dumb charging/solar panels question

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TempoHouse

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We have rooftop solar at home and, over the last twelve months, have produced a little over 1600kWh more electricity than we have used (the kids have grown up, left to seek their fortune, and taken a lot of energy consumption with them).

The SE battery pack seems to max out at 28.9 kWh. I'm guesstimating a full charge, at home, would use something like 36 kWh. There's some loss of energy in charging the batteries but I don't know how much. I'm estimating about 25%. I am not now nor have I ever been an electrical engineer. Anyway, at 36 kWh a pop, I should be able to charge the car 44 or 45 times, at home, without using any more power than our panels are already producing. Does that sound right? Am I missing something? Am I overestimating or underestimating the energy lost in charging/converting to DC?

Of course I don't plan to run the battery down to zero before charging. If the numbers are right, we could charge from 20% to 100%, about once a week. We're looking to add a few solar panels to the roof, to account for charging the SE, and I'm trying to figure out how many to add.
 
First off, here's a source for charging efficiency. Efficiency depends on a bunch of variables but on average level 2 charging (240V) was 89.4% efficient or 10.6% loss.

Probably a better way of looking at this than number of charges is just miles driven on your 1600kWh:
  • 1600kWh * 89.4% = 1430.4kWh into the batteries.
  • The MINI gets an EPA rating of 31kWh/100 mi or 3.23 mi/kWh (I get ~4.2 mi/kWh)
  • 3.23 mi/kWh * 1430.4kWh = 4620.2 mi of driving from your solar (or more likely ~6000mi using my 4.2mi/kWh)
 
Carsten Haase said:
First off, here's a source for charging efficiency. Efficiency depends on a bunch of variables but on average level 2 charging (240V) was 89.4% efficient or 10.6% loss.

Probably a better way of looking at this than number of charges is just miles driven on your 1600kWh:
  • 1600kWh * 89.4% = 1430.4kWh into the batteries.
  • The MINI gets an EPA rating of 31kWh/100 mi or 3.23 mi/kWh (I get ~4.2 mi/kWh)
  • 3.23 mi/kWh * 1430.4kWh = 4620.2 mi of driving from your solar (or more likely ~6000mi using my 4.2mi/kWh)
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I agree, this is a good way of approaching the question. One minor nit, though - the first bullet is unnecessary. The second bullet includes the charging losses (it is measured "wall-to-wheels"). So your final calculation has double-counted those losses. This should make sense if you think about it. The car is rated at 110 miles on 28.9kWh usable, yet it requires 31kWh to drive 100 miles (or 34.1kWh to drive 110 miles). The difference is the charging losses (28.9/34.1 = 84.6%). Note, however, that the 4.2 mi/kWh you get does NOT include those loses since the car does not count them. It is only battery-to-wheels that the car measures.

Bottom line, assuming the rest of your numbers are correct (I didn't double-check), it's actually 3.23 mi/kWh * 1600kWh = 5,168 miles.
 
It isn't just inefficiency, there is also an overhead. The car needs to power up the charging systems, and manage any battery heat. Along with inefficiency, I worked out I lose around 400W when using low power charging. The 8A (240V) charger that came with the car puts about 1600W into the battery. When I use the 10A charger in the garage, it pumps in around 2kW, so I lose 400W to inefficiency and the cars systems. This results in something like 18-19 hours to fully charge (from empty) on the 8A and about 14 hours from the 10A.

So, the SE gets around 5mi/kWh (for my driving) - and we know that at 10A, you put 2kW into the battery from a 2.4kW source. This would mean about 4.2mi/kWh from the charger's point of view. If you generated 1600kWh from solar you should be able to travel around 6700mi. The EPA's figures are quite pessimistic, so the good news is that you should easily get the 5,168 miles that GetOffYourGas calculated.
 
Tempo - sorry if to personal but are you located in the states or elsewhere? What is your cost per kWh for your electricity? Basically, I’m wondering what the cost to charge comparison looks like between solar and your provider. I’ve seen some other attempts at this but since this is specific to our Minis it will be much more relevant to us.

Thx - mike



Sent from my iPhone using Inside EVs
 
GetOffYourGas said:
I agree, this is a good way of approaching the question. One minor nit, though - the first bullet is unnecessary. The second bullet includes the charging losses (it is measured "wall-to-wheels"). So your final calculation has double-counted those losses. This should make sense if you think about it. The car is rated at 110 miles on 28.9kWh usable, yet it requires 31kWh to drive 100 miles (or 34.1kWh to drive 110 miles). The difference is the charging losses (28.9/34.1 = 84.6%). Note, however, that the 4.2 mi/kWh you get does NOT include those loses since the car does not count them. It is only battery-to-wheels that the car measures.

Bottom line, assuming the rest of your numbers are correct (I didn't double-check), it's actually 3.23 mi/kWh * 1600kWh = 5,168 miles.
Click to expand...

Thanks, I wasn't aware the EPA was accounting for the charging losses already
 
mikeg0305 said:
Tempo - sorry if too personal but are you located in the states or elsewhere? What is your cost per kWh for your electricity?
Click to expand...

We're in Hawaii. It's about 29 cents/kWh now where we live. I guess Hawaii has the highest rates for electricity and lowest average consumption of all the states, which isn't too surprising.
 
29cents…wow. No wonder solar is so attractive for you. Not only do you get clean energy but the breakeven isn’t terrible. I’m in MD and we pay 11cents all in. A weeks worth of driving of 300 miles costs me ~$9 for a “fillup” on the Mini vs close to $50 in one of our ICE vehicles. And solar is free once you pay the upfront, which had been slowly coming down.


Sent from my iPhone using Inside EVs
 
I have been thinking about if it would be better to charge my Mini directly from the solar panel power (using e.g. a Tesla Powerwall 2 battery) but then I checked the daily price variation of electricity here in Stockholm (source https://www.nordpoolgroup.com/Market-data1/Dayahead/Area-Prices/SE/Hourly/?view=table ):
upload_2021-10-6_15-20-38.webp
It appears that I get paid quite a lot more (a factor of 2 in my case) for selling electricity during the day, than it costs to buy electricity at night. So I will happily keep using the grid as storage and charge the Mini at night (cheapest between 01 and 07 AM apparently).
If I charge at work I have to pay 300 öre per kWh compared to 40 at home at night.
Fast DC charging costs 800 öre per kWh, which equals just under 1 USD per kWh.
 
I came across your post and thought it was a fascinating look at the calculations involved in solar panel and EV coordination. Your point about not running the battery down to zero is well-taken. It's always best for the longevity of any lithium-ion battery to keep it in the middle of its charge range as much as possible.
 
Whenemont said:
I came across your post and thought it was a fascinating look at the calculations involved in solar panel and EV coordination. Your point about not running the battery down to zero is well-taken. It's always best for the longevity of any lithium-ion battery to keep it in the middle of its charge range as much as possible.
Click to expand...
If you're thinking of adding more panels to your rooftop setup, have you considered enhancing your storage capabilities as well? ACE Battery offers a battery called EnerCube, which could be a real game-changer for you. It would allow you to store any excess energy for those days when the sun isn't cooperating or when you might need more power for your SE.
 
Whenemont said:
If you're thinking of adding more panels to your rooftop setup, have you considered enhancing your storage capabilities as well? ACE Battery offers a battery called EnerCube, which could be a real game-changer for you. It would allow you to store any excess energy for those days when the sun isn't cooperating or when you might need more power for your SE.
Click to expand...
Are Ace's EnerCube systems less expensive than a Tesla Powerwall or a Ford F-150 Lightning?
 
ericy said:
Yikes. I have read stories about "mislabeled" batteries that are all over the place in China. I would be really careful about ordering these things.
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Yes they run rampant in the LFP server rack style battery market. Usually the savings are from discount B & C grade batteries.

One major off-grid inverter/battery company in USA even claims (S&P/Moody's/Fitch) Grade AAA battery rating.
 
IMG_1100.webp
insightman said:
Are Ace's EnerCube systems less expensive than a Tesla Powerwall or a Ford F-150 Lightning?
Click to expand...
To install two Tesla back up batteries with 13.5 KW +13.5 KW is about $18000 if you have sufficient solar panels and charge a SE you need 32.6 KWh usable 28.9 KWh if you without power in your house you use that at once to charge SE from back up battery. Better if you have second gas car and use battery’s for run a fridge and lights and let SE to rest .I was going to install back up batteries but I think they just useless with price and storage capacity they last only 10 years I drive gas car for necessity when is power outrage and is easier to keep 10 gallons of gas in storage than $18000 tag of batteries.I own solar panels for for 8 years and this is my output: 5.5 KW system install by contractor but by my choice of panels inverter and optimaizers .I have in a system my own meter showing me in KWh what I produce in 8 years system is grid connected outrage no electricity.in 8 years I produce about 56000HWh of electricity which is about 7000 KWh a year and basically I don’t pay for electricity at all .$20 surcharge for meter.My bill is $30-40 once /twice a year . House is very sufficient from gas heating condensing boiler efficient appliances and LED lights AC I run from April to September .Best of all was a price for solar system $16000 pay outright: $7500 tax deductible,$1000 for 4 year’s property abatement,$1000 for 4 year state tax deduction.System cost me $500 and not paying in8 years for electric bills additional $14000 in a pocket.I’m driving For past 7 yers EV my driving I limited to 5000 miles year . To me batteries backup for solar panels is waist of money you never take advantage of it with today’s technology.This is my TWO cents to this subject.One more point :Taking advantage from Ev car to charge your house backwards from EV batteries in case of outrage of power/disaster- this is the way to go but so far nobody in manufactures going this directions bc warranty of you EV will be void ! Some owners of EVs keep battery charge 50 % to charge again which is wrong those new EVs and SE have a buffer up or down 10% you can’t damage a battery’s .IMG_1097.webpIMG_1096.webp my system of solar at 3.30 PM is still producing 1300 watts with low sunshine in Staten Island NY.
 
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In 2012 after SANDY hurricane hit NewYork very badly with a lot of damage to every neighborhood I decide to buy generator I lost electricity only for day and half and I survived just barbecued all my meat to prevent from spoiling and I could survive not eating for day and half too .My generator 4.5 KW is triple source of power gasoline /NG /LP Honda engine and I’m restarting that baby every year and changing synthetic W5-30 oil too.The most interesting think is I NEVER USE THAT BABY ONCE !!! What is conclusion of that forget about backups batteries for solar/generators / just burn a wood as you need it for heat or cooking as ancestors did before !
 
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