Can you put a ground loop for a geothermal system below the basement floor?

Q: For new construction, would it be possible or make sense to put in loops for a ground source heat pump below the structure? 

A: I have not seen this done, but I have thought about it for new construction. Since you are excavating for the basement anyway, why not just go down a few feet more and put in ground loops? I think this would greatly reduce the cost of adding geothermal. However, I do not know if this would reduce the cost enough to compete with air-sourced heat pumps. The downside I can see is that your basement would get colder which could lead to increased condensation in summer when it is humid. 

Will a battery like a Tesla Powerwall last for 3-5 days?

Q: How much can a Tesla power wall store? Will it be enough for 3-5 days in case of an outage?

A: One Tesla Powerwall stores 14kWh of electricity. If you are using fossil fuels for heating then you are probably using about 20kWh a day to run all the lights and appliances in your house. If you are using heat pumps to heat or you are using AC in the summer then you are probably using about double this. Hence it is not practical to use a Powerwall (or any other type of battery) to run your entire house loads. However, this is not how most people use a Powerwall (see my blog post on uses of batteries). Most people use them as an alternative to using a diesel or propane back-up generator. So the battery or generator is powering an emergency panel which is usually the fridge, the furnace burner circuit and the circulating fans or pumps, plus some lights and a few outlets. This is typically under 1kW in total continuous load so a Powerwall can last about a day. This is usually enough to get you through a power outage. To last 3-5 days you would probably need 2 Powerwalls and to reduce the load on your batteries to just the lights, the wifi, a few outlets and the fridge.

Net metering rules are very different in my state.

Q: In Colorado, our grid-tied solar is restricted to 120% of historical kWh usage. Your solar panel system is massive. Is your system restricted in size OR are you actually using that much electricity? Where is most of your electrical usage in your home?

A: Net metering rules (and the subsidy rules, which are different entirely) vary a lot by state and even by town within a state and even then by utility company within a town. In MA, net metering is generous but there are many limits to net metering. Under 10kW you get 100% credit for any power you export. Above 10kW and below 25kW you get only 60% credit for the excess power not 100%. Above 10kW the utility has to approve your system and if there is, say an overloaded transformer on your street, they will not approve it. The SMART subsidy (the MA subsidy for solar power) drops to almost zero above 25kW, but then it goes up if you add a battery or install the array as a canopy over a parking lot. If you install it over a farm field and maintain the agricultural use, it goes up again. So you just have to pick through your local rules and try to optimize financially. In my experience in MA today, the financially optimal array is just under 25kW with battery back up.

My arrays generate slightly more electricity than I use throughout the year. Most of that electricity now goes on our heat pumps because we use almost no heating oil.

Why do you not recommend solar hot-water panels?

Solar photovoltaic panels are sometimes called solar P.V. panels to distinguish them from solar thermal panels, or solar hot-water panels, which use the heat from the sun to directly heat water. Solar thermal panels can be over 70% efficient, which sounds great compared to solar P.V. where the maximum commercially available efficiency is 22%. However, if you are using that solar electricity to power a heat pump hot water tank (please see page 66 in Chapter 2), with its 400% efficiency, you get a total heating efficiency of 84% for the solar P.V. panel that is heating your hot water with a heat pump. This is better than the efficiency of a solar thermal panel. Because of net-metering (please see page 82), solar P.V. panels can generate the electricity in the summer, and you can use it in the winter. This is not possible with solar thermal panels, which generate little hot water in winter, which is right when you need it.  Also, the solar-P.V.-plus-heat-pump-hot-water-tank option has no pipes and hence cannot leak. Better overall efficiency, energy “storage” via net-metering, and no burst pipes make solar P.V., in my opinion, a far better solution than solar thermal panels. 

What is the source of the $20 house-price increase for every $1 in utility bill savings?

Q: Can you link to the DOE study on home value?

You can download it here:

It was published in The Appraisal Journal in October 1998 and authored by Rick Nevin and Gregory Watson

Q: for this DOE study of $20 gained for $1 savings – is that $1 per annum or $1 per month? [Ken Calligar] []

It is $20 for every $1 in annual bill savings.

How long do solar panels and inverters last?

Q: What is the life span of the PV array panels and inverter?

A: The panels are warrantied for 25 years to produce at least about 90% of their initial power production. This varies a bit by manufacturer. They will probably last for many years beyond that. My inverter is warrantied for 15 years but new ones today come with a 25 year warranty. 

What about buying 100% clean energy from my utility? Is that better than solar panels?

Q: Do you still prefer installing solar panels rather than purchasing 100% clean energy from National Grid through suppliers like Eligo Energy. 8.9 cents/Kwh for 6 months. Have you done a financial analysis of paying 9 cents to 14 cents/kwh to receive 100% clean energy from National Grid compared to installing solar panels on roof?

A: I am assuming that the 9c per kWh is the cost of generating the electricity. Utilities charge separately for distributing that electricity, often about 12c in MA. This means that you actually pay about 20-24c per kWh after you add in all the other charges, including the $7 a month they charge you for being a customer. When you add solar panels you eliminate the entire bill (except the $7 a month which is effectively what you pay to maintaining the option of drawing power from the grid which you need it) so your cost drops from 24c/kWh (what I am paying today for Eversource electricity) to between 4c and 11c depending on how much shade you have on your roof and which subsidies you get. So generating your own solar power is far cheaper than even the generating cost of electricity from Eligo, let alone the full cost of that electricity.

If your roof is so shady that solar panels on your roof will generate electricity at more than the full cost of electricity from your utility (21c/kWh in the above example) then buying 100% clean power may make sense for you. However even a half shaded roof (I have one) generates electricity at 11c per kWh which is less than half of what I pay Eversource today.

Shouldn’t air sealing come first, even before insulation?

Shouldn’t air sealing come first, even before insulation?

Q: since this is talking about what makes sense financially, it would be useful to add air sealing to your fab 4, and I think you would find that this is the best investment, even better than making your basement cold (insulating the ceiling). I think the conclusion that you hinted at is that you should not “wing it”, but do the shortest payback measures first, and this results in the least expensive net zero result. Anyone can be net zero by adding solar collectors, but the question is how to get there most cost effectively. So, things like doing air sealing, should be done first, always. Yes? 

A: I believe air sealing is important, it just wasn’t a big issue on our house. This is because both our roof and our walls were already well sealed. I have seen houses where air-sealing alone has cut the energy bill 25%.

Our house has a flat roof with a rubber membrane waterproofing layer on the top of the roof. Under this waterproofing layer are two layers of 2” ISO boards. This makes my roof air-tight as well as water-tight. This makes my roof far more draft-proof than a typical roof with sloped sides, shingles and a lot of small air cracks between the walls and the roof. 

Also, our siding is vertical boards with tongue and groove connections, and it is well painted. This makes my siding almost impenetrable to wind. This cannot be said of unpainted shingles or standard shiplap horizontal siding, both of which allow a lot of drafts. Also, because our house is two stories high with a flat roof, the top of our house is about 20’ high which is much lower than the 30’ or so of a house with a pitched roof. The low height of our roof reduces the “stack” effect which is where rising warm air forces air to leak out of the top of the house and sucks cooler air into the basement. So, our house did not have many of the causes of drafts in typical houses. In contrast, our windows were terribly drafty. Our windows were all replaced with well-sealed and well-insulated triple-glazed windows. We paid particular attention to making these windows air tight.

The biggest source of drafts through the walls in our house was along the sill plate, which is the place where the top of the concrete of the basement joins to the wood studs of the walls. I sealed obvious drafts with a few cans of spray foam and I weather-stripped the bulkhead door.  I also stuffed the fiberglass that I used to insulate the ceiling of the basement into the sill plate and this cut down the drafts.  Since this was done at the same time as I did the insulation, I could not separately measure the contribution of the draft sealing compared to that from the insulation. Hence, you can say that the money savings I attribute to insulation alone are actually due to both insulation and air-sealing. I just think the insulation was by far the bigger contributor because the drop in my energy bills was almost exactly what was predicted from my energy model which directly accounts for insulation but, back then, did not account for drafts.

Early on in my zero-carbon renovation, I added weather-stripping to my external doors, but I could not detect any change in the energy bills from sealing the drafts on the doors alone.  That is why I do not call it out as one of the fab four. That does not mean that draft-sealing is unimportant. In fact, on most homes with sloped roofs, shiplap siding and no sealing of the top or bottom of the walls, drafts can be a major factor in heat loss. Air-sealing is generally cheap, easy to do and highly effective. Unlike other things like heat pumps, solar and triple-glazed windows, it is something that you can do yourself, which makes it a very good return on investment.

There are quite a few other things I did that did not warrant being called out specifically (I wanted to keep it simple) such as: insulating the hot-water pipes in the basement, insulating the ductwork in the basement, replacing an old fridge and adding a heat-pump hot water tank. I think all of these had very good returns on investment, but they were too small for me to be able to quantify with any confidence (except the fridge which paid for itself in 18 months on the electric bill savings). So, I do think they are important, and they have high ROI’s, but they each only cut my carbon footprint by relatively small amounts.

Can you use HITS to build a new house with a zero-carbon footprint?

Q: Does HITS apply to new construction?

A; The HITS recipe makes it fairly easy to make money by cutting your carbon footprint dramatically on existing houses. It is far easier to do the same on a new house. This is because it cost very little more to install 6” cavity walls and fill them with insulation than it does to install 4” cavity walls. If the sheathing (plywood) outside layer is made from structurally insulated panels (which are boards made of an insulating layer like a 2” ISO board glued to a plywood structural layer which is painted in the factory to have a water proof and vapor proof layer on the outside which then has the seams between the panels taped and sealed on site) you can prevent water penetration, vapor penetration, drafts, gain rigidity and insulation in a single installation. This takes far less labor time to install than it does to install each component separately. Adding triple-glazed low-E windows costs only a few % more than double-glazed windows. Hence, it costs very little extra to build new a house with an excellent thermal envelope that will dramatically cut the carbon footprint and heating bills than it does to build a standard house. Since the heating and cooling loads are far lower than in a code-built house, the house probably needs smaller heat pumps to heat and cool the house, which saves money compared to a standard house. If the house is designed to have the back facing south with no shade then the roof can generate all the electricity needed to go zero carbon zero bills. 

The moderator on our call, Bruce Sullivan, built his own house with 10” thick walls. He heats it entirely with a single air-sourced heat pump, even in the depths of winter in St. Louis. He powers the entire house with solar panels on his roof. He pays no utility bills. 

While I have not built one of my own, I think the ROI on newly built zero carbon, zero bills houses is excellent.