Aptera › Community › Aptera Discussions › kwh/mile and wh/mile

I can’t get the concept to settle in my head. I expressed fuel efficiency all my adult life as miles per gallon (distance from a certain amount of fuel). Now the concept includes a factor of time? Now it’s amount of energy AND time AND distance? It doesn’t make sense to me. I only used time when discussing speed. Why isn’t miles per kilowatt or watt used? What does an hour have to do with it?

I agree but maybe it should be the number of kilowatts used per 100 miles. This is similar to how “mileage” is determined in metric countries: liters per 100 kilometers.

Yeah, I can wrap my head around the reverse, kilowatts per mile, but not with time in it.

The trouble is that a watt is a unit of power (energy per unit time, or Joules per second). For some reason, we are multiplying it by time (kwh) and using it as a unit of energy (instead of just using Joules). Perhaps ‘kwh’ is convenient for some reason one of our fellow Apterans can muster.

Electricity is sold by the KWh, look at your electric bill. Gas is sold by the gallon, a gallon of gasoline is equivalent to 33.7 KWh.
If you are interested in your cost per mile then want to see the energy efficiency expressed as KWh per mile, so for example if you are paying 25 cents per KWh then the Aptera, which uses 100Wh/mile, will cost you 2.5 cents per mile. If you are interested in range then you want to see the reciprocal, i.e. miles per KWh. The Aptera is 10 miles per KWh so you can travel 400 miles with a 40KWh battery.
 This reply was modified 1 week, 2 days ago by Joshua Rosen.

I pay 7.9 cents for the first 450KWh, then 12.9 cents for anything over, but that’s not going to be relevant since I will use the suncharge most days. What I don’t get is why does anyone put “hour” in the label? It’s not a function of time, is it?

A kilowatt is an instantaneous measurement (technically, a watt is one joule per second and a kilowatt second (vs hour) would be one joule per second for one second (kind of like acceleration). The “kilowatthour” would equate to the total energy the system now contains rather than the rate of energy being put in. Kilowatt=rate; kilowatthour=total elapsed.
Another way of thinking about it, if you’re filling a bathtub, your water flow rate is in liters per second (kilowatts), but you want to figure out how much water is in there, you need to know the total number of seconds (elapsed time) and the flow rate (kilowatts) to calculate the liquid volume.
 This reply was modified 1 week, 2 days ago by Jared Cormier. Reason: stupid typo

Thank you. I still haven’t got it. I have to think about your explanation some more.

OK, but I wouldn’t call what I measured literhours, I would say I dispensed so many deciliters or Kiloliters, not deci or kilohours.

From basic Physics: Energy = Power times Time. Electrical power is measured in watts.
Electrical Energy can thus measured by multiplying power (in watts or kilowatts) times time (in seconds or hours). Why that unit? History, I guess.
Here’s an example that might help. Say you switch on a 100 watt (old fashioned incandescent) lightbulb. 100 watts is a measure that determines how much electrical power your light will draw and how bright the light it will be. Leaving your light on for one hour requires 100 watthours (= 0.1 KWH) of energy, (enough to drive the Aptera 1 mile). If you had a 200 watt light bulb, it would demand twice the power. However if you left it on for only half an hour, it would use the same 100 watthours.
If you drive an Aptera at 50 miles per hour, it will go 50 miles in an hour, using about 5 KWH in that hour.

This concept doesn’t want to fit in my head.
If I turned on a 200watt bulb for 30 minutes, I would say I used 100 watts, not watthours. It wasn’t on an hour.
The watthour thing feels like a misnomer, an unnecessary complication.
You guys don’t have to take your time to make me understand. It’s one of those things that I could never encompass and actually incorporate into my head. I don’t “see” it. I drove my math teacher crazy too.

One more try from this old Physics major and I’ll leave you alone.
A 100 watts lightbulb uses 100 watts every instant it is on, whether for a minute, an hour, a day, or all year. Watts doesn’t measure energy used. Energy use isn’t independent of time. It takes time to use energy. Watthours is one measure of that.

Every second? So, the kilowatthour is a simplistic way of saying watts are flowing over a certain amount of time. It feels like a misnomer because an hour didn’t actually pass.
If I were to accurately describe the 200 watt light bulb burning for 30 minutes, for CONVENIENCE I say 100wh, because “half a 200watthour” or “200watthalfhours” sounds clunkier or makes the equation not as cleanlooking. I do like equations reduced for understanding.
Is that it?

Watts are like the speed that the energy is coming. It’s energy per time. Energy is not the same as power. Energy is like a distance and power is like speed. Let’s create a unit of speed called the Maniac and define the Maniac to equal 100 Miles per hour. If you drive at a speed of 1 Maniac for an hour, you could say you travelled 1 maniachour total = 100 miles.

@Lane Costilow Not quite. Let’s try this analogy: Think of the 100 Watt bulb as a hose. Every instant the tap is open, that hose is pumping out 100 amounts of water. If the hose is open for 1 hour, it’s pumping out 100 amounts of water continuously for one hour. If it’s a 200 Watt bulb, the hose is pumping out 200 amounts of water continuously. If the tap is open for 1 minute, it’s pumping out 200 amounts of water for one minute. If it’s open for 30 minutes it’s pumping out 200 amounts of water for 30 minutes.
The use of “hours” is just a convenience – because hours are a standard measure of elapsed time everywhere on the planet.
So, if you have a battery pack with a capacity of 100 kWh (kilowatt hours), it has the theoretical ability to pump out 100 kW of power continuously for one hour. This is why fullthrottle “launches” of EVs drain batteries so quickly: Launching makes them pump out a lot of power very quickly. Normal driving, however, doesn’t draw that much power continuously.

That reminds me of the Steven Wright joke:
the police stopped me for speeding, and they said, “Don’t you know the speed limit is 55 miles an hour?” I said, “Yeah, I know, but I wasn’t gonna be out that long.”.
So if he was going 100mph, but was only driving for half an hour, he was really only going 50mph, right?

But we don’t say x number of miles per .5 gallons. EPA standards is MPG. For EV’s it’s MPGe.


Harry, the problem is with the notations of KWh & Wh. Using your example of a 100 watt light bulb on for 1 hour, the light bulb consumes 100 watts PER hour or 100w/h. Whoever came up with the Wh & KWh notations either was too lazy to use the “per” symbol (/) or, most likely, didn’t think it through for nonphysicists. You know how the various professions have their own internal shorthand jargon! Of course the professions will fight anything that helps “the great unwashed” understand said jargon.
Unfortunately, the electric companies also use KWh for your TOTAL MONTHLY consumption, but in this case it shows your consumption as the equivalent of using the monthly total in 1 hour, not that you are using that amount in each hour of the month. Using my recent bill as and example, I used 706 “KWh” over 30 days or 720 hours (well actually 718.0335 hours). Dividing the 706 KW by the number of hours gives me an average consumption of a tad over 0.98 KW per hour.
But getting back to vehicle usage, we measure speed in distance per hour (mph & kph) units and liquid fuel consumption in either distance per volume or volume per distance (mpg & l/100km) units, so for electricity we can use something similar. Since we are already using KW per hour for home electricity, perhaps that might be the easiest. Alternately, you can do what you do now with liquid fuel and use distance per amount consumed: write down the odometer reading when you first charge the battery, then drive until you need to recharge the battery and again write down the odometer reading. Subtract the first odometer reading from the first to get the distance traveled. Take the number of kilowatts need to recharge your battery and divide that into the distance traveled to get M/KW or L/KW.

So your energy usage should more properly say 706 KW leaving the “hour” out of it?

Correct. I used a TOTAL of 706 kilowatts OVER 718.0335 hours. I’ve always mentally dropped the “h” at the end of my consumption amount shown on the bill.
BTW, when I look at my HOURLY usage online, the electric company shows it as just “KW”. For July 29th my lowest hourly consumption was 0.18 KW and my highest was 1.82 KW (air conditioning was on)
 This reply was modified 1 week, 1 day ago by Dennis Swaney. Reason: Added usage info

I am frequently very literalminded. People don’t always say exactly what they mean. THEY know what they mean and expect me to know also. Tech jargon is not always accurate, but it should be.

Tell me about it. Think of some of our military jargon.





For me to understand this may be a matter of osmosis. I feel like I have a slippery grip on the front of it, but maybe not all of it. I may lose what little grasp of it I have, but I have this conversation to remind me where I got to.
An Aptera will be my first electric vehicle. I was so excited about it when they started the first time because AT LAST someone was putting out a viable solarpowered vehicle. I couldn’t take advantage of it then, even if they had stayed in business (divorce, digging out of debt). I have a second chance.
Thank you all very much for helping me.

For me whenever I canâ€™t grasp how a certain technology works I imagine that itâ€™s running on magic. The best way I associate electric cars to gas cars is cost to refill/recharge and how far that takes me.
My gas car goes 350 miles on a $90 refill so .25 per mile
My Tesla goes 230 miles and costs $32 to recharge so .13 per mile
Aptera will go infinity miles and cost $0 to recharge. Good deal

Lane, you misremember slightly, the Aptera from the first years (Before 2012) was not a solar vehicle, although it did have a minimal amount of solar panels on the roof to run a parked ventilation system.

Others in SoCal were experimenting with solar cells on cars, so I probably mashed them up in my mind.



This explanation from Google may help (I’m learning this crap myself (MPGe))…
Kilowatt hours (kWh) are the units in which electrical power usage is primarily measured â€” they are what you see on your electricity bill. The reason that this unit is used to measure your usage rather than kilowatts on their own is because kWh is a measurement of absolute power consumed over time. It’ll take absolutely 1kwh to run a 100watt bulb for an hour.
EDIT: my bad… bulb for 10 hours
 This reply was modified 1 week, 2 days ago by Selvan Poothamby.

I’d gotten less uncomfortable. And my mind didn’t object to what you stated — until the last phrase: 1kwh to run a 100watt bulb for an hour.
60 seconds x 60 minutes x 100 watts = 360,000 watts (360kw)
Now I’m confused again.

If you prefer there is another unit of energy, the joule. A joule = watt second so a KWh is 360KJ. Electric companies don’t bill in joules they bill in KWh so the KWh is the most convenient unit to use because you can translate it into cost and dollars and cents are the units that people are most interested in.

My conclusion: I’m going to try to just leave the “h” (hour) out of my thoughts of watts. I know electrons are moving and I don’t need to use badly named terminology to tell me so. It goes against the grain to not read the “h” but it’s not my fault all tech terms aren’t perfect. It’ll be just watts and kilowatts.

All you have to do is recognize that the ‘h’ is there merely to denote there’s a period of time involved. Like a month in a utility bill.

Except is is only accurate to denote an HOURLY rate of consumption. KWH should properly be written KW/H and should NOT be used for a total over more than 1 hour. I did NOT use 706 KW/H each of the 718.0335 hours of my last bill; I used 706 KW TOTAL for the 30 day billing cycle.

Dennis, That’s almost correct. The hard part is keeping ENERGY and POWER straight in our heads. People often make a mistake and write KW when they mean KWH and visaversa.
POWER (watts) = ENERGY / TIME, the rate of using energy.
Power is not measured in watts per hour, it’s watts in each instant of time. Watts and Kilowatts measure INSTANT POWER = the RATE of using ENERGY.
POWER (watts) = ENERGY (watthours) / TIME (hours), the rate of using energy.
Residential users pay for the ENERGY they use each month (plus fees for connection to the grid and the delivery of that energy). POWER times TIME = ENERGY. (Kilowatts x Hours = KilowattHours). See how symmetrical the units and relationships are?
Lightbulbs, heaters and motors are rated for the POWER they use, in watts and kilowatts. For instance the Aptera motor is rated at 50 KW. (That’s the peak power it can sustain – not the average power it uses.)



Lane, the terminology is correct, not bad. It’s math.
Think of kW as horsepower. You don’t fill up an ICE car with horsepower, but with gasoline which gives you the possibility to crank out of the engine a certain amount of horsepower over a certain time.


Outside of a few keyboard warriors on various forums, nobody is going to care in real life if you leave the ‘hour’ part off.
My car has a 100KW battery pack
I had to charge my car with 50KW today
I’ll use 25KW driving those 250 milesThey all are technically incorrect, but explain what you mean exactly.

Since we’re used to MPG, the EPA should’ve used MPK (and let it be standard that the k symbol here is 33.7kWh. You can drop the Wh.