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Model 3 supercharger rate example, help me with the math

dhanson865

Well-Known Member
Feb 16, 2013
5,600
17,674
Knoxville, Tennessee
OK so the Semi will get 7 cents per kWh guaranteed (meaning it could get cheaper than that someday). So my next thought is when what rate would it take for the Supercharger 10 miles from my house to be cheaper than charging in my garage (still more convenient to charge at home, this is a hypothetical exploration of years down the road).

Givens:

Assume I'm getting at the supercharger at 25% SOC and adding 40 kWh and then leaving (time required would vary by Tesla used)
$0.16 per minute above 60 kW
$0.08 per minute at or below 60 kW

So the question is what time would be logged and the resulting price per kWh for

S60 (2014)
S85 (2014)
Model 3 LR (2018)

This would be so much easier in a state with per kWh fees, for example Colorado is $0.13 per kWh so no matter the time used it'd just be 40 x 0.13 = $5.20 or just keep it at $0.13 per kWh to begin with since I'm comparing vs my home cost to charge.

Does anyone feel like doing the math and showing your work?
 

TeszLA

New Member
Jan 6, 2018
1
0
LA
Assume I'm getting at the supercharger at 25% SOC and adding 40 kWh and then leaving (time required would vary by Tesla used)
$0.16 per minute above 60 kW
$0.08 per minute at or below 60 kW


You’re confusing power (kW) for energy (kWh). The rate table is tiered by power, i.e. the rate at which you draw energy. You pay one price or anoher for the same amount of energy, depending on how quickly you draw it, which is dependent on the supercharger, your battery size, state of charge etc. You pay $0.16/kWh for as long as you draw at 60kW. Once your draw rate drops below 60kW, you pay $0.08.
 

dhanson865

Well-Known Member
Feb 16, 2013
5,600
17,674
Knoxville, Tennessee
You’re confusing power (kW) for energy (kWh). The rate table is tiered by power, i.e. the rate at which you draw energy. You pay one price or another for the same amount of energy, depending on how quickly you draw it, which is dependent on the supercharger, your battery size, state of charge etc. You pay $0.16/kWh for as long as you draw at 60kW. Once your draw rate drops below 60kW, you pay $0.08.

No I'm not confusing kW for kWh.

Go to Supercharging and see
  • In North America, pricing is fixed within each state or province. Internationally, pricing is fixed within each country. All prices include taxes and fees.
  • Where possible, owners are billed per kWh (kilowatt-hour), which is the most fair and simple method. In other areas, we bill for the service per minute.
  • When billing per minute, there are two tiers to account for changes in charging speeds, called “tier 1” and “tier 2”.
    • Tier 1 applies while cars are charging at or below 60 kW and tier 2 applies while cars are charging above 60 kW. Tier 1 is half the cost of tier 2.
    • Tier 1 also applies anytime your vehicle is sharing Supercharger power with another car.

and change the state to any of these:

Alabama
Arizona
Connecticut
Delaware
Georgia
Indiana
Iowa
Kansas
Kentucky
Louisiana
Michigan
Mississippi
Missouri
Montana
Nebraska
Nevada
New Hampshire
New Jersey
New Mexico
North Carolina
Ohio
Oklahoma
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Vermont
Wisconsin
Wyoming

to see the per minute rate. It is literally per minute not per kWh. If it were as simple as you suggested I wouldn't have created this thread.

3/5ths or roughly 60% of the states don't allow Tesla to charge per kWh. It's a billing thing not a power vs energy thing.
 
Last edited:

dhanson865

Well-Known Member
Feb 16, 2013
5,600
17,674
Knoxville, Tennessee
price per kWh = price per h / power = price per min * 60min/h / power

in >60 kW part: 16 ct/min * 60 min/h / 96~60 kW = 10~16 ct/kWh
in <60 kW part: 08 ct/min * 60 min/h / 60~20 kW = 8~24 ct/kWh

That's a start but how many minutes does it take for the 3 example cars to get 40 kWh?

I could do the math once for paired to assume the lower rate per minute and again for unpaired but for the unpaired you need to know the supercharging curve for the 3 example vehicles and figure out how much time (if not all the time) will be in the higher price bracket.

Starting from 25% SOC do you think the 3 cars can all do 60 kWh+ for the time needed to do 40 kWh.
 

dhanson865

Well-Known Member
Feb 16, 2013
5,600
17,674
Knoxville, Tennessee
does it matter? You charge enough for next SC or destination + some buffer, overthinking doesn't change the state of grid, charger or battery :)

1 obvious conclusion if possible you should not charge at <30 kW it's most expensive part

This math isn't about making it to another supercharger. This is about comparing cost of supercharging vs cost of the local grid as priced for residential use. Thinking ahead to a paradigm shift and I wanted a benchmark to say where the tipping point would be.
 

T34ME

Active Member
Mar 31, 2016
2,262
3,872
Inland Empire
This math isn't about making it to another supercharger. This is about comparing cost of supercharging vs cost of the local grid as priced for residential use. Thinking ahead to a paradigm shift and I wanted a benchmark to say where the tipping point would be.
Another alternative is go PV on the roof. I had a Solar City system installed almost three years ago, At that time my breakeven on the system was around 9 years. Due to increases in local electrical utility rates, my breakeven has lowered to pretty close to 7 years. I expect breakeven to drop even further with future increases in utility rates. With advances in PV components, the initial cost of a PV system has fallen further from just 3 years ago, further reducing the breakeven point.

I will rarely use the SuC system when it makes so much sense for me to charge at home in terms of: time spent, convenience, and cost. I will wake up each morning to a full (80%), 'free' tank of fuel.
 

swaltner

Active Member
Oct 13, 2012
1,822
2,100
Kansas, USA
I'm having a little bit of trouble coming up with a way to explain the logic behind the math, but it's hidden in the units. Here's a way to quickly check if you're better off charging at home than at the supercharger for a specific charging rate. This is useful in states where local laws only allow the electric companies to charge customers per kWh. At different points in the charging cycle, you'll be above or below the break even point.

Assumptions (Kansas rates):
- $0.18 per minute above 60 kW
- $0.09 per minute at or below 60 kW
- Residential rate is $0.117 per kWh (all taxes and fees included ignoring the basic service fee for the meter)

Tier 1 Charging Breakeven point:
$0.09 / $0.117 * 60 = 46.2 kW

Tier 2 Charging Breakeven point:
$0.18 / $0.117 * 60 = 92.3 kW

So, for those stated assumptions, you either want to charge at higher than 92.3 kW or between 60 kW and 46.2 kW. But, you have no real control over the charging speed you're getting during a specific supercharger session. Tesla priced these rates so they'd be very close to what your residential rates are. Without knowing the taper that you'll see (supercharger pairing, battery temp, ambient temp, connector temp, final state of charge, etc...), you can't really predict your final per kWh rate at one of these stations.

Also be sure to factor in the round trip expense to get to the supercharger if you're making a special trip to the charger. If you drive 20 miles round trip to recharge, you need an additional ~5 kWh of energy, just for the round trip. This is why I found apps like Gas Buddy or gas discounts at the grocery store not terribly useful. If you want me to drive 4 extra miles in my pickup to save 2 cents per gallon of gas, it's not a net gain. I'll spend at least 50 cents in gas alone (ignores the wear/tear/depreciation) to save 40 cents on 20 gallons of gas for the pickup. Put that up to the IRS number of ~50 cents/mile and you're really losing money trying to save money.

In other words, charge at home if you can and enjoy the drive. :)
 
I'm having a little bit of trouble coming up with a way to explain the logic behind the math, but it's hidden in the units. Here's a way to quickly check if you're better off charging at home than at the supercharger for a specific charging rate. This is useful in states where local laws only allow the electric companies to charge customers per kWh. At different points in the charging cycle, you'll be above or below the break even point.

Assumptions (Kansas rates):
- $0.18 per minute above 60 kW
- $0.09 per minute at or below 60 kW
- Residential rate is $0.117 per kWh (all taxes and fees included ignoring the basic service fee for the meter)

Tier 1 Charging Breakeven point:
$0.09 / $0.117 * 60 = 46.2 kW

Tier 2 Charging Breakeven point:
$0.18 / $0.117 * 60 = 92.3 kW

So, for those stated assumptions, you either want to charge at higher than 92.3 kW or between 60 kW and 46.2 kW. But, you have no real control over the charging speed you're getting during a specific supercharger session. Tesla priced these rates so they'd be very close to what your residential rates are. Without knowing the taper that you'll see (supercharger pairing, battery temp, ambient temp, connector temp, final state of charge, etc...), you can't really predict your final per kWh rate at one of these stations.

Also be sure to factor in the round trip expense to get to the supercharger if you're making a special trip to the charger. If you drive 20 miles round trip to recharge, you need an additional ~5 kWh of energy, just for the round trip. This is why I found apps like Gas Buddy or gas discounts at the grocery store not terribly useful. If you want me to drive 4 extra miles in my pickup to save 2 cents per gallon of gas, it's not a net gain. I'll spend at least 50 cents in gas alone (ignores the wear/tear/depreciation) to save 40 cents on 20 gallons of gas for the pickup. Put that up to the IRS number of ~50 cents/mile and you're really losing money trying to save money.

In other words, charge at home if you can and enjoy the drive. :)

The math you did is a start, we need supercharging curve data to plug in and finish the math.

As to miles to the superhcarger vs charging at home, this is a theoretical exercise of where the tipping point will be. For many people home charging isn't an option or if it is home solar PV isn't an option. At some point down the road charging in public will be cheaper than charging at home for those users. I doubt that will ever be true for all users as solar PV at home can get cheap enough to beat the grid price easily and shows no sign of stopping the decline in price per kWh any time in the foreseeable future.

But charging home being more expensive for those people absolutely opposite of now. I pay less for electricity at home in TN than you do in KS. Every public charger is outrageously expensive by comparison. I expect that to flip in the future and at some point be cheaper than my home grid connection. Even then it would have to be considerably cheaper to deal with the hassle of going offsite to charge.

I'm just trying to do the math for a purely hypothetical situation. I'm very happy laying in bed and letting my car charge in the garage below.

Some day when I'm bored with other stuff or interested in this and feeling energetic I'd try and tie it all together in a more presentable form.
 

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