What is the ROI (return on investment) of a Tesla Powerwall battery?

Q: Please comment on the utility/ROI of Tesla power walls. 

A: In general batteries have two uses. 


The first is to take advantage of TOD (time of day) tariffs. A TOD tariff is where your electricity company charges different rates for electricity at different times of the day. You can use this TOD tariff to buy electricity at a cheaper rate and sell it back at a higher rate. We do not have a TOD tariff where we live so I have no direct experience of using batteries for this application. 


The second use is as an alternative to a diesel or propane emergency back-up generator for when the power lines go down in a storm. I have not bought a battery for this application but I am planning to do so. 


The Tesla Powerwall is considerably cheaper (at about $500 per kWh of storage) per kilowatt-hour of storage than other batteries from companies like Sonnen and Simpliphi. I had a quote from Simpliphi that was $1,333 per kWh). However, at about $7,000 it is considerably more expensive than a diesel generator which costs $2-3,000. However, I have a propane back-up generator and it costs me $500 a year to get it serviced so that it actually works when we need it. I have learned this lesson the hard way – our previous back-up generator was not serviced and it stopped working right when we needed it. Over 10 years this maintenance cost is $5,000. So the full cost of a back-up generator is about the same as a Powerwall. 


Also, in Massachusetts, the new SMART subsidy program for solar panels includes an extra subsidy (an “adder” they call it) for a battery. This is currently about 4c per kWh generated by the solar panels. If you have a 10kW array generating about 10,000 kWh a year then this is worth $400 a year for 10 years which brings the cost of the Powerwall down to about $3,000 which makes it similar to the upfront cost of a diesel back-up generator. Only now you have no maintenance cost. This is why I intend to install a Powerwall to replace my defunct back-up generator in the near future. The regulations on qualifying for this adder are very complicated!

Should I insulate my doors?

Q: What about insulated non-glass doors – are they am important adjunct to triple glazed windows?

A: A typical wood door is only about R1-2 or only slightly better than a single-glazed window. So, having insulated doors will help cut your carbon footprint. However, I do not know of any way to improve the insulation of a door other than by replacing it. So you would need to do the calculations to see how much carbon footprint and money you would save. Very roughly, if you improved the R value of a door by R1 (say from R2 to R3) you might save about $40 a year on heating bills. Before you do this though, I would make sure the door has no drafts around it. Sealing these drafts with weatherstripping is cheap, easy and effective. 

Is it OK to measure the R-value of a window at the center of the glass?

Q: How are you calculating the R value? are you using the center of glass U value only? should you consider the frame? 

A: The R-values I measured for the window inserts are taken in the center of the glass or plastic. As you can see from the infrared photographs in Chapter 3 of Zero Carbon Home, the frames are all considerably warmer than the glass or plastic sheet. The frames also have far smaller area than the glass or plastic sheet. Hence I think that the center of the glass measurement is a good approximation of the overall performance of the window insert. 

What is the embodied carbon footprint of a solar panel?

Q: Can you discuss the embodied energy and natural resource use of the active systems, such as solar panels, that are necessary to achieve a “net zero” status? 

A: The embodied carbon footprint of solar panels (i.e., the carbon dioxide released by making the panel) is about equal to 18 months worth of the carbon footprint avoided by generating zero carbon electricity. So, from a carbon footprint  perspective, you can think of the solar panel paying for itself in about 18 months. Financially, ours will pay for themselves in about 6 years. 

Is it worth adding insulation to a ceiling with 8″ already?

Q: Could you talk a little about financials of adding iso board to a pitched roof-cathedral ceiling with 8″ cellulose in rafter bays and shingles that otherwise wouldn’t be ready for replacement?

A: If you have 8” of insulation already, then adding more insulation is unlikely to save enough money on the bills to pay back the investment in a reasonable period of time. I have found with my client work that even with 6” of insulation already, the savings on going to 12” is only about $150 a year. If you have no insulation today it is worth adding 12” but going from 6” to 12” will not save very much money. Please see also my other recent post on cathedral ceilings.

Insulating a cathedral ceiling

Q: Insulation recommendations for cathedral ceilings?

A: Cathedral ceilings are tricky to insulate. Most building codes require them to be vented to remove the inevitable condensation caused by warm, moist, air rising to the top of the ceiling and penetrating the ceiling through light fixtures, skylights, cracks and just from diffusion through the drywall. The moisture in this air then condenses when it hits the cold roof surface. If this does not evaporate you will get mold and rot. You can now get vapor barriers especially designed for this situation. One supplier is Majrex in Switzerland. I have not used this myself but I intend to use it when converting unfinished space in our rental property to finished space. With a proper vapor barrier you can either add insulation behind the barrier or in front of it (lowering the ceiling). Check out Martin Halladay’s posts on this topic on Green Building Advisor.

How do I improve the insulation on historic windows?

Q: Do you know about the availability of high efficiency windows that will be acceptable for homes in historic districts with strict regulations to adhering to historic accuracy?


A: I think probably the best way to deal with this is to add the window inserts you can see in Chapter 3 of Zero Carbon Home. These are invisible from the outside so historical commissions usually are OK. If you use the glass ones and get the low-E version you can add about R2 to an existing window. If the existing window is R1 you now have an R3 window, which isn’t great compared to modern low-E triple-glazed windows at R4-5 but it is 50% better than a plain double-glazed window. They are also far cheaper than replacing windows and you can fit them yourself, you don’t need to hire a contractor. 

A heat pump for a greenhouse?

Q: What about installing the heat pump in a greenhouse? Maybe removable for the summer.

A: I have not done this in a greenhouse  but I think it is a good idea. Effectively I have done this by bringing plants indoors in the fall and keeping them in our sunroom. The sunroom receives a very small amount of heat from our house heat pumps (it is at the end of the ductwork) but, using LED grow lights (powered by my solar panels) I was able to get red ripe peppers at Christmas. They tasted great!

I have no insulation in my walls

Q: So your walls are insulated with 4″ fiberglass batting?  Earlier I had the impression that your house has no wall insulation.  What about homes much older that ’74 with NO wall insulation? 

A: Yes our house has 2” by 4” stud cavity walls filled with fiberglass. The only answer, John, is to get wall insulation. You can do this without having to remove and replace all your siding by blowing in spray foam or dense packed cellulose from the outside. This requires drilling holes in the siding but these can be patched afterwards or a small section of the siding can be replaced. If you have no insulation today in your walls you are losing money through the  walls the way that water runs through a sieve. 

Should I insulate a crawl space?

Q: What about sealing/insulating crawlspace walls and plastic on soil to insulate crawlspace? 

A: Insulating walls in crawlspaces is a very good idea. It is a lot easier to insulate the underside of the floor than to try to insulate the soil because you can just push fiberglass in between the joists. 

Response: Hi David, Thank you for replying. I had noticed condensation problems in the crawl space due to uninsulated HVAC ducts sweating in the summer. Venting the space in the summer allows humid air into the space where colder pipes sweat badly. Researched this and found a revised opinion on what to do with crawl spaces. The advice I read (sorry, no references) was to seal the wall vents, insulate the inner walls and cover the soil base with heavier plastic. Wetness due to water inflow may require drains and a sump pump. I am moving in that direction (DIY) having previously insulated the joists under the floor and ducting but still experiencing the moisture issue. Thank you again for sharing.

Reply: It sounds like you have two sources of moisture in the crawl space. The vents and the soil. I think you will need to deal with both to stop the water condensation on the HVAC ducts when the AC is running. Sealing the vents will help but without dealing with that wet soil it will probably not be enough. I think the plastic sheeting on the soil will help, but it is a band aid, not a cure. The real question is why is there so much moisture in the floor in the first place? Is the ground water high near you? If not, are your gutters in good shape? Overflowing gutters can put a lot of water in the soil right by the house and this will wet the soil under the crawl space. I have seen this and repairing the gutters (I put a perforated metal plate on the  top of the gutter to keep the leaves out) and replacing split downspouts (and adding extenders to the bottom of the downspouts to keep the water away from the foundation), worked. I also sealed cracks in the basement concrete with a can of spray foam – cheap and effective as a water barrier as well as an air barrier. Once you have solved this problem, I think replacing your hot water tank with a heat-pump hot water tank will help. It will not only cut your bills and carbon footprint but it will dehumidify the basement air too. Once you have done all this, I would insulate the HVAC ducts, but if you don’t deal with the moisture first, you will risk getting dampness and mold on the insulation.


Yours sustainably,
David Green
Make money by cutting your home’s carbon footprint to zero. We have done it.