Cooling Strategies with PV + ESS

[MJ_CA_2019]

Background:
I am on PG&E TOU-B (peak 3p-9p M-F, all other times off peak)
I have a 12.6 kW (DC) PV (majority of my panels facing west) and 3 PW2. Currently set to Time Based Control / Balanced with a 15% reserve.
I have a two story home with old (2005) AC units. My home office has a 5500 btu/hr unit that vents to a window. Draws about 950 w.
(We have NG Dryer, Furnace, Stove/Oven, water heater)

I have been effectively off-grid since February with only a handful of days where I have needed any significant grid draw.
February: 719 kWh usage / 1066 kWh PV
March: 863 kWh usage / 1523 kWh PV
April: 835 kWh usage / 1884 kWh PV

This will flip eventually where We will see days needing 80 - 100 kWh and only producing 75 kWh (last year underperformed so I'm guessing here).

Situation / Question:
May is the beginning of Air Conditioner season for us. I use a whole house fan to try to cool the home as much as possible first thing in the morning.
If I leave the ACs off in the unoccupied areas of the house (everything except my home office until school is out for the summer) The 2nd story ends up being pretty hot by the time the kids need to go to bed. This means the AC is running pretty hard during peak times. PV is exporting and the PW are running the house.

You will see in these two screenshots that on May 3rd I let the thermostats run normally where the AC doesn't kick on until close to when we need to go upstairs for bedtime.
Yesterday, I tried "pre-cooling" by turning the thermostats down right after the PW reached 100% charged.
I also am currently running my pool pump during the day, it doesn't start until I know the PV will be higher than total home load.


Should I be trying to use directly as much PV as possible to avoid efficiency penalties (90% round trip)?
Does this only matter until I reach a point when my PV produces less than my daily usage?

My home doesn't have great insulation so I'm wondering if the 10% efficiency gained from not doing the round trip would be wiped out by cooling the upstairs earlier than needed.

What would you do? Set it and forget it? Or try to manage it more actively?

 

[getakey]

Generally better to not let the house get above the temp you want it to be at bedtime. Its more work to cool everything down than to keep it at desired temp. That said, I'm not sure how that would play with your PV and PW situation. I have 3 PWs and 11.8kW PV. In the summer, I just set it to desired temp and forget about it. I still get negative NEM in the summer months

 

[jjrandorin]

Where I live, we have hot summer days but the nights tend to be color. Its the perfect situation for an whole home fan here ( in fact the founder of quietcool whole home fans and the corporate headquarters is in my city, thats how perfect attic fans are where I live).

Before I got the whole home fan last year, I would set the AC to the temp I wanted the home to be and just forget it. That temp, however, was like 80 degrees. My home doesnt get hot during the day as its insulated pretty well. what happens is, it gets hot at night from the left over heat generated during the day that is now trapped in the home.

So, for my case, where the attic fan advertisements state "up to 90% reduction in AC", for me, that was the case. I can open a window around 7pm most nights, and by bedtime (10:00-11:00pm), its cool in our bedroom which is the window we open when we turn the fan on.

Last summer was on the hot side, but I used a lot less electricity. I have 2 AC units, one for downstairs and one for upstairs, but the are builder grade, 2005 units. I was going to look into replacing them, but now that I can use the whole home fan to cool the home (mostly) I can wait till they fail.

I think this is going to be another one of those "depends on your specific situation" things, but in general, I think its less energy to maintain a temp than it is to try to cool down a hot home to your temp, unless you have extenuating circumstances, like I do with generally cooler nights that work well with whole home fans.

(*whole home fans are slightly different than attic fans, so I am not sure if you have a regular attic fan or a whole home fan, or if your area is conducive to whole home fans, or you can tolerate the extra dust those units tend to bring into the home)

 

[MJ_CA_2019]

Where I live, we have hot summer days but the nights tend to be color. Its the perfect situation for an whole home fan here ( in fact the founder of quietcool whole home fans and the corporate headquarters is in my city, thats how perfect attic fans are where I live).

Before I got the whole home fan last year, I would set the AC to the temp I wanted the home to be and just forget it. That temp, however, was like 80 degrees. My home doesnt get hot during the day as its insulated pretty well. what happens is, it gets hot at night from the left over heat generated during the day that is now trapped in the home.

So, for my case, where the attic fan advertisements state "up to 90% reduction in AC", for me, that was the case. I can open a window around 7pm most nights, and by bedtime (10:00-11:00pm), its cool in our bedroom which is the window we open when we turn the fan on.

Last summer was on the hot side, but I used a lot less electricity. I have 2 AC units, one for downstairs and one for upstairs, but the are builder grade, 2005 units. I was going to look into replacing them, but now that I can use the whole home fan to cool the home (mostly) I can wait till they fail.

I think this is going to be another one of those "depends on your specific situation" things, but in general, I think its less energy to maintain a temp than it is to try to cool down a hot home to your temp, unless you have extenuating circumstances, like I do with generally cooler nights that work well with whole home fans.

(*whole home fans are slightly different than attic fans, so I am not sure if you have a regular attic fan or a whole home fan, or if your area is conducive to whole home fans, or you can tolerate the extra dust those units tend to bring into the home)

We are in the Central Valley. I have the whole home fan and have it setup on a timer switch so I can set it for 1 hr, 30 min etc and let it run.
Works great right now when I get up and it's 60 or colder outside. The issue is when the temperature differential between the Low temp and my desired indoor temp decreases then they lose their efficiency. Also, I'm not going to get up at 2:00 am to open all the windows. (I'm not complaining though... lived in AZ where we would essentially seal the house for the "heat season" and never crack a window for months on end.)

Summer is tricky. I have two little enough to need a nap and if it's hot upstairs that is out of the question. It's challenging to cool a space that would otherwise be empty but it does take a while to get that floor cooled off if the AC is not running throughout the day.

 

[MJ_CA_2019]

Generally better to not let the house get above the temp you want it to be at bedtime. Its more work to cool everything down than to keep it at desired temp. That said, I'm not sure how that would play with your PV and PW situation. I have 3 PWs and 11.8kW PV. In the summer, I just set it to desired temp and forget about it. I still get negative NEM in the summer months

Do you have a floor (or zone) that is basically unoccupied during most of the day in the summer?
I might think more about keeping it cool if I knew that the space was used more during the day.
Our typical in the summer is the kids come down for breakfast and don't really go back upstairs until sleep time.
They spend a lot of time in the pool.

 

[aesculus]

You will see in these two screenshots that on May 3rd I let the thermostats run normally where the AC doesn't kick on until close to when we need to go upstairs for bedtime.
Yesterday, I tried "pre-cooling" by turning the thermostats down right after the PW reached 100% charged.

We see the charts but what about the numbers and comfort? Were you more comfortable one way or the other? What was the differential in energy? Also what about the export differential and loss of trade value for the 2nd day? Any way to quickly estimate your costs between the two days?

 

[getakey]

Do you have a floor (or zone) that is basically unoccupied during most of the day in the summer?
I might think more about keeping it cool if I knew that the space was used more during the day.
Our typical in the summer is the kids come down for breakfast and don't really go back upstairs until sleep time.
They spend a lot of time in the pool.

I wasn't speaking specific to my home, rather in general it easier to keep at a temp than let it get significantly above that temp and then try to cool down. Perhaps, you could set your upstairs a little higher than desired and then cool it from a smaller delta.
In my case, I have a somewhat unique house. I'm on a hill and the house goes down 3 levels not up. The "upstairs" is street level and has living room, kitchen, dining, master, and wife's office. The upstairs is the only level that has a west exposure due to the hill. First level down has large family room and the other bedrooms. 3rd level down is about 1/3 the footprint of the upper levels and has my office and an area for sitting/drinking wine. Upper level has dedicated AC. Lower levels share AC. Home is well insulated and we also have a whole house fan. Even on hottest days, I usually don't have to turn upper AC on until afternoon and lower level after that. In the summer, I cannot use the whole house fan until after 9pm as it stays fairly warm. We will have stretches wher eit stays above 100 for days. We are already in the 90s today.

 

[h2ofun]

I wasn't speaking specific to my home, rather in general it easier to keep at a temp than let it get significantly above that temp and then try to cool down. Perhaps, you could set your upstairs a little higher than desired and then cool it from a smaller delta.
In my case, I have a somewhat unique house. I'm on a hill and the house goes down 3 levels not up. The "upstairs" is street level and has living room, kitchen, dining, master, and wife's office. The upstairs is the only level that has a west exposure due to the hill. First level down has large family room and the other bedrooms. 3rd level down is about 1/3 the footprint of the upper levels and has my office and an area for sitting/drinking wine. Upper level has dedicated AC. Lower levels share AC. Home is well insulated and we also have a whole house fan. Even on hottest days, I usually don't have to turn upper AC on until afternoon and lower level after that. In the summer, I cannot use the whole house fan until after 9pm as it stays fairly warm. We will have stretches wher eit stays above 100 for days. We are already in the 90s today.

I have days in the summer that it does not cool down enough until after midnight, and some times not enough to even get cool air.

So I still have not figured out how I will deal with summer. Our bedroom is downstairs, so that helps. BUT, wife does not like a hot kitchen, which is upstairs.
So for now, trying to bank money. Last month I banked 250 bucks to get back to net zero. Should be able to bank even more this month, since still keeping AC off, even though today its tough, but temps supposed to drop.

Am hoping I have enough credit, and solar, that soon, I will just crank on the AC, and not have to worry about it. Shall see.

 

[holeydonut]

Generally better to not let the house get above the temp you want it to be at bedtime. Its more work to cool everything down than to keep it at desired temp. That said, I'm not sure how that would play with your PV and PW situation. I have 3 PWs and 11.8kW PV. In the summer, I just set it to desired temp and forget about it. I still get negative NEM in the summer months

I agree with this... in my limited attempt to "game" the AC, did not make sense with my PWs.

I have Emporia Vue CT sensors on my 2x outdoor air condensing units, and on my indoor 2x air handlers.
House is two story with vaulted ceilings (no real attic space although I did sneak some radiant barriers in between my rafters in the limited attic crawlspace)
Roofing tiles are flat concrete so they absorb a ton of heat
I have a southward facing front elevation so the afternoon sun just bakes two upstairs bedrooms and roofline.

Just for funskies the other day (90 F high outdoor ambient peak), I tried to pre-cool the upstairs before 4pm. It only got down to 66F, and I wanted to see how hot it got upstairs if I didn't run the AC again. By 7pm it was back up up to 77F and my wife told me to stop my stupid experiment. I ended up blowing through 6 kWh of energy to do this silly pre-cooling thing by the time the ACs permanently stayed off at 8pm.

But yesterday we were back up to 90F, and I just let the thermostat do whatever it took to peg things at 73F with no interruption. (Please don't say anything; I'm like WTF 73F is so goddamn low at 5pm!!! WHYYYYYY). So the things naturally took energy that previous peak time that I could have been exporting to the grid. And this approach used 6 kWh as well in the same timeframe. So yeah, the cost savings may be there; and I may get better round-trip with the pre-cooling method. But man, it's not worth it to wonder WTF it's so cold, then WTF it's so hot, then WTF the wife is not happy.

 

[h2ofun]

I agree with this... in my limited attempt to "game" the AC, did not make sense with my PWs.

I have Emporia Vue CT sensors on my 2x outdoor air condensing units, and on my indoor 2x air handlers.
House is two story with vaulted ceilings (no real attic space although I did sneak some radiant barriers in between my rafters in the limited attic crawlspace)
Roofing tiles are flat concrete so they absorb a ton of heat
I have a southward facing front elevation so the afternoon sun just bakes two upstairs bedrooms and roofline.

Just for funskies the other day (90 F high outdoor ambient peak), I tried to pre-cool the upstairs before 4pm. It only got down to 66F, and I wanted to see how hot it got upstairs if I didn't run the AC again. By 7pm it was back up up to 77F and my wife told me to stop my stupid experiment. I ended up blowing through 6 kWh of energy to do this silly pre-cooling thing by the time the ACs permanently stayed off at 8pm.

But yesterday we were back up to 90F, and I just let the thermostat do whatever it took to peg things at 73F with no interruption. (Please don't say anything; I'm like WTF 73F is so goddamn low at 5pm!!! WHYYYYYY). So the things naturally took energy that previous peak time that I could have been exporting to the grid. And this approach used 6 kWh as well in the same timeframe. So yeah, the cost savings may be there; and I may get better round-trip with the pre-cooling method. But man, it's not worth it to wonder WTF it's so cold, then WTF it's so hot, then WTF the wife is not happy.

What ever temp the boss wants, the boss gets!! The amount of money I have spent on this stuff is NOT worth telling her but its too cold, or in the winter, too hot!

 

[getakey]

I don't think pre-cooling saves. I've seen several studies that confirms that

 

[MJ_CA_2019]

We see the charts but what about the numbers and comfort? Were you more comfortable one way or the other? What was the differential in energy? Also what about the export differential and loss of trade value for the 2nd day? Any way to quickly estimate your costs between the two days?

We see the charts but what about the numbers and comfort? Were you more comfortable one way or the other? What was the differential in energy? Also what about the export differential and loss of trade value for the 2nd day? Any way to quickly estimate your costs between the two days?

May 3rd (the no change day)
Home Usage: 42.4 kWh
Solar Energy: 70.3 kWh
From Powerwall: 23.6 kWh
To Powerwall: 16.1 kWh
From Grid: 0.9 kWh
To Grid: 36.3 kWh (15.9 kWh of this was during Peak)

May 4th (the pre-cool day)
Home Usage: 46 kWh
Solar Energy: 69.4 kWh
From Powerwall: 19.7 kWh
To Powerwall: 25.8 kWh
From Grid: 2.1 kWh
To Grid: 19.4 kWh (15.5 kWh of this was during Peak)

Comfort maybe slightly lower on the first day because the AC is running longer around bed time.

 

[holeydonut]

I don't think pre-cooling saves. I've seen several studies that confirms that

Yeah, it's just a load shifting thing to try and get the kWh out of peak time. But, it totally sucks.

 

[holeydonut]

What ever temp the boss wants, the boss gets!! The amount of money I have spent on this stuff is NOT worth telling her but its too cold, or in the winter, too hot!

Lol you should have seen the look on her face in 2019 when I told her "hey, PG&E says we should line dry our clothes and turn down the brightness of the TV to save energy. Sound good?"

 

[MorrisonHiker]

Where we live, evaporative coolers are popular and effective. Our Breezair Icon provides 37580 BTUs of cooling and only uses 1.22 kW at maximum. Normally, that's too cold so we run it at a lower fan speed and it only uses 400W to 600W to cool about 2200 square feet. The 1200 square feet walk-out basement typically doesn't need to be cooled.

For those who don't know what an evaporative cooler is, it's kind of like a whole house fan but it blows cooled air through the entire house and you just crack a window in whatever room you'd like cooled. It can also be installed with ducts. Water is passed through aspen pads which evaporates and creates the cooling effect which can lower the temperature by 40 degrees or more.

As it cools down outside at night, it doesn't even need to use water anymore and will just blow the cool air from outside throughout the house.

 

[h2ofun]

Lol you should have seen the look on her face in 2019 when I told her "hey, PG&E says we should line dry our clothes and turn down the brightness of the TV to save energy. Sound good?"

Just like when I said to the boss she needs to run the electric dryer in the mornings, not evenings, I was toast. She is not happy I put a switch on the TV power she has to turn on and off. I then said whatever you want, boss

 

[aesculus]

May 3rd (the no change day)
Home Usage: 42.4 kWh
Solar Energy: 70.3 kWh
From Powerwall: 23.6 kWh
To Powerwall: 16.1 kWh
From Grid: 0.9 kWh
To Grid: 36.3 kWh (15.9 kWh of this was during Peak)

May 4th (the pre-cool day)
Home Usage: 46 kWh
Solar Energy: 69.4 kWh
From Powerwall: 19.7 kWh
To Powerwall: 25.8 kWh
From Grid: 2.1 kWh
To Grid: 19.4 kWh (15.5 kWh of this was during Peak)

Comfort maybe slightly lower on the first day because the AC is running longer around bed time.

Well by these numbers you should NOT do pre cooling IMHO. It got you nowhere.

 

[aesculus]

I don't think pre-cooling saves. I've seen several studies that confirms that

Yeah, it's just a load shifting thing to try and get the kWh out of peak time. But, it totally sucks.

I have seen studies where it makes sense financially but not for all the other factors. With PWs it probably makes no sense now.

But I do shift my temps by 2 degrees during peak periods. A carryover from before the PWs. I should probably do some investigation to see if that makes sense anymore.

Another factor I have is that I have a lot of thermal mass downstairs where we really live. That can act as a great thermal flywheel that really lasts for about 3 days. But beyond that it can hurt as much as help. It will keep the house warmer if the ambient temp lowers but there is no breeze etc. Whole house fan would have to run for days to make a difference and I hate that thing.

 

[getakey]

Well by these numbers you should NOT do pre cooling IMHO. It got you nowhere.

doesn't that prove my point?

 

[sorka]

I don't think pre-cooling saves. I've seen several studies that confirms that

I think it depends on when and how much. If I run my AC from 2 to 3pm which is still off peak at 19 cents / kwh, I expect not to have to run it at all for the first 2 hours in the 49 cents / kwh price range.

But now that I have my energy monitors setup on every circuit and temperature logging, I plan to do these comparisons back and fourth during the summer.

I'm in a similar situation. In Merced (central valley). I have a 48" belt driven Triangle whole house fan that I installed 20 years ago when I bought the house to replace the wimpy 30" direct drive fan that was loud and buzzy. The 48" not only moves 4 times as much air but does so nearly silently.

It's on somewhat automatic control based on internal and external temperature differences and how many windows I have open. The algorithm uses 1/2 of the square footage of each window to determine what speed the fan can run at. I really should add external and internal barometric sensors to detect when there's no enough airflow from the open windows to provide an automatic safety shutdown. But at the very least it will shut off sometime in the morning as the outside e temperature rises and then I get a notification to close my windows.

As to the OPs question it really matters what your TOU rates are. I'm on EV2A which is a 2.5 x differential between off peak and peak, so 100% of all my peak solar production goes back to the grid.

If you're on any of the other TOU rates, there's so little difference between peak and off peak that it seems self consumption while the sun is shining would be the best and then grid consumption off peak AND when the sun isn't shining. But to achieve that, you'd have to mess with the app settings twice a day.