Smart Solar Charging

ridgebackpilot

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Having just expanded my 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 (Two Mustang Mach-Es and a Lightning).

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 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?
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JohnFoxeSheets

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I only have 13 panels on my roof (and no room for more šŸ˜¢) and the max output is about 3.8kW. I don't have PowerWalls or any other type of whole house batteries. For the first few years after we had our solar installed in 2015 we ran net positive (sold more electrons than we bought) on an annual basis. But now that we have the MME (and make heavier use of electric rather than gas for cooking), we are definitely no longer net positive.

I charge my EV at night when the rates are the lowest. I use FP to keep the rate schedule current (it knows the rate plan we have from the power company), and just charge during the off-peak time periods. I have a "standard" (non-EV) residential TOU rate plan; I did some number crunching using my after hourly electric usage over the year since I bought my MME and it is 87% cheaper to not use the EV rate.

EDIT: found an important error in my spreadsheet and while TOU-C is still less expensive than EV2, it's only about 30% less, not 87%!
 
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Mach-Lee

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You'll need an EVSE that ramps charging speed to match solar output. That sounds like a good application for an Emporia EVSE and Vue energy monitor. The energy monitor will monitor the solar output and tell the EVSE to adjust charging speed to match the panel output in real time.

I'm curious if the variable charging rate will work with the Mach-E or if it will trigger a charge fault.
 
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ridgebackpilot

ridgebackpilot

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I charge my EV at night when the rates are the lowest. I use FP to keep the rate schedule current (it knows the rate plan we have from the power company), and just charge during the off-peak time periods. I have a "standard" (non-EV) residential TOU rate plan; I did some number crunching using my after hourly electric usage over the year since I bought my MME and it is 87% cheaper to not use the EV rate.
Interesting, thanks! I have the EV2A rate, which I assumed would work better for me than the simple TOU rate plan.

But based on your experience I'm going to have to check on that to make sure I'm getting the best rate. Here on the Central Coast, we have a community energy association and I switched from PGE to Central Coast Community Energy last year. Their rates are generally lower than PGE but I'm going to have to compare the EV2A and TOU rates to see which might work better for me.

Incidentally, when I purchased my EVs, I got rebates from both PGE and CCCE, which helped defray the costs of the vehicles.
 

markboris

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I have a similar setup to yours except a bit more of everything. 39, 320W Silfab panels and 12, 370W Silfab panels totaling just under 17kW with a SolarEdge 11.4kW inverter. 4 PowerWalls and like yours they are also set to self-powered so they take over when there's no solar production.

I installed the Emporia Vue Energy monitoring system and two Emporia EV chargers (one on each side of the garage for connivence). The chargers are hard wired and set to the 48 amp setting so they will charge the car at max if need be. Like Lee mentioned, I have them set to charge the car ONLY with "excess solar". The EV charger constantly varyies the charge amperage depending on excess solar production. If I'm using more power than it's producing or there's a temporary cloud cover, the EV charger pauses. If I need the car to charge at full amperage instantly at any time, I can tell it to do so and it will charge at the max 48 amp setting. Also, the charging amperage can easily/instantly be adjusted on the app.
 
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skiingj

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I pretty much follow your current plan. I have a 8kW with two Powerwall's and one EV the Mach-E. Once the Powerwall's are close to or at 100%, I will charge my Mach-E usually with a slower amperage. Especially when its hot out and we are using AC. My city utility rate is .9kWH anytime. Unfortunately I am not at home most weekdays.

It definitely takes some work but charging with solar is the way to go. I ordered my solar before I had an EV and my city only allows a system size of 100% of the past year usage. Not worth putting in another 8kW system and another 1 or 2 Powerwall's.

I'll be interested in what others say. I believe what we are doing is the only way unless you buy a huge solar system. I plan on selling my Mach-E or turning it in under the hybrid loan/lease program. It has too many issues. I might get a BMW i4 M50 which has a two year Electrify America plan.
 
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ridgebackpilot

ridgebackpilot

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I installed the Emporia Vue Energy monitoring system and two Emporia EV chargers (one on each side of the garage for connivence) and like Lee mentioned, I have them set to charge the car ONLY with excess solar production. The EV charger constantly varyies the charge amperage depending on what is being used in the house. If I am using more than the is producing at any time, the EV charger temporarily pauses and as soon as there is more than 1kW of excess solar, it starts up again. The chargers are hard wired and set to the 48 amp setting so they will charge the car at max if need be. If I need the car to charge instantly at any time, I go to the app on my phone and select Charge Now and it will charge at max 48 amp. The charging amperage can be instantly adjusted on the app also.
Based on what you and @Mach-Lee have said, it sounds like I need to replace my Level 2 wall charger with an Emporia unit. I'd love to be able to automate variable charging with solar power only, with the option to charge from the grid when necessary.

Thanks for the tip about the Emporia charger and energy monitor. That would be a relatively inexpensive upgrade to my current system, and my power company offers rebates for EVSE installation.
 

SpaceEVDriver

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I think you have it pretty well dialed in without spending more money. The only thing I can think of is that my TOU rate plan (in AZ) has weekends and holidays as off-peak, so we extend our charging session time (we don't have one of the automatically adjustable EVSEs) on the weekends.
 

JoeyD

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You'll need an EVSE that ramps charging speed to match solar output. That sounds like a good application for an Emporia EVSE and Vue energy monitor. The energy monitor will monitor the solar output and tell the EVSE to adjust charging speed to match the panel output in real time.

I'm curious if the variable charging rate will work with the Mach-E or if it will trigger a charge fault.
I had and used the ChargePoint Home Flex charger for over a year and it worked great. Only reason I switched to the Emporia EV charger was for the option to charge only with excess solar output. It works great.
 

SpaceEVDriver

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I had and used the ChargePoint Home Flex charger for over a year and it worked great. Only reason I switched to the Emporia EV charger was for the option to charge only with excess solar output. It works great.
Yeah, I have the CP Home Flex. I really like it, but I'm building a workshop right now and was looking to install another, 60A or greater, hard-wired EVSE into the garage and move the CP Home Flex out to the workshop. Might get the Emporia EV charger for this purpose.
 

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In Virginia weā€™re on net metering for solar. No TOU rate differences. It doesnā€™t matter when I charge. Only concession to high demand periods is allowing the power company to reduce my charging rate during times of high demand. They pay me $40 per year for that.

All my power produced goes to my house and any excess is pushed out to the grid. My system was sized come as close to my prior usage and how much good roof space I had.
 

KPal64

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In Virginia weā€™re on net metering for solar. No TOU rate differences. It doesnā€™t matter when I charge. Only concession to high demand periods is allowing the power company to reduce my charging rate during times of high demand. They pay me $40 per year for that.

All my power produced goes to my house and any excess is pushed out to the grid. My system was sized come as close to my prior usage and how much good roof space I had.
Agree; net metering here too with no off-peak advantage when needing power from the grid so I donā€™t think it matters when I charge; it all works out the same cost-wise.
 

jlauro

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I set my powerwall to Time-Based Control and set it to reserve 40% for backup and gave it the buy and sell based time of day rates and let the powerwall optimize for when to charge batteries and when to send to grid and when pull from grid or pull from batteries. I charge at lower rates even if it means charging from batteries instead of panels, but for me my buy/sell rates match although change based on time of day.

For you, I would still recommend time-base control instead of self powered, but charging when you have good sun and 100% charge would probably make most sense, or at lowest rates if batteries never get fully charged when the Sun is up.
 

v8318cid

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I am lucky enough to have a power company that doesn't differentiate between time of day and power generated. I get essentially a one-for-one exchange with my local coop. What I cannot tell you is how much I've generated vs what I've used (I know I've generated more than I've used since months where I use more than I generate I still only pay the base load rate of 21/mo). Regardless of that, I charge my MMe only on the weekend since I seldom use more than 50% of the battery over the course of a week. I also have a Nissan Leaf and a Jeep Grand Cherokee 4xe that get charged almost daily. So far I have managed to keep my usage below what my solar array (~12kw) generates in a given day (assumes sunny weather). We are also in the process of adding a battery, but that hasn't happened yet. I can only hope that my coop doesn't decide to reduce my reimbursement rate anytime soon.
 

Mach-Lee

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Agree; net metering here too with no off-peak advantage when needing power from the grid so I donā€™t think it matters when I charge; it all works out the same cost-wise.
You're one of the lucky ones. Here we only get a fraction back, have to pay extra fees per month, upgrade to a 320A meter, and have to carry $300k liability insurance to get a grid connection.
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