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Community and Q&A

Power utilization: Review

Alex Stanton | Posted in Energy Efficiency and Durability on

Alright, so one of my first posts despite having read I think every article and post here over the last three years during design and construction of our new house.

Here’s the punchline: I did not set out to build a Passive or even true Pretty Good home, but I did try to incorporate a wide range of concepts to make my completely-too-big-home not completely suck. I’m proud of how awesome the place turned out, but surprised (not shocked) that we’re running such a high kWh utilization. My ego wants my mini mansion to use less than 1000kWh/mo with no sacrifices.

Quick facts:

– zone 6
– 3500sqft
– two stories, no basement
– pretty tight, ~3.0ACH
– Walls @ R21+R6.6 (ZIP-R 1.5″, Vault Ceiling @ R50, Other ceilings @ R74, over unconditioned Floors @ R48, R10 under slab areas)
– Lots of glass, mostly Milgard Ultra fiberglass dual pane w/ 4th Surface, one large wall of “storefront style” thermally broken aluminium frame w/ 1″ dual pane glass (overall u calc @ u .24)
– Mitsubishi mini-splits with a head in literally every room (9 internal heads), 3 outside units (our diamond contractor overbuilt this I am sure)
– Electric oven, gas range
– Electric HW tanks, 50G x 2
– Heated tile floor in master bath, towel warmer, HW recirc pump (scheduled)
– LED everything
– Smarthome controls for everything, PoE switch for cameras, alarm system, home audio, etc.
– Designed to maximize S & SW exposures for solar heat gain (worked AMAZING in the cold of winter, cozy on a 20F day with HVAC off)

Because someone is going to ask:

– MXZ3C24NAHZ outdoor unit, running (3) FH06 indoor units to BEDROOM 2, LAUNDRY, AND FOYER on first floor
– MXZ4C36NAHZ outdoor unit, running (3) FH06 indoor units to BEDROOM 3, OFFICE 1 AND OFFICE 2  on second floor and (1) FH18NA in LIVING ROOM
– MXZ3C30NAHZZ outdoor unit, running (1) FH18NA2 to DINING ROOM (first floor) and (1) horizonally ducted PEAD-A12AA7 in MASTER (second floor)

Open floor plan, master over dining room, dining open to kitchen/living room, 20’+ ceilings in great room w/ open loft upstairs.

I’ve include a copy-paste and I hope you can ignore the amazing power rates we’re running here (<$0.06/kWh), they won’t be that way forever! Check out a few months below:

Billing Periods
Billing Days:
Days in Period:
Total Usage Charge:
Total Monthly Charges:
Total Usage:
Average Daily Usage:
Max Temperature:
Min Temperature:
Avg Temperature:
Jan 2019
3,719.00 kWh
128.24 kWh
Feb 2019
4,193.00 kWh
131.03 kWh
May 2019
2,048.00 kWh
66.06 kWh
Jun 2019
1,709.00 kWh
55.13 kWh

Should I just be happy, or should I grab a power utilization system and try to figure out what is soaking up all the power? I’ve done experiments like turning off one of the HW tanks, killing the recirc pump, turning off all extras like the heated floors and etc. with little impact.

My assumption on high winter usage is due to the mini-splits being improperly engineered. I should have rolled up my sleeves and done some calc work myself but we had no backup heat source and they were worried about the winters. Pretty confident now that we were hitting hardcore short-cycling issues in the winter, especially during the day with the SHG.

Besides eviscerating me for wanting to have my cake and eat it too, any thoughts?

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  1. Expert Member
    Zephyr7 | | #1

    Those numbers don’t look too bad to me. You’re possibly running some electric auxillary heat during the coldest days, and that resistance heat will soak up a lot of power. You’re basically all electric too, so you probably have a very, very small gas bill.

    I expect a big chunk of your heat loss is through all the glass at night. Probably not much you can do about that assuming good windows and good air sealing. The heated floor might be sucking up a fair amount of power too, so tighten up the schedule on that (and the towel warmer, if it’s a separate zone from the floor) as much as you can.

    You’ll probably hear from Dana after the weekend, he’s great with this stuff and will probably have a lot more detail for you.


  2. Trevor Lambert | | #2

    You've got 90kBTU/h of heating. That is a lot. It's likely not running all that efficiently. Did you have a heat loss calculation done?

    The windows are probably the biggest hit, depending on what you mean by "lots".

    3ACH50 isn't all that tight, it's code minimum.

    You've got monthly usages between 1700 and 4200kWh. The only way you'll get close to 1000kWh average is if you turn off the HVAC and just wear a coat all winter.

  3. kjmass1 | | #3

    I’d be complaining to the diamond dealer since they sold you on 1 head per room which is crazy oversized.

    Best bet is to reconfigure your outdoor units/heads to downsize and pick the units that are remotely sized for their loads. Tough trying to cover 3500sf though.

  4. BFW577 | | #4

    I’ve include a copy-paste and I hope you can ignore the amazing power rates we’re running here (<$0.06/kWh), they won’t be that way forever! Check out a few months below:

    That kind of usage here in New England would cost a fortune. Your Jan 2019 bill would cost me over $900 here in CT!

  5. Charlie Sullivan | | #5

    I really rough interpretation of your numbers is that you use 2000 kWh/mo for heating during the coldest months, in addition to 2000 kWh/mo for other stuff. That 2000 kWh/mo for other stuff is what surprises me. I would be inclined to chip away at that number rather than worrying about the heating. I doubt there's any one thing that gets it up that high but things that could be contributors would be incandescent lights left on a lot, hot-water usage, maybe laundry and high-flow showers, electric clothes dryers, maybe a lot of electronics adding up to a significant contribution if left on? For 120 V plug loads, a "kill-a-watt" meter is an easy way to do some sleuthing.

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