Calculating KWh usage from 120v outlet

[AlanSubie4Life]

kW is power. That’s an instantaneous thing, like horsepower.
kWh is energy. It’s just power x time.

1 kW for 30 minutes is 0.5 kWh
2 kW for 30 minutes is 1.0 kWh
1 kW for 60 minutes is 1.0 kWh

You have 8.8 kWh daily budget. You use 4-5 kWh per day.

8.8 kWh minus 5 kWh max daily use before charging = 3.8 kWh left for charging.

If you charge at rate of 1.44 kW, the math is:
3.8 kWh / 1.44 kW = 2.6 hours, 2h38m.

This is maybe 13 miles of rated range added.

I hear an echo. Should I add the bit about miles added per day to my post too? I was thinking about it before I saw yours!

 

[darth_vad3r]

I hear an echo. Should I add the bit about miles added per day to my post too? I was thinking about it before I saw yours!

It should prompt you to read super-recent posts that got posted while you were drafting your own post

I might have beat you if I did less math. Suffice it to say my 13 miles guesstimate was pretty accurate and backed by math which got deleted. First I was “way” off because I forgot about 250 W overhead.

 

[eustachio]

Hat tip and thanks to you both, splendidly clear explanations!

 

[AlanSubie4Life]

Suffice it to say my 13 miles guesstimate was pretty accurate and backed by math which got deleted.

AWD...70% efficiency...see my math further up...but I think it works out to be more like 11 miles in that 2.6 hours. But it’s all a bit of a guess even with your handy chart. I could be wrong. Shrug.

before hitting the 8.8 limit.

I would suggest you not worry too much about the bump up to 28 cents from 22 cents. Seems not too punitive. Likely not worth fiddling around over and you can then charge considerably more at home. If you do get worried about it and it is possible for you, consider solar. Long payoff and there are many variables, so it may not make sense for you. California is pretty good for it though.

I think the payoff for a 240V install might be shorter than solar, though, if it is typically priced. Not sure though. Could be a long payoff too - but there is actual additional convenience/utility (faster charging!) to that. If you did all your charging at home (which you do not), and you drive 10k miles per year, a 14-50 install would save you something like $170 a year in electricity costs, if I am calculating correctly.

 

[darth_vad3r]

AWD...70% efficiency...see my math further up...but I think it works out to be more like 11 miles in that 2.6 hours. But it’s all a bit of a guess even with your handy chart. I could be wrong. Shrug.

I shoulda just left it at “about 13 miles”

I was using the underlying less-rounded 2.64 hours number.

I get ~13 for SR+ using my own real world numbers which are less than 1.44 kW due to voltage < 120 V.
I get ~12 multiplying 13.4 by 219/245 to adjust SR+ to AWD.

If I use 1.44 kW instead of my empirical data the AWD goes up to a rounded 13 .[/QUOTE]

 

[justsomeguy]

This gets me every time. I don't understand the distinction.
1.44 kWh per hour expands to "1.44 kilowatt hour per hour"?

The other two answered it really well.

Just think about the units.

1.44kW * 1h = 1.44kWh
So the phrase 1.44kWh per hour translates directly to 1.44kWh / 1h which is just a ridiculous way of saying 1.44kW

 

[srs5694]

Currently we use about 4-5 kw per day. That means I charge for about 2 hours at home before hitting the 8.8 limit.

Is this really measured on a daily basis, or per billing cycle (monthly)? It'd be a pain to have to track electricity use on a daily basis.

In any event, 8.8kWh per day works out to 264kWh in a month, which isn't much. I've just checked my own TeslaFi data, and for the past several months, my car has been using 170-210kWh per month in home charging, plus more in away-from-home charging (Supercharging on road trips and the occasional Level 2 charge at local public EVSEs).

Looked at another way, the Model 3's EPA fuel economy figure is 116 MPGe, or 29 kWh/100 miles (290 Wh/mile). If you were to drive 1,000 miles in a month (about average for the US), you'd use 290 kWh -- just over the limit to get your lowest electricity rate. Of course, if you do a significant amount of charging away from home, you might still be able to keep your home use low enough to not incur additional charges.

Another detail to consider: If you exceed your 264kWh/month, does the rate go up for all your use for the month, or just for your usage that exceeds the 264kWh? If the latter, then using, say, 300kWh in a month won't really be that big a deal -- you'll be paying an extra $0.06/kWh on just 36kWh, or $2.16. If the rate goes up for all of it, though, that'll be an extra $18.00. If the excess is billed on a daily basis, then the computations get trickier. Even if your non-EV electricity use brought you exactly to the limit, a Tesla being charged enough to drive 1,000 miles would cost you an extra $17.40/month for going over the limit. That's pretty much the limit for what exceeding your limit for your car will cost, assuming average US driving habits and 100% home charging -- and also assuming that you still pay $0.22/kWh for your usage below the limit. (The total cost for charging 290kWh at $0.28/kWh is $81.20.)

I'm trying to ultimately figure out how much I can charge at home without going into the higher tier. I can charge at work for free, but typically only get about 9 kw of charge, and only drive to the office 3 days a week.

This sounds like Level 1 charging (off a 120v outlet). If your office has a Level 2 EVSE, then you ought to be charging at about 6kW, for about 48kWh of charge during an 8-hour work day. You may want to check PlugShare to see if there's public Level 2 charging available near your work, or near other places you frequent, for that matter. Level 2 EVSEs are often situated near parks, malls, movie theaters, and so on. Some of these are free, but some of them cost money.

This gets me every time. I don't understand the distinction.
1.44 kWh per hour expands to "1.44 kilowatt hour per hour"?

Others have posted explanations, but here's another way to look at it: kW is a flow rate, like gallons/minute; kWh is a total quantity, like gallons. The time unit ("h" for "hour") gets used as a multiplier in electricity, but it's a divisor in most units with which we're familiar. If you're not used to electrical units, this seems backwards and counter-intuitive, but mathematically, both approaches are equally valid.

 

[AlanSubie4Life]

Model 3's EPA fuel economy figure is 116 MPGe, or 29 kWh/100 miles (290 Wh/mile). If you were to drive 1,000 miles in a month (about average for the US), you'd use 290 kWh -- just over the limit to get your lowest electricity rate.

As mentioned above, the typical number for all users in mild climates is closer to 33kWh/100mi, and for the OP it’s probably closer to about 38-40kWh/100mi, due to 120V charging.

 

[darth_vad3r]

1.44kW * 1h = 1.44kWh
So the phrase 1.44kWh per hour translates directly to 1.44kWh / 1h which is just a ridiculous way of saying 1.44kW

It’s all pretty ridiculous too when you consider that a watt (W) is joules per second (J/s), and joules is energy already ... so the wonkiness plays out by using watt-hours (joule-hours per second) for energy instead of joules directly.

1 kWh = 3.6 MJ ... doesn’t that sound better? MEGA joules, or kilowatt-hours? No contest. Who doesn’t like MJ.

3.6 MJ/hr also makes sense as a charge rate when people like to think in “per hour” terms and avoids the double hour per hour weirdness. I think it would actually be less confusing for people.

Joules for energy. “Your charge session added 20 MJ.”
Watts for power. “The motor uses 50 kW to go X mph.”
Joules/hour for charge rate (power) in human-relatable per-hour terms instead of watts. “Charging at home on 120 V adds ~3.6 MJ/hr to the battery”