Peak Utility Awareness

Peak Utility Awareness

Hi Tesla fans,

I've convinced my wife that this is the year to trade in her gas guzzling ginormous SUV for a Model S. As part of my research I have a question I was hoping someone might be able to answer.

My current utility bill is based on two components; usage and peak demand. During the summer time (I'm in Phoenix) the peak cost jumps from ~$.05/kwh to almost $.09/kwh. More importantly the demand 'penalty' component to my utility plan takes my single highest on peak hour over the month and charges me $13.50 per kwh for that single hour.

So my question is this: Is the car capable of not charging itself, even if plugged in, based on time of day?

I realize I could just leave it unplugged during peak hours but beyond the inconvenience, no doubt mishaps will occur where we forget to charge it overnight, or we forget to unplug it and I watch the money fly out of my wallet to the electric company. Given the structure of the utility plan it would only take a single mistake during a month to send my bill sky high.


rlarno | 23 aprile 2013

a bit of perspective: my peak costs 0.21 euro/kWh

And yes, you can schedule the charging times.

olanmills | 23 aprile 2013

Right now, the car supports scheduled charging. That is, you can set a start time, and even if plugged in, the car will not begin charging until that time. However, once it starts charging, it will continue until fully charged.

What you cannot do instruct the car to avoid charging at certain times, though that would be an interesting feature.

You can use the mobile app to remotely instruct your car to stop and start charging as you wish. It actually wouldn't be all that difficult to write a custom mobile app that you could use to achieve a the behavior you want.

Also, either in the car or with the mobile app, you can reduce the amperage of the charge and since Watts = Volts x Amps, the amount of power used in a single hour will be reduced.

Your electricity rates seem relatively low, except that 13.50/kwh charge for the one hour will totally undo the savings from gas. The normal charger uses 10kW, and if you drive normally every day, you will need at least one hour of charging a day, which virtually guarantees that you will be hit with that $135 charge every month (if you don't reduce the charging amperage as described above). However, even if you do reduce amperage, your bill will still be high. If you drive around 50 miles a day, that usually requires 2-3 hours of charging. Suppose you reduce the amperage so that it takes 10 hours to charge, then still your peak energy usage will be 2-3 kW, and so that "one peak hour" will still cost you $30-50.

That's crazy. I understand that the power company may not change their billing model overnight just for you, but have you considered writing to them? Their billing model really doesn't make sense for EV owners, or I should say, it completely discourages EV ownership. I get that their "one peak hour" model is designed to make you not necessarily reduce your overall electrical consumption, but rather, reduce the rate of your electrical consumption, but with charging an EV, that's not really possible.

olanmills | 23 aprile 2013

Also, I meant to add, I don't see how charging or not charging on a certain schedule will help your situation.

You said the expensive hour is based on your household's peak energy usage, not the peak energy usage of all customers collectively.

So it would seem to me that it doesn't really matter when you charge. It's true, you will save yourself a little money by avoiding charging your car while you are up in your home using stuff, but in general, the amount of kW you're using inside your home at any instant is going to be much smaller than the amount of kW used by charging a car. Although, you are in Phoenix, and I guess air conditioners do use a lot of power. Still, I have heard that the average home uses 1 kW, and let's say it's 2 kW for your air conditioning. Wow I guess that is significant. Still, the car (with the normal charger) uses 10 kW. If you reduce the amperage to spread it out over a night, it will still use 2-3 kW.

olanmills | 23 aprile 2013

Sorry I keep rambling on, but I just wanted to add that, if I understand your situation correctly, reducing your charging rate is much more important than charging at a specific time.

ghillair | 23 aprile 2013


I think you missed, he said the 13.50 is on highest on peak hour. As long as charging is restricted to off peak he should be fine.

NetDan | 23 aprile 2013

Yes, sorry. I live in Phoenix so peak electricity time is always 3-7PM when we all have cranked our A/Cs up to avoid cooking. So the power company only applies that $13.50 'deamnd charge', as they call it, to the highest hour usage of the month between 3-7PM M-F.

So they are trying to discourage usage during that time of day. And since both my wife and I work from home, it's hard enough doing it on our own, let alone having the car sucking off the 50amp circuit in the garage.

But the fact that we can set the start time is a big plus. This does beg the question, does the car have an API? I'm happy to write an app the control the charging. I have a small server that does a number of time bases activity for me already and this would be trivial to implement.

NetDan | 23 aprile 2013

Actually, I think it's noon-7PM. But the same still applies. Avoid usage because a higher rate applies during those 7 hours and avoid a massive draw because it will increase the peak demand cost.

frmercado | 23 aprile 2013

You should go Solar. In your case it makes sense even without owning an EV; add to that the fact that you live in Phoenix (Plenty of SUN) and that you want to get an EV. Its a no brainer...

olanmills | 23 aprile 2013

Yes, the Model S has a fairly simple REST API accessible over the internet and authenticated with the same account you use on It is not "public" per se, but it has been reverse engineered by observing what the official iOS and Android Tesla apps do. Unofficial Tesla apps for Windows Phone have been created using this API.

More info:

Brian33 | 26 aprile 2013

This is all great stuff. (I geek on this stuff...not normal but that's me) I have worked with my local utility on structuring a fair net metering rate for solar and have learned the basic cost structure. Utilities in New England pay four basic charges Overhead ($.04), energy ($.04 to $.07 this days), transition ($.015 or $7 per kwh during that hour) and capacity ($.02 or $46 per kwh during that hour) (all $ based on all kwhs sold). Overhead and energy are charged to the utility on a per kwh basis. Transition and capacity are based on the peak demand hour. (Transition is monthly peak and capacity is one yearly peak) These fees help fund upgrading the transmission lines and building additional power generating capacity. Both fees are expected to grow to the point were they are the bulk of the per kwh cost.

I would prefer a demand charge or peak hour usage fee on my bill because I can control when I consume kwhs. I'm glad to see a utility doing it at the residential level.

If anyone is interested, Powerhouse Dynamics make a product call the eMonitor that lets you know what device consumed the kwhs the utility billed you for and when. (35% to the EV, 25% to HVAC, 9% TV, 7% fridge...)

For those super geek, check out the ISO-NE site to see grid demand.;jsessionid=80E58785DB34...

NetDan if you had solar on your roof and sent kwh to the street during a peak hour would the utility credit you the demand rate? You would be helping them avoid a cost.