Category Archives: zero energy home

Some day houses for sale will have MPG stickers on them

There is sorta such a thing today.  It’s called HERS.  100 is a “normal” house.  0 is a “net zero energy” house.  And negative means you have even more PVs than you need.  Nice.  Someday websites like,, etc. will let you search on such things.

The best house I’ve seen is this -33 HERS of Carolyn and Kyle Cave in Hadley, MA.   It’s also nice to know what a house is pre-PV to get an idea of how efficient the house and it’s occupants are.  Oh, and a house in Maynard MA is -8 HERS.

Anyway, good work Caves!  Your house follows the important rule of thumb I now encourage people to use — Build (or pick)  your house with a lot of good roof space for PVs.  Small footprint houses like ours are a little more efficient, but we don’t have nearly as much room for PVs.  Dumb.

(Oh, and our house is nearly 0 HERS.  I am not exactly sure what — I forgot to ask for the pre-PV score and if I recall correctly the PV offset used the wrong KW total.)

(Oh, and read about the limitations of HERS at the link at top…)

Leave a comment

Filed under contrarian, erik-green, passive house, solar, zero energy home

To figure out how much electricity/energy we use each month…

Like many/most people who have grid-tied PV solar panels…. to figure out how much electricity/energy we use each month I have to do some math.  That’s because the smart meter doesn’t know how much electricity we use directly from the panels.  Some electrons never even hits the electric co’s meter, which can only show numbers for 2 things: (1) the extra KWh flowing out and (2) the extra KWh we need that is coming in (at nights, clouds and cold cold weather)

That’s not enough.  I also have to read (3) the total produced by our PV solar panels.  And then do some math.  The pain in this is that since one’s electricity bill is usually not calendar month, and our smart meter isn’t being read automatically by any device, I have to remember to “read the meter” near the beginning/end of the month.  I can’t use numbers on my electricity bill.


The basic idea:  IN KWh = OUT KWh


Solve for X and I’ve got it.

Additionally I think it makes sense to divide by the number of occupants in your house before comparing with your friends.  And maybe adjust by things like HDD and CDD (heating degree days and cooling degree days) if they live in a different part of the Earth.

So that’s KWh used per person per month.   We have averaged under 800 KWh per month year round on average for 28 months.  And we have 4 people here.  With 8 living with us for 10(?) months in 2011.

So under 200 KWh per person per month.  For everything, including heating and AC, cooking, lawn mowing and 2 home offices.

It’s hard to compare to most people in the northeast because most people don’t know their grand total since almost no one heats their house with electricity (geothermal or air-source heat pumps) like we do.  So they’d need to add up their gas/propane/wood/oil BTUs used and convert to KWhs.

Leave a comment

Filed under energy, energy-efficiency, erik-green, solar, zero energy home

New house? Changing exterior door knobs? — use levers

Are you building a new house or fixing one up — passivhaus, zero energy, or otherwise energy efficient to some degree? Some simple advice: Use levers not knobs. At least for the exterior doors since those are the ones that are going to be sealed up tight with gaskets and such. And also since your house might be quite tight, especially in the winter when all the windows are closed… you are pushing/pulling against this “vaccuum”. So no draft, but the door is also a little harder to open and close.

Maybe not noticable for an abled adult, but it is noticable if you have little kids. Yes, you say, but turning the knob/lever is different than pushing/pulling the door open and I am only changing the opener not the gaskets. But I am telling you. FROM EXPERIENCE… they are related!!! My 4 year old cannot easily twist the knob and simultaneously push/pull our exterior doors open. I think if they were levers (like a storm door we have)… it would be much easier.

To me this is both a safety issue and a respect for children issue. And you’ll be happy too (and guests) if you are maybe older or injured and are having trouble with the knobs. General “accessibility” I guess the word is.

I’ve heard levers are better. It’s now obvious to me that this is quite correct and important and not to be ignored. Do it!

Leave a comment

Filed under green, kids -- freedom and responsibility, passive house, superinsulation, zero energy home

What we pay for electricity

Our 6/15/2012 electricity bill was $15.86 even though we made approximately 200 KWh more than we used. That’s because we don’t have net metering here yet. (Yet, as in… I assume some day there will be.)

And it looks like we’ve paid approximately $630 for electricity in the last year. If you pretend we have net metering, then that would have been roughly $630 / (11,000 – 8,400) KWh (used vs generated) = 24 cents/KWh. A bit expensive, but not outrageous for Massachusetts.

What this $630/yr makes me wonder is what it would cost to add a smaller PV array with batteries that is off-grid to power us at night so we’d have even closer to $0 electricity bills.

Eh, money probably much better spent on something practical for the family. Like a pool. Or our kids’ school tuition.

Leave a comment

Filed under erik-green, solar, zero energy home

Our PECs: site vs source (primary) energy

Another good way to weigh one’s home energy usage is per person. (not arbitrary per sqft or per HDD, etc.). PEC=Primary Energy Consumption. Makes sense if we are trying to be “green”.

That’s what Marc Rosenbaum is proposing here.

If I am understanding the gist of what he is saying it is as follows as applies to us:

1. Here in MA/New England, power plants are about 33% efficient. They waste 2/3 of the energy (mostly fossil fuels) in making a unit of electricity. The exact number Marc uses is 1/2.7, not 1/3. (37% efficient source to site.) So… since we have used approximately 11000KWh/year, that is 29700 KWh/year in source energy that we REALLY used.

(Worry about PVs later… in step 3.)

2. We are 4 people, so look up our “fair share” according to his “people = bedrooms+1″ equation and we should be trying to meet 13,600 KWh/year in source energy. For everything… heating, cooling, hot water, appliances, cooking, lighting.

3. One should be able to offset some percentage of hot water and electricity usage from onsite generation. Marc explains (I believe) that the Passivhaus PHPP allows up to 20% offset for a traditional solar hot water system, so in his mind, why not allow up to 20% of electricity use as well. And indeed… we heat our hot water with an air-source heat pump too, so I am lumping it all together. And actually, since I am dealing with real data, not estimates, I see from my record keeping that in the last year we have exported 5600 KWh to the grid. So that is the equivalent of 2.7 that much in “source energy” that we have offset = 15,120 KWh

So if we reduce our 29,700 by 15,120 => 14,580 KWh/year

4. 14,580 > 13,600 KWh (Marc’s limit for 4 people)

So we didn’t meet Marc’s proposed criteria for PEC for a Passivhaus in New England, even assuming my generous PV offset based on our grid export numbers.

Pretty close though. Why didn’t we meet it? I assume:
- Our house is too big (1741 TFA via PHPP)
- 2 of us work at home and waste energy
- Windows and HRV not efficient enough
- We should use more solar thermal heating (I have plans on this front)

‎”Your house is a leaky bucket and the sun is a hose. To raise the water level, you need fewer leaks or a bigger hose.” — Nick Pine in a discussion of his “box on the lawn” solar collector design in

Leave a comment

Filed under erik-green, passive house, solar, zero energy home

Our house vs passivhaus / passive house

BTW, as a followup to my last post. Our house misses the passivhaus PHPP standard for Specific Space Heat Demand.

= 4.75*1000*1741/(3412) = 2423 KWh used per year is what the PH Certificate requires as a max for “Specific Space Heat Demand” (for our 1741/sqft TFA)

Give that we *used* an estimated 2300 KWh (2338) for heating, and given we can estimate our (non hyperheat/h2i) heat pumps at 2.25 COP overall (as a 15% adjustment from the “North Carolina” temp zone rating–I think I looked up once for our unit’s seasonal HSPF… 15% adjustment: as they seem to do in Canada since the COP is temp dependent.)

That would be:

5260 KWh of actual heat *delivered* from the 2338 KWh our minisplits *used*.

So we are 1/2 or 1/3 as good as a passive house I guess. Probably 1/3 since it was a mild winter.

So you can see why people think passivhaus might be extreme. We are talking about $350 (our house) vs $120 (a similar-sized passivhaus) in heating given $0.15/KWh electricity. I still think it makes sense, especially since people are getting practiced at doing it panelized/modular. (See Green Mountain Habitat for Humanity in VT for example.)

- roughly 68F max in winter, roughly 78F max in summer
- I should do a per HDD calc

FUDGE FACTORS (+ or -) in my “1/2 or 1/3 as good as a Passivhaus” calc:
- Weather/climate in PHPP is a 30-year avg. This 2011/2012 winter was mild.
- COP of our heat pump is a guess (maybe I guessed way too low?)
- Previously mentioned guess for % (10%) of heat used for hot water heating
- Some inaccuracy of the emonitor device (vs direct submetering)
- Any errors made in the PHPP (our house used a number of non-certified products… windows, HRV so guesses had to be made)
- Related to above… our windows and HRV are also not as efficient as those typically used in a passivhaus.

That said, obviously there are some houses that don’t use Passivhaus” equipment — like this one — that do MUCH better than the PH standard. Occupant behavior matters a lot.

Leave a comment

Filed under erik-green, passive house, zero energy home

2011/12: The year in heating

We have an emonitor gizmo that tracks our home energy use by circuit. One circuit is the air-source minisplit heat pump (the heat and cool in the house).

For the YEAR ending April 2012, the heatpump circuit shows:

3,009 KWh (total for year, heating and cooling and some hot water)
2,598 KWh (Oct-Apr — 7 mostly heating months)

Remember that we also heat our water with an air-to-water heat pump in the conditioned basement, so during the heating months, it is stealing heat from the house. So the 2598 includes some of that. Let’s pretend it is 10% of our total load (no idea) so that would be

2338 KWh (Oct-Apr — mostly heating months. HOME HEATING ONLY)

If we pretend the price we pay for electricity is $0.15/KWh (it’s more complicated than just a simple number like that with this and that charges) but close… then that is:

$350.70 (our estimated heating bill for winter 2011/2012)


(Well, and actually… minus some significant fraction of that which is covered by our PVs (electric solar panels). We don’t have net metering, so our electric bill is rarely $0 even in the summer. I just don’t unclude the PVs cause I generally think of them as an offset. Not an important part of the house.)

- The silly 20KWh/month our minisplit uses whether it is on or not. Nothing to do about that at least in the winter. But I could flip the dip-switch for 5 months of the year.
- Someday I will add a submeter for the minisplits since the emonitor is probably 10% off in some direction. (I believe that’s the spec I’ve seen.)
- More PVs, perhaps this string with a central inverter and small battery for:     – night-time (since no net metering) and
    – power outages (we have a well so it would at LEAST be nice to have running water when the power goes out.)

Leave a comment

Filed under energy, energy-efficiency, erik-green, HVAC, passive house, superinsulation, zero energy home

We’d be net zero but…

Our house would be net zero source energy but…

- well filters: we avg 54 KWh / month
- well pump: 10 Kwh / month
- radon fan: 40 KWh / month
- 2 home offices: 60 KWh / month
- Mitsubishi minisplit: 20 Kwh / month (EVEN WHEN OFF DUE TO A COIL HEATER THINGY!)
- electric lawn mower: (not much, but just sayin’)

What else did I forget?

So that’s 54+10+40+60+20 = 184 KWh / month
= 2208 / KWh per year

… that we can’t help that some other net-zero types of house don’t have since our house has a well and 2 home offices and a mitsubishi mr slim air-source heat pump with what I would consider a design flaw!

We use approximately 10,000 KWh per year for everything (heat, hot water, lighting, cooking, etc.) And our 6.9 KW PV panels make about 8,400 KWh so if we didn’t have the extra 2200 KWh, we’d be easily net zero.

BUT… then there is driving. Someone who lives in a city in and walks everywhere is blowing us away. We drive maybe 15000 miles per year at 20 MPG (minivan). There goes net-zero.

Leave a comment

Filed under contrarian, energy-efficiency, erik-green, HVAC, passive house, superinsulation, thinking, zero energy home

March energy report

KWh used in March: 799.
KWh made in March: 801.
We made electricity (net).
This is for everything in the house. Heating, hot water, appliances, lights, all the water filters and radon filters the house needs.
721 HDD (base 68F — which is roughly what we aim for during the day in heating months)
It was 17% warmer in March 2012 than in March 2011 in HDD (base 68F)

Leave a comment

Filed under erik-green, solar, zero energy home

Overheating, Passivhaus Style

One limitation of the PHPP modelling used in the Passivhaus / Passive House certification process is that the only kind of solar thermal heating which is modeled is windows (passive solar… BTW, not to be confused with “passive house”… 2 different things!) and not fancier (active solar) stuff with anything movable insulation and/or small fans and/or externally located solar-air-heaters like:

1 – commercially available solar air-heater SolarSheat or Sunmate (great example here)
2 – low-mass thermally isolated sunspace ala Nick Pine / Norman Saunders / William Shurcliff
3 – “solar siding” ala Nick Pine (essentially a very large solar-air heater, kinda like SolarWall or solar tempering)
4 – DIY downspot heaters ala Scott Davis
5 – a solar “yard furnace” ala Nick Pine (see messages in the SolarHeat yahoo group — always free membership required).
6 – Commercial or DIY solar water heating used for heating (via radiant floor heating or an water-to-air heat exchanger)


So the only thing you can do in PHPP is (in New England) optimize the windows for high SHGC (for the winter-time gains needed, winter being the main energy hog here in 2011) and add overhangs (ideally movable, like a trellis of greenery) or exterior shades (like they used in switzerland and france) to deal with the summer risk of overheating (since it still gets hot and sunny here). Also problematic for overheating are periods of the fall and spring when the sun is still low and leaves are not on the trees but it’s warm outside. Yes, you can open the windows. That will help a bit. Yes, you can install a concrete floor. That will help a bit to even out the swings, though it’s slow to react. But this is what Nick Pine and others like to call “living inside the heat battery”. Temps swing around a lot and you have little control over it besides turning on the heat or AC or moving shutters and insulating shades and such around manually. If you have the time.

A better way is keeping the solar collection on the outside of the thermal envelope of the house and optionally automatically store some for later in a huge highly insulated water tank in the basement (though that gets more complicated and/or expensive) (Getting close to 100% solar heating means being able to get thru quite a few days of no sun, so do your BTU/KWh heat load and storage capacity calculations over at the SolarHeat yahoo group.)

1. You can have pinpoint control over how much of that solar heat you let into your home!
2. Not blinded by all the light pouring thru lots of windows
3. Not as limited in architecture. Want bedrooms to the south but not wanting light blocking shades, etc, etc. Bad view on the South? Just add a huge air collector… no windows needed!
4. Easier to add solar heating existing homes/retrofit

So back to the overheating. To summarize the reasons to think carefully about cooling/preventing overheating in a passivhouse or otherwise superinsulated home:

1. too much passive solar. Big windows on S with high SHGC and no overhangs? Look out!

2. point source cooling on first floor (A BE2012 presentation about the VT Passivhaus by Habitat for Humanity detailed the warm 2nd floor)

3. warm bedrooms in summer. (related to point 2). bedrooms are often on second floor. If you are using air-source mini-split heat pumps to heat/cool your house and there is not an inside head in a bedroom, then guess what… on those summer nights when it doesn’t cool down outside and you need to keep the windows closed, it’s going to get warm in the bedroom… you’ve got 300Watts per person and warmish air coming in thru the fresh-air ventilation system (HRV or ERV) and how is it going to cool off? That’s right… it’s not. People should worry A LOT about this. Winter time is no problem with point source heating downstairs (or down the hall in our case). It’s a tad cooler in the bedroom, but the body heat — 300W per person — helps mitigate. Plus most people like it a “little” cooler for sleeping.

4. Global Warming. Not to get too pessimistic, but some scientific predictions are than NH weather will be like NC in 30 years. LINK. And Southern VT is already like PA in the 1960s. Yikes. It’s worth considering!

That said, the basic idea of worrying more about the heating load than the cooling load in New England and the midwest is a valid one. There are many many more HDD (heating degree days) than CDD (cooling degree days). So optimize for heating first. But have a cooling plan too! It still gets very HOT AND HUMID in Massachusetts and Minnesota!

Also not allowed in PHPP is counting PVs as a solar hot water heater. Instead of buying an expensive and complicated traditional solar hot water heating system that still might only provide a 60-70% overall solar fraction for the year, I explored building one myself for $1k and ultimately decided to just increase the KW of our PV (solar-electric) array to meet the hot water demand (I calculated) a full 100% (net for the year). Marc took the same route described here.

I am of course very open to correction on my assessment of the current state of the PHPP (circa 2010-12) with regard to solar.

Leave a comment

Filed under contrarian, erik-green, passive house, simple, solar, superinsulation, zero energy home