Calculating most efficient electric use.

[Jerry1]

Our energy provider just switched to peak/off peak pricing. Between 3pm and 7pm electricity is .06 per kilowatt hour higher than the rest of the day M-F. I’m trying to figure out if it’s worth it to react to this. It’s easy to do the dishwasher and laundry in off peak hours so the only significant electrical draw is the air conditioner. If I turn off the AC from 3 to 7, the house will get hot and the AC will have to work harder for awhile to get the house cooled back down. If I let it go up only a few degrees, it feels like the savings won’t be worth it.

How would you go about figuring out how best to handle the AC during peak time? If I had some clue as to what the AC uses in terms of kilowatt hours, it would be helpful, but I don’t have a clue. The thermostat captures usage but only on a 24 hour basis. I don’t need an exact measure, but I’d like some idea if it’s worth being a bit uncomfortable for a few hours to save some money. It doesn’t make sense to be uncomfortable for an extra dollar per day (though even that adds up to $20/mo) but if it’s $5 or more per day, I’d be inclined to be a little uncomfortable.

 

[Koolau]

Our energy provider just switched to peak/off peak pricing. Between 3pm and 7pm electricity is .06 per kilowatt hour higher than the rest of the day M-F. I’m trying to figure out if it’s worth it to react to this. It’s easy to do the dishwasher and laundry in off peak hours so the only significant electrical draw is the air conditioner. If I turn off the AC from 3 to 7, the house will get hot and the AC will have to work harder for awhile to get the house cooled back down. If I let it go up only a few degrees, it feels like the savings won’t be worth it.

How would you go about figuring out how best to handle the AC during peak time? If I had some clue as to what the AC uses in terms of kilowatt hours, it would be helpful, but I don’t have a clue. The thermostat captures usage but only on a 24 hour basis. I don’t need an exact measure, but I’d like some idea if it’s worth being a bit uncomfortable for a few hours to save some money. It doesn’t make sense to be uncomfortable for an extra dollar per day (though even that adds up to $20/mo) but if it’s $5 or more per day, I’d be inclined to be a little uncomfortable.

I'm guessing NW-Bound will be along soon and tell you how to calculate your AC electrical usage. I can't do it.

What about cooling the house a degree or two extra before the 3:00PM witching hour then let it get a degree or two above the normal nightly setting before turning on the AC again.

 

[Telly]

.... If I had some clue as to what the AC uses in terms of kilowatt hours, it would be helpful, but I don’t have a clue. The thermostat captures usage but only on a 24 hour basis. I don’t need an exact measure, but I’d like some idea if it’s worth being a bit uncomfortable for a few hours to save some money....

You could use your electric meter. Pick a time, turn off other major loads. Run AC continuously by dropping thermostat set point well below present room temp. Run it for 30 minutes straight. Take the delta in watt-hour meter before and after readings, multiply by 2 to determine per-hour kwhr usage of AC compressor and blower motor.

For the test, no need to disconnect wall-worts or stuff like that, they're very low power compared to AC. But turn off pool pump, refrigerator (-25 points if you forget to turn it back ON!), etc. This will give you the Kwhr of an hour of continuous run.

If your thermostat records hours of run time per day, I'm sure you can get a pretty good guesstimate of what fraction of that time was in whatever period of day you are interested in. Like, little late at night through day break, around here heavy run time starts about mid-afternoon till shortly after sun down. Our max run is from ~4 PM to ~8 PM.

 

[Lewis Clark]

Our electric company has a program that allows customers to choose what they call an "energy saving program" which varies the cost of electricity by time of day, and by day of the week. It sounded good so we signed up. As part of the program they installed a programmable thermostat we could control from their website. And we could also control the run hours of our water heater.

We were good at shifting some loads to off-peak times: washing and drying clothes, dishwasher. But the big loads of air conditioning and electric heat are difficult to reduce if someone is home during the daytime without changing the thermostat setting. It's easier to adjust the thermostat to reduce daytime (peak cost) heating and cooling and turn off the water heater if you are out of the house during those hours. We asked the electric company to do a cost analysis on our usage over several years and were told the "energy savings progam" was actually costing us slightly more money than keeping our rate constant all the time.

If you have a programmable thermostat, and can reduce your cooling and heating load during peak pricing (perhaps by adjusting your schedule to be out of the house while you adjust the thermostat setting) you might benefit the most. Good luck!

 

[DrRoy]

Our energy provider just switched to peak/off peak pricing. Between 3pm and 7pm electricity is .06 per kilowatt hour higher than the rest of the day M-F. I’m trying to figure out if it’s worth it to react to this. some money.

Us too! Are you in SE Michigan? I am going to try programming the thermostat for a couple degrees in the peak time. Then we'll see.

 

[ERD50]

Our energy provider just switched to peak/off peak pricing. Between 3pm and 7pm electricity is .06 per kilowatt hour higher than the rest of the day M-F. I’m trying to figure out if it’s worth it to react to this. It’s easy to do the dishwasher and laundry in off peak hours so the only significant electrical draw is the air conditioner. If I turn off the AC from 3 to 7, the house will get hot and the AC will have to work harder for awhile to get the house cooled back down. If I let it go up only a few degrees, it feels like the savings won’t be worth it. ....

Lots of variables, but while you are getting a better number on the ON draw of your A/C (and air handler fan), here's a rough estimate, and a process for when you get better numbers:

A quick search says that a 'higher end typical' number for A/C is ~ 4 kW, so:

$.06 *4kW = $0.26/hour; times 4 hours = $0.96/day delta, assuming the A/C was running 100% of the time.

So you need your number for draw, and some estimate of the % run time (duty cycle) of your A/C. But I can't imaging it being anywhere near $5/day.

RE:

.... If I turn off the AC from 3 to 7, the house will get hot and the AC will have to work harder for awhile to get the house cooled back down. If I let it go up only a few degrees, it feels like the savings won’t be worth it. ....

Remember, the "work harder" to cool back down isn't an issue, that's offset by the time the house stayed cool after first shut down. The savings come from the fact that the average temperature over that time was lower.

As mentioned, if it's easy to set a schedule, dropping the temp a degree or two an hour ahead, and then dropping back a degree or two lower than normal, and then back to 'normal', should help a small amount.

-ERD50

 

[ERD50]

... Remember, the "work harder" to cool back down isn't an issue, that's offset by the time the house stayed cool after first shut down. The savings come from the fact that the average temperature over that time was lower. ...

Quoting myself to better demonstrate this point, w/o turning the earlier post into TL/DR.

Easier to use heating numbers. Say you set the heat to 70F for 12 hours during the day, and 60F for 12 hours at night. While it's true that the system will "work harder" in the AM to raise the temperature, that was offset by it being off while the house retained heat at night.

The real savings is from the fact that your average temperature is now 65F instead of 70F. Your home isn't losing as much energy on average. The greater the average temperature delta, the higher the amount of energy lost. It simply takes more energy to keep a house heated to 70F than it does to 65F.

-ERD50

 

[isisdave]

Use the meter on two days, as close as possible to each other regarding temperature profile. Record reading at 3pm and 11pm. On one day, run AC as usual. On the other, try Plan B -- either waiting till after 7, or pre-cooling, or both.

Entrance of heat into the house is directly proportional to the difference in temperature between inside and out. I am guessing that between 3 and 11, with identical outside temperature, you use just about the same amount of energy no matter how you do it. It'll just seem warmer because it heats up and then takes a while to cool down. If your house faces west and has a lot of windows, however, your results may vary a little.

By the way, your AC has only one speed: ON. Turning the thermostat lower will not make it work harder or cool faster. Many people don't realize, or at least don't want to believe, this.

 

[Jerry1]

Us too! Are you in SE Michigan? I am going to try programming the thermostat for a couple degrees in the peak time. Then we'll see.

Yes. I live in western Wayne County.

 

[Koolau]

By the way, your AC has only one speed: ON. Turning the thermostat lower will not make it work harder or cool faster. Many people don't realize, or at least don't want to believe, this.

Yes, like my wife! She gets either hot or cold (at the old homestead) and ups the the temp (or lowers it) 5 degrees - so it will go faster. Fortunately, in Paradise, she can only open or close the windows.

 

[Jerry1]

Thanks for the information. This is interesting I don’t think the savings is there. Said another way, the cost of keeping the AC running during peak hours isn’t that much. Maybe the company is playing somewhat of a mind game. By saying it cost more between certain hours, people will shut off their appliances or use them less without really thinking about it. That’s kinda of where I was heading.

I’ll do some more information gathering, but I think I’m going to just shift my usage of the easy things like laundry and dishwashing to off peak and not worry about something like AC, which is a comfort thing. It might also be helpful to see my first bill when they break out the peak and off peak usage.

 

[Jerry1]

By the way, your AC has only one speed: ON. Turning the thermostat lower will not make it work harder or cool faster. Many people don't realize, or at least don't want to believe, this.

I get that. Work harder is a poor choice of words. Run longer would probably be more appropriate.

 

[Koolau]

I get that. Work harder is a poor choice of words. Run longer would probably be more appropriate.

Heh, heh, that's what I try to tell DW but she just doesn't buy it.

 

[ERD50]

I'm also going to put in a cautionary tale when using things like clothes dryers and dishwashers at off-peak times. I suggest that you don't let these run after you go to bed, or go out of the home. They occasionally start fires, and if you are asleep or out, you may become a victim of smoke inhalation, or just have a very destructive fire.

I saw pictures from a relative who had a friend with a dishwasher that over-heated when they were out. No fire spread, but there was such a thick layer of soot from the smoldering fire/heat in the entire townhouse, it required a full rehab. Could have killed them if they were there. Smoke alarms are sometimes insensitive to certain types of smoke, no guarantees.

So off-peak is fine, but don't extend it to bedtime hours... my 2 cents.

-ERD50

 

[Koolau]

I'm also going to put in a cautionary tale when using things like clothes dryers and dishwashers at off-peak times. I suggest that you don't let these run after you go to bed, or go out of the home. They occasionally start fires, and if you are asleep or out, you may become a victim of smoke inhalation, or just have a very destructive fire.

I saw pictures from a relative who had a friend with a dishwasher that over-heated when they were out. No fire spread, but there was such a thick layer of soot from the smoldering fire/heat in the entire townhouse, it required a full rehab. Could have killed them if they were there. Smoke alarms are sometimes insensitive to certain types of smoke, no guarantees.

So off-peak is fine, but don't extend it to bedtime hours... my 2 cents.

-ERD50

Agreed. We don't use any appliances when we sleep or leave the apartment. At the Old Homestead, we DO use the HVAC system while gone or sleeping. The actual heating and cooling unit sits outside. Inside there is only a fan.

Sorta funny story: Years ago, I stopped at my parents house. Dad was in his favorite chair, asleep. In front of him, the TV was joyously in flame. I grabbed a heavy towel and pulled the plug out of the wall. Dad and I grabbed the TV (it was in a console) and set it outside. There was just a bit of soot damage on the wall, but the burnt plastic smell lasted a month or more.

 

[Mark1]

Well if you assume that you use 2000 kilowatt-hours per month and half of them are in the 3-7:00 window, then it would be $60/month more. My guess is that you actual usage might be ~50% of that so maybe $30/month. I would run the dishwasher outside the window, etc., but wouldn't mess with the AC enough to be uncomfortable.

 

[misanman]

Interested in this discussion since I'm affected as well. Haven't figured out what action, if any, we should pursue.

 

[REWahoo]

Jerry1, we changed over to time-of-use pricing three years ago and saw our summer bill increase roughly $5/mo even though we made no change to our thermostat settings. Overall our monthly electric bill the remainder of the year went down slightly as the economy rates at night more than compensated for the peak rate during the afternoon.

YMMV of course.

 

[Koolau]

I'm sure most electric suppliers think they have it figured out when they go to programs like this. I guess we'll see how effective they are. Please report back on your utility's success or failure with folk's electric usage.

My electric company (the most expensive electricity in the nation) came up with the ability to remotely shut down electric hot water heaters for brief periods. They paid us $3/month for that opportunity. If electric utilities did that with other big users of electricity (primarily AC or heat - but maybe refrigerators, pool heater, etc.) as well, I would think they could achieve their goals of NOT adding more capacity but rather smoothing usage by strategic shut down of individual high-demand equipment for short periods. It's probably more labor intensive and computer driven as well. But, how much more would it cost to set up this system than to build in monitors within electric meters? Just a thought.