(Nearly) Passive House Electric Backup

[superman22x]

Hello all, I'm looking for some tidbits of advice. We are constructing a house this year, and it is intended to be nearly passive. I'm not going through the expense of certifying, etc. But I based my targets for insulation, air sealing, windows on the PassiveHaus energy consumption goals. So for those that aren't familiar, the house will have a very low heating/cooling load. In the region of 1 ton at design temp, or 12kbtu. With that said, I intend to have solar at this house (Michigan) that will cover me for most of the year except the worst of winter when we see the sun once a month. We will still be grid tied, but I would like to have most of the house covered by backup power. I do not need a grid interactive inverter however, as my utility has made that basically useless. So I'm stuck at choosing a path for the inverter... Initially I was thinking an EG4 18k, the 200A pass through would be easy to setup and have the whole house backed up. But it really might be quite a bit more power than we need except on occasion. Two EG4 6000xp's seems like it would be more than enough power for half the cost. But a subpanel would be required. Maybe not going EG4 at all. It seems their latest inverters are a good leap ahead though.

We will have a heat pump with max current draw of 3000watts
Heat pump Dryer
Well Pump - shallow, only 40ft to water
Electric water heater 4500watt
Induction Stove
Electric Oven

Those are the big ticket items I can think of.

For the most part, one 6000xp could cover everything. The water heater and stove/oven have some pretty large power draws though. So either they would have to be on the not backed up panel or possibly two 6000xps feeding a 100amp panel would be sufficient.

Appreciate any input! Thanks

 

[prometheanfire]

There are electric water heaters that run off 120V, rheem proterra, so that could help a ton. For things like the well pump, think of startup load.

 

[zanydroid]

HPWH costs the same as a 6000XP and shrinks your battery and solar needs by 3x on energy basis and 10x on power basis, so slam dunk in my book.

The stove and oven will use a lot of power but not much energy. So I would personally just add a 6000XP to be able to cover that. Simplified the system design a bit and gives more resilience

 

[superman22x]

Yes, HPWH may be in the picture. My mechanical room is small, so I'm not tying myself to that just yet, but it would be ideal. HPWH needs a decent bit of air space.

 

[zanydroid]

Yes, HPWH may be in the picture. My mechanical room is small, so I'm not tying myself to that just yet, but it would be ideal. HPWH needs a decent bit of air space.

It doesn’t need that much space. If it is a room and not a closet, probably good. The manufacturers have detailed air volume instructions.

 

[superman22x]

They do, the room is too small for open air, but I may be able to duct it somewhere that it is fine.

 

[Quattrohead]

An 18kpv or pair of 6000XP or equivalent units will cover your loads, less than that you will be disappointed.
You might be willing to think about Max load and manually shed but others won't. Laundry and cooking tend to happen together in my home ?
The 18kpv handled 13.6kw for a minute before I decided to stop the dryer.....I bottled it before the inverter did ? but it would have switched to load assist or bypass if it needed too.

 

[Quattrohead]

HPWH does work well for us but it is in a Florida garage, works fantastic in the summer and still fine even at around 66 ambient ATM.
Supposedly they are good into the 40's but will switch to the resistive elements if required.
You could have fun ducting heat to it in the winter and the cool discharge to the house in the summer ?

 

[superman22x]

Ok, that's good to know! From a power perspective, two 6000xps is pretty similar to an 18k, slightly more operating losses I guess. That's why I'm torn between the options.

The other problem is that a hpwh uses heat from the house in the winter, which will be fed by my primary heat pump. So there is a decent efficiency loss there. Still better than resistance.

 

[shazam]

Just a few ideas from a dozen years experience in a super-tight, 1800 sq/ft w/ full basement ICF home in NE OH. Our Energy Star certifier had to buy a smaller aperture for his blower-door test equipment we're so tight ?

Strongly urge you to include a HPWH in your plans. When we replaced our 80 gallon electric with the 80 gallon HPWH we went from 6 kWh a day to less than one. Yes, there is an issue with the cool air vented into the house being heated again. The flip side that most folks don't realize is that in a very tight home, an issue all year (yup, even in the winter time) is too much moisture in the air. Common practice in our area is to use a heat-recovery ventilator to lower moisture content...essentially, a HRV tries to pull the heat out of ejected air and transfer it to the incoming. Pretty efficient, but still warm out-cold in. The HPWH exhausts very low moisture air, helping with that issue.

We heat with a small wood stove (it's 76 in the living room as I type), with a whole-house ducted heat pump (passe techno now) for back-up. The HP hasn't run yet this year.

We have the same issue with our POCO as you do..they just don't want to play nice. We're in the process of installing a SA 15K, 16 kW of 460W bifacials, and 25 kWh of SOK batteries. We will use the 200 amp pass-thru in the SA direct to the existing MEP for grid-backup of last resort, zero-export to the grid. Solar should do us most of the year, and plan B is our 4-cylinder diesel gennie (ex-mil MEP803 10kW for those that follow those things). When we are on-line next winter, we'll adjust the battery pack size upward as needed to meet the need.

It's an adventure!
Jim

 

[superman22x]

Awesome, that's good to hear! I think I will figure out how to fit the HPWH in one way or another then. My primary home heat pump is going to likely be an A2W, but I'm not sure if it will be able to do the job or not for hot water. I'm still ironing out the details there.

 

[shazam]

Awesome, that's good to hear! I think I will figure out how to fit the HPWH in one way or another then. My primary home heat pump is going to likely be an A2W, but I'm not sure if it will be able to do the job or not for hot water. I'm still ironing out the details there.

I hadn't anticipated A2W...that should handle everything heating-water-wise. We looked @ A2W to heated downstairs floors before we built...a nice option. When you spec the house, make sure it specs HRV, not ERV. Our HVAC guy made that mistake, and we fought moisture in the air continuously til we swapped it out.

It's an adventure!

Jim

 

[superman22x]

It should be able to handle it all, they sometimes struggle to get water above 120*F I think, I'd rather not have a preheat and final tank if possible. I suppose any resistance backup elements could be powered from the main panel instead of the sub panel too. Most cold climate heat pumps maintain pretty decent capacity down to the coldest temperatures we get here in Michigan.

 

[gramos100]

I was interested in this post as I just built a " passiv haus " , in principle , not certification ( that alone was quoted at 22000 euros )
but we are totally off grid so the similarity ends there ,
Good luck with your project and do,nt listen to anyone who says ooohh that ,ll be expensive , its not !