Smart Solar Charging

[ridgebackpilot]

Having just expanded my Sun Power rooftop solar array and added a third Tesla Powerwall battery to my house, I'm trying to figure out the optimal charging plan for my three EVs (Lightning and two Mustang Mach-Es).

Previously, with 20 solar panels producing 6 kW maximum, I wasn't able to charge my EVs solely by sun power. So I usually charged at night to take advantage of off-peak electric rates from the grid. But now that I have 36 panels making over 11 kW maximum, I'm able to power my Level 2 charger solely by the solar array during midday.

Obviously, I save money by charging my Lightning and other EVs with solar power as opposed to charging them from the grid. However, the nuances are important in this calculation: It costs me $0.076/kWH for power from the grid during off-peak hours in winter. But when I send solar power back to the grid, I'm only paid $0.047/kWH no matter the time of day.

By my calculation, I'm nearly 3 cents/kWH better off charging my cars with solar power than sending that surplus solar energy back to the grid.

These days, my daily regime is: The Powerwalls are set to "Self-Powered" so that they provide power for my home at night or anytime the solar array isn't producing enough. When the sun comes up each morning, I wait until the solar array charges my Powerwalls to 100 percent. As soon as that happens, I plug in an EV and allow it to charge using solar power. By 3 pm when the sun starts to go down and peak grid rates begin, I program my EVs to cease charging. When the sun sets, the Powerwalls take over and power my home.

With 36 solar panels, I'm essentially off-grid for the summer months, including charging my EVs. During winter when the solar array produces only about one-fourth of the power it does in the summer, I'll obviously have to rely more on the grid for charging during off-peak hours.

For those of you with rooftop solar arrays, I'm curious how my experience compares to yours. What is your daily routine? And what tricks and practices do you use to maximize the interplay between your solar array, battery storage, grid power, and EV charging?

Sponsored

 

[queuewho]

Your experience is very similar to mine, because I also have Tesla solar. I've got 24 panels on my roof that can do a max of 8kw. I have two powerwalls for a total of about 26kwh of battery. I have also found, like you have, that it works out to being more cost effective to use excess solar for charging rather than solar credits, even though in my case I am paid one-to-one for energy I consume or sell back. There are line fees and grid improvement project fees that they charge if I use power from the grid, and I don't get those back when I use credits. So, I try to charge during the day, sometimes de-rating my EVSE to charge slower, rather than emptying the powerwalls into the truck. I do not have peak/off-peak here. So really I just try to be as off-grid as I can in general.

I use less power than you due to only having a single EV. My system matches my use, in the summer I wrack up credits that I use deep into winter. Power bills in the summer are a couple bucks, and they are seldom more than $20 in the winter.

I'm glad I got solar before it got more expensive. People ask if it is worth it, even if mathematically it doesn't work out to "paying for itself" for quite a few years, I find value in the battery backup for potential ice storms in the winter, and thunderstorms in the summer. Having an EV is also even more affordable with solar.

 

[COrocket]

Wasn’t Tesla supposed to come out with a feature where your Tesla EV charger could communicate with your powerwall and solar array to essentially dump the excess solar into your EV after the powerwall is charged, to minimize the amount of power flowing too/from the grid?

I have 1:1 net metering without batteries so I can push a bunch of power during the day and draw a bunch to charge the truck overnight and it doesn’t matter on the bill. But if the day ever comes where that’s not the case, I’m probably going to invest in the Enphase bidirectional EV charger which does the “charge on solar” setting where it takes the solar production, subtracts the house use, and continuously adjusts the EV charger power so the utility meter sees zero consumption or production.

 

[ridgebackpilot]

I have 1:1 net metering without batteries so I can push a bunch of power during the day and draw a bunch to charge the truck overnight and it doesn’t matter on the bill. But if the day ever comes where that’s not the case, I’m probably going to invest in the Enphase bidirectional EV charger which does the “charge on solar” setting where it takes the solar production, subtracts the house use, and continuously adjusts the EV charger power so the utility meter sees zero consumption or production.

Apparently the Emporia Level 2 EVSE has the ability to vary the charge rate based on solar production when paired with the company's energy monitors.

 

[Blochead21]

Apparently the Emporia Level 2 EVSE has the ability to vary the charge rate based on solar production when paired with the company's energy monitors.

I can confirm the emporia level 2 evse does have this feature and I’ve tested it and it does works.

 

[Yellow Buddy]

Having just expanded my Sun Power rooftop solar array and added a third Tesla Powerwall battery to my house, I'm trying to figure out the optimal charging plan for my three EVs (Lightning and two Mustang Mach-Es).

Previously, with 20 solar panels producing 6 kW maximum, I wasn't able to charge my EVs solely by sun power. So I usually charged at night to take advantage of off-peak electric rates from the grid. But now that I have 36 panels making over 11 kW maximum, I'm able to power my Level 2 charger solely by the solar array during midday.

Obviously, I save money by charging my Lightning and other EVs with solar power as opposed to charging them from the grid. However, the nuances are important in this calculation: It costs me $0.076/kWH for power from the grid during off-peak hours in winter. But when I send solar power back to the grid, I'm only paid $0.047/kWH no matter the time of day.

By my calculation, I'm nearly 3 cents/kWH better off charging my cars with solar power than sending that surplus solar energy back to the grid.

These days, my daily regime is: The Powerwalls are set to "Self-Powered" so that they provide power for my home at night or anytime the solar array isn't producing enough. When the sun comes up each morning, I wait until the solar array charges my Powerwalls to 100 percent. As soon as that happens, I plug in an EV and allow it to charge using solar power. By 3 pm when the sun starts to go down and peak grid rates begin, I program my EVs to cease charging. When the sun sets, the Powerwalls take over and power my home.

With 36 solar panels, I'm essentially off-grid for the summer months, including charging my EVs. During winter when the solar array produces only about one-fourth of the power it does in the summer, I'll obviously have to rely more on the grid for charging during off-peak hours.

For those of you with rooftop solar arrays, I'm curious how my experience compares to yours. What is your daily routine? And what tricks and practices do you use to maximize the interplay between your solar array, battery storage, grid power, and EV charging?

After maxing out my first system at 36 panels, I got tired of these calculations and futzing with the roof. Solar got cheap enough that I went ground mount and added another 84 panels on the ground for 40kWp of solar, but I’m in a net metering state so any money for batteries was better spent on more panels.

 

[ridgebackpilot]

I can confirm the emporia level 2 evse does have this feature and I’ve tested it and it does works.

Does your Lightning (or other EV) throw a charge fault when the Emporia charger varies the rate of charge according to solar production?

 

[Blochead21]

I only tested it once with it and the lightning didn’t have an issue accepting the charge.

 

[VTbuckeye]

Does your Lightning (or other EV) throw a charge fault when the Emporia charger varies the rate of charge according to solar production?

Probably not. I was at a shared level 2 charging station 6.6/3.3 and someone plugged into the other plug on the same station. Charge dropped from 6 down to 3 for 45 minutes, then back up to 6. The truck should be able to handle the changes.

For me with rooftop solar in Vermont with sort of 1:1 net metering and discounted EV charging off peak I don't use excess solar for EV charging. We have two electric meters on the house. One for net grid use and one for solar production. No time of use simply how much did you produce and how much did your house use. In the December I produced 250kWh, so far in May I'm up to 1240kWh. Whatever excess solar is credited at 0.17xx per kWh (buy price is 0.18xx per kWh). The net metering credits can be used to pay for kWh used in the winter. They expire 12 months after generation. There is no financial benefit to using your own generated electricity versus grid power. If grid power goes out we have powerwalls backup and I can plug in cars to use excess (if I am home).

 

[MM in SouthTX]

Having just expanded my Sun Power rooftop solar array and added a third Tesla Powerwall battery to my house, I'm trying to figure out the optimal charging plan for my three EVs (Lightning and two Mustang Mach-Es).

Previously, with 20 solar panels producing 6 kW maximum, I wasn't able to charge my EVs solely by sun power. So I usually charged at night to take advantage of off-peak electric rates from the grid. But now that I have 36 panels making over 11 kW maximum, I'm able to power my Level 2 charger solely by the solar array during midday.

Obviously, I save money by charging my Lightning and other EVs with solar power as opposed to charging them from the grid. However, the nuances are important in this calculation: It costs me $0.076/kWH for power from the grid during off-peak hours in winter. But when I send solar power back to the grid, I'm only paid $0.047/kWH no matter the time of day.

By my calculation, I'm nearly 3 cents/kWH better off charging my cars with solar power than sending that surplus solar energy back to the grid.

These days, my daily regime is: The Powerwalls are set to "Self-Powered" so that they provide power for my home at night or anytime the solar array isn't producing enough. When the sun comes up each morning, I wait until the solar array charges my Powerwalls to 100 percent. As soon as that happens, I plug in an EV and allow it to charge using solar power. By 3 pm when the sun starts to go down and peak grid rates begin, I program my EVs to cease charging. When the sun sets, the Powerwalls take over and power my home.

With 36 solar panels, I'm essentially off-grid for the summer months, including charging my EVs. During winter when the solar array produces only about one-fourth of the power it does in the summer, I'll obviously have to rely more on the grid for charging during off-peak hours.

For those of you with rooftop solar arrays, I'm curious how my experience compares to yours. What is your daily routine? And what tricks and practices do you use to maximize the interplay between your solar array, battery storage, grid power, and EV charging?

I applaud your effort to go off grid as much as possible. I have solar on my roof as well, even though the payout for me looks like 17 years with the cheap electricity where I am. I am confused about your goal of cost savings, after spending well over $50,000 on your system. Depending on how much the 3 cars that you have get driven (are there 3 drivers, how much do they drive etc.), we are talking about a difference of a few hundred dollars per car. Do what's convenient, and be happy that you are drawing so little from PG&E.

I do try to charge on sunny days when the A/C is not running. That's never in the summer. I don't have battery backup because the cost isn't worth it. I am usually using more than the 20-panel Enphase array produces anyway.

 

[ridgebackpilot]

I applaud your effort to go off grid as much as possible. I have solar on my roof as well, even though the payout for me looks like 17 years with the cheap electricity where I am. I am confused about your goal of cost savings, after spending well over $50,000 on your system. Depending on how much the 3 cars that you have get driven (are there 3 drivers, how much do they drive etc.), we are talking about a difference of a few hundred dollars per car. Do what's convenient, and be happy that you are drawing so little from PG&E.

I do try to charge on sunny days when the A/C is not running. That's never in the summer. I don't have battery backup because the cost isn't worth it. I am usually using more than the 20-panel Enphase array produces anyway.

Good points, although my focus on cost-savings is driven in part by my desire to recoup my solar investment as quickly as possible.

It appears that my utility (PGE) pays me retail rates for solar power used to offset the grid power I consume each month. But any excess solar energy I produce over what I consume each month is paid at the end of the year true-up (net surplus compensation) at the paltry rate of $0.047/kWH, far below retail rates.

So I'm better off using as much solar power as possible for charging cars, etc. rather than sending too much back to the grid so that I generate a surplus and get paid less for it. And, since my family members and I drive a lot each month, the more we can charge cheaply at home, the less we'll have to use expensive Level 3 DC fast chargers on the road!

 

[MM in SouthTX]

Good points, although my focus on cost-savings is driven in part by my desire to recoup my solar investment as quickly as possible.

It appears that my utility (PGE) pays me retail rates for solar power used to offset the grid power I consume each month. But any excess solar energy I produce over what I consume each month is paid at the end of the year true-up (net surplus compensation) at the paltry rate of $0.047/kWH, far below retail rates.

So I'm better off using as much solar power as possible for charging cars, etc. rather than sending too much back to the grid so that I generate a surplus and get paid less for it.

(Edited to address an error)
Not sure how it works where you are, but I will walk through my bill and my solar production for April 2023 to give an example that may help. Starting with my solar production. Lots of rain last month, so lower than usual:

Then my bill:

So, I produced 730kWH but only used 730 - 381 kWH. I sold 381k WH back to the grid. At $0.09485 per kWH, the 730 - 381 = 349 kWH of solar production saved me $33.10 in energy cost, and $15.00 in transmission and delivery cost (see TDU line on the bill, at $0.043299/kWH plus a small base charge). So, $48.10 total. Another way to calculate it would be to add the energy and TDU charges together, and say that I am paying 13.8 cents per kWH.

The energy that I produced but did not use (381 kWH) was sold back at $0.0649, netting me $24.76. This does not include the transmission and delivery costs, but it also does not reflect the retail price for electricity, but instead the wholesale price that my co-op electric provider pays for energy.

That's my best job at understanding it. The grand total savings is $73.86 last month. Bottom line is, you are far better off using what you produce than selling it to the grid. In my case, by a factor of 2.

It's going to take a long time to break even on the solar installation.

 

[ridgebackpilot]

Bottom line is, you are far better off using what you produce than selling it to the grid. In my case, by a factor of 2. Problem is I am often not home when the sun is shining!

Fascinating, thanks! Looks like in almost any case, most of us are better off using the solar energy we produce than selling it back to the grid utility.

Smart chargers like the Emporia Level 2 EVSE solve the problem of not being home to manually charge your EV when the sun is shining. You can connect the charger to an Emporia energy monitor and set it to charge the EV with any available surplus solar power.

 

[MM in SouthTX]

Fascinating, thanks! Looks like in almost any case, most of us are better off using the solar energy we produce than selling it back to the grid utility.

Smart chargers like the Emporia Level 2 EVSE solve the problem of not being home to manually charge your EV when the sun is shining. You can connect the charger to an Emporia energy monitor and set it to charge the EV with any available surplus solar power.

Make sure you read the edited post. I initially did not back out the energy sold to the grid from my energy savings.

 

[MM in SouthTX]

When it gets hot around here...

...we use it all. Savings about $130. The Power Perks adjustment is co-op redistribution of profit.

Sponsored