# Complicated control of multiple AC units?

We have a large house with three separate AC units (downstairs, upstairs, and FROG apartment). We also have solar panels which means we are on a TOU-D electric rate plan. Part of this plan is the peak charge, which is based on our maximum draw during peak period hours.

So anything we can do to reduce our peak draw is a big win, and synchronizing the AC units so only one of them is on at a time is the obvious low-hanging fruit.

The basic algorithm would be something like:

During peak periods, every 5 minutes, poll the thermostats, pick the zone that is most out of temperature range, and have that zone on while all the other zones are off.

We probably still want the thermostats to run an internal schedule to set the current target temp rather than centralizing that (at least at first). All the automation needs to do is figure out what AC unit has priority.

We're pretty sure that a Hubitat can do something like this, but it's entirely unclear what thermostats are up to the task of integrating with it. AFAICT we don't need anything fancy like a Nest, but maybe we're wrong.

I'd like to start off with a question. Why do you assume that? If you have one 2 AC units in the house and they have to run of X minutes to cool the space, wouldn't only one AC unit require at a minimum 2X minutes to cool the house, therefore drawing the same amount of power? But we all know that one unit is going to be less efficient than 2 so it's actually going to be something like 3X as long. Therefore drawing MORE power if you only run one.

The real difference would be in raising your thermostat temp during peak hours. That's the only thing that's going to get you savings. Running only one AC will just make the other one work harder.

And I don't understand why you would think that cooling the main house would have an effect on the FROG apartment. It's going to take twice as long to cool that space once the AC comes back on if it's been allowed to heat up far beyond the point the AC should turn on. Again, raising the thermostat is the way to save power.

I apologise if I wasn't clear enough. We are on a TOU-D electric plan. This means we pay a lower rate per KWH, but we also pay a peak demand charge. This is about \$5 per KW. So if our maximum momentary draw during the peak rate hours in any month is 10KW, the peak charge for that month is \$50.

Thus, this isn't about saving total power usage; the total number of KWH will be about the same. It is about shaving the peak charge by not running multiple heavy draw devices during peak (for example, we don't use the washer and dryer during peak rate hours). If we can shave 3KW off the peak, down to 7KW, that's \$15 a month in savings.

Our peak load point during any month is almost certainly a few short periods where all three ACs just happen to be on at the same time. If we can cut that down to two or even one AC at a time, we'll reduce the peak. That is the goal.

It's probable we won't be able to maintain the desired temperature on the hottest days with only one AC unit running at a time, but there may be optimizations, like cooling the house in the morning of hot days a few degrees lower than normal and using it as a thermal battery to ride through the peak cooling period. However, I am pretty sure one AC at a time will work at least 9 months out of the year. Saving \$15 a month 9 times a year is still a nice win.

On a side note, if you have heavy draw devices and you can control when they consume power (ie: charging an EV), a TOU or TOU-D rate plan can result in significant savings.

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What you don't understand is that during that peak usage time, you can have one AC unit pulling power for 10 minutes or 2 units pulling power for 5 minutes. Aren't those the same number of KWH? It's not like 1 AC will be able to cool the same space within the same time as 2 do. You have to raise your thermostat during peak times to get any real savings. Don't worry so much about WHAT is doing the cooling, get both of the units to be shut off more by raising the temperature setting on the thermostat. That will get you real savings. You can always cool to a temp a few below your normal setting right before peak time. Just make sure that you set back to the higher setting before peak begins or you'll be shelling out a lot of cash.

I think he wants to prevent all 3 from simultaneously coming on. If you have 3 @ 15KW (plus other household stuff) that would push you into a gross consumer category with a penalty.

If you could do a round-robin, then you would spread the consumption over the course of an hour.

I agree though, raising setpoints would help here. My Honeywell natively does this if you program peak/off peak times.

I fail to see how you can accomplish that without raising the thermostat. If one unit runs for 3 times as long to cool the space, won't that consume MORE power, not less?

That won't happen because each unit will have to run for the same amount of time it does if you're not trying to round-robin them. It's all about total usage during the peak and if you don't change the setpoint you are going to use the same if not MORE power screwing with which unit is running.

It's about KW not KWH. If he goes over a certain KW at any second in that time windows he gets charged extra. The KWH may even be higher over the whole day as long as he stays under that KW peak power consumption at once.

I think they meter by peak, not by average. I.E. they look at total consumption at one point in time vs how much you use per hour or day.

I think the power company wants people to do more like steady state "highway driving" instead of "stoplight to stoplight". The constant spikes are what screws up power generation and distribution.

There would be a different but beneficial side effect to this. Longer run times are better for dehumidification. He might be able to even further raise setpoints if there is low humidity.

The temp swings would be awful for general comfort, but that doesn't seem to be the goal here.

How would longer run times result in Lower humidity for the space as a whole? I don't think you understand how AC works. There's a reason your house has 2. If you take one away, the remaining one is going to have to work twice as hard to cool the house.

If the remaining one is capable of working twice as hard. But since he has two, it probably isn't which will result in his goal to not go over the max peak power use at once.

How will that happen if the one unit runs twice as long?

He want to stay under a peak of KW. Not KWH.

No.....he doesn't. Where do you get that?!?

Ryan, total KWH use is not relevant in my situation. Even if doing this raises my overall consumption slightly, this is completely overwhelmed by the potential savings in peak charges.

This is not about the total amount of energy used. It is about the maximum amount of energy drawn at any particular moment in time. That happens when all of the ACs are on at the same time.

Perhaps it would help to focus on the how instead of the why. The key questions for my potential use are:

• What is a good thermostat that can report its state (current temp, target temp, current mode) to the Hubitat, and in return be told to change mode? We don't need anything fancy here.
• Is it possible to write a script that implements some sort of round-robin strategy?

Max vs steady state. If he hits the magic number, lets say 50KW, in a defined time period (lets say one minute) he will have to pay extra. They don't care if he consumes 49KW all day long, in fact this plan encourages it if I understand correctly.

It is more of a "speed limit" thing, 55 MPH is fine, but you get a ticket if you go 56 MPH.

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Uhm....amount of power over a period of time is the very definition of KWH. KW is not over a period of time. It is a momentary measurement.

Let's get back to the questions shall we?

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Back to the basic problem:

I am awful with Rule Machine, but here is what I envision.

• Trigger would be time of day
• IF A/C 1 is running, THEN turn A/C 2 and 3 off.
• If A/C 2 is running, THEN turn A/C 1 and 3 off.
• IF A/C 3 is running, THEN turn A/C 1 and 2 off.
I think you would need a
ELSE IF return A/C to Auto mode

There is a step or two I am leaving out, but that is the basic logic I would use.

You also are incorrect about TOU plans. It is not about peak usage but the amount of electricity consumed during the peak times. You get charged one rate for power during peak hours and one for power during off-peak hours.

" TOU rate plans are different : your per-kWh rate changes based on your total monthly electricity use and the time of day you use it.

There are specific times during the day when electricity is more expensive to generate โ these are known as โpeakโ hours. When you draw electricity from the grid during peak hours, SCE charges you more for it. Alternatively, you can experience lower electricity rates if you consume energy during the specified โoff-peakโ hours."