency.)
So long as rooftop PV in a city is less than consumption in the city, we are saving PG&E long distance transmission losses as well.

Don't know how the tariffs are worked out there but here such factors are included in the tariffs. Here they are called "Transmission Loss Factor" and "Distribution Loss Factor". Combined they average out at ~ 10% losses from grid power stations to end customer. The HV network is pretty efficient, so any additional factor is going to be a small portion of that 10%. We are talking fractional cents/kWh.

The problem is none of the excess energy we export during the day helps meet the peak demand requirements in the early evening. That peak demand carrying capacity is what the grid is built to manage and where a lot of the costs are.

 

[wattmatters]

And one more thing, an adjustment, a multiplier.
You come up with a price for power delivered by wholesaler to the grid.
Then you determine how many kW you have to buy from the wholesaler to deliver 1.0 kW to consumers (transmission efficiency.)
So long as rooftop PV in a city is less than consumption in the city, we are saving PG&E long distance transmission losses as well.

BTW, in one state here, Victoria, the government sets the minimum export tariff retailers must apply.

This includes an allowance for the reduction in transmission demand costs, the carbon offset benefit, the health benefit of renewables (they burn a lot of brown coal there) as well as the weighted average wholesale energy generation value.

That minimum FIT for the 2023-24 financial year is set at A$0.049/kWh or US 3.25c/kWh.

As a result most retailers in Victoria offer the minimum feed-in tariff.

Meanwhile in other states with no mandatory minimum FIT, the average FIT is higher. The exception is WA but they have a govt owned single retailer with fixed pricing.

 

[Hedges]

The problem is none of the excess energy we export during the day helps meet the peak demand requirements in the early evening. That peak demand carrying capacity is what the grid is built to manage and where a lot of the costs are.

Here, utilities talk about the "Duck's back curve", power demand peaking in the evening. It is a lie.
Power demand actually peaks mid afternoon. PV peaks at noon (oriented West could peak early afternoon.)
Total of all demand minus PV is what yields the duck's back curve. (You could subtract any once source from all load and make a different curve.)

So far, time of use rates have not been imposed on consumers, so they turn up the A/C when they get home. Given the abundance of PV, what we want is for consumers to pre-cool their houses when surplus PV is available, storing that energy in the thermal "battery" that is their house.
A simple pager that receives a probability figure broadcast to all could knock a percentage of A/C offline or a while, thereby adjusting A/C power draw to utilize no more than available PV. Much cheaper to switch a 24V control signal than to build grid-scale or distributed batteries to store the same power. Also, power generation and transmission is cheaper if load operate at closer to system capacity, rather than having excess idle headroom.

I say that PV generation, distributed or otherwise, is extremely valuable because it saves building power plant and transmission lines for the increased usage. But because PV has been built out slowly and has no consolidated negotiating power, it has not been assigned true value.