Random bit of math.
In California circa 2018, the
overnight carbon intensity of our electricity is around 0.275 mTCO2/MWh or, if you prefer, 275 grams of CO2/KWh (bless you metric). During the day the CO2 intensity varies with season, but is around 130 gCO2/KWh
in the spring, and somewhat higher in the other seasons -- though almost always lower than overnight values. On this nice clear, January day, it's at
205.
So, 130 to 275 gCO2/KWh. The best selling electric car in the US averages 0.270 KWh/mile, thus emitting
35 to
74 gCO2 / mile depending on when you charge.
A 2018 Toyota Camry (chosen as it's the best selling passenger car), in hybrid form (because I wouldn't even think of anything less) gets 52 MPG combined. A gallon of gas emits
8890 gCO2/gallon, thus emitting
170 gCO2 / mile. A non-hybrid Camry (with the 2.5 L engine) gets 34 MPG, or
260 gCO2/mile. A Chevy Silvarado,
445 gCO2/mile.
And, of course, an electric bicycle emits approximately
0 gCO2/mile, or close enough so as to not matter.
All numbers above are just the direct emissions. Without being too rigorous, I think it's safe to assume that the emissions associated with transporting natural gas to California are lower than those for oil transportation and refining, though significant methane leakage could certainly complicate the picture.
So, if you
need a car, electric is clearly better. And clearly it's better to charge during the middle of the day, since the emissions are roughly half then.
Or
does it matter? I'm not so sure it does, and that's because
average emissions aren't the interesting thing. It's marginal emissions that really matter. When you start charging, how is the additional electricity that's required being generated? In almost all cases, it'll be via natural gas. The exception to that is if you charge during the early evening peak, when solar has quit for the day, and demand is at its highest. During those hours, California imports a
lot of electricity, and furthermore, it imports
enough that coal starts coming into the mix. California imports are a mix of natural gas fired and coal fired, but natural gas tend to be dispatched first for economic reasons, and most hours of the day there's relatively little coal generation feeding those imports. For example, when looking at CAISO's current emission values (midday) -- they don't break down the imports by generation type, unfortunately -- but I can see that the imports and our in-state natural gas have approximately the same intensity. Which is as expected, since imports are relatively low right now.
So, anyways, if you add an electric car, the added electricity load is going to be met with
natural gas:
Natural gas GHG content – 117 lb/MMBtu --> 53070 g / 293 KWh --> 181 g/KWh -->
49 g/mile
Incidentally, for coal:
Coal GHG content – 208 lb/MMBtu --> 94347 g / 293 KWh --> 322 g/KWh -->
87 g/mile
Edit: I think I misinterpreted the above numbers for gas and coal. They need to be converted from thermal energy to electrical, using appropriate power plant efficiencies. There's a somewhat fuller explanation in a later post. </edit>
Hang on a sec...
And this is where I think I have to change my mind. California's average,
overnight intensity is 275 g/KWh. That's a
lot higher than natural gas's intensity of 181 g/KWh. That means that we're importing quite a bit of coal to meet our overnight demand. And, given that natural gas tends to dispatch first, that means that any additional overnight load is being met by ... pure coal.
Well, fuck. I'm driving a coal-powered car. Thanks to the Model 3 being supremely efficient it's still less carbon intensive than even a hybrid Camry (87 vs 170 g/mile). But, still.
So, it
does matter if you charge midday vs overnight. Your additional load still won't be met with solar (we don't have enough yet), but at least it'll be met with natural gas instead of coal.
By the way, while I think the focus on marginal generation type is the most interesting for analyzing emissions, there are
other considerations. Adding additional midday load may not get met with solar right now ... but it does help keep midday electricity prices from declining further, which helps keep the financials stable for deploying more solar.
