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A: 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 of Zero Carbon Home), 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 of Zero Carbon Home), 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 don’t generate enough hot water in the winter and generate far too much in the summer.  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.

A: 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 of Zero Carbon Home), 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 of Zero Carbon Home), 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 don’t generate enough hot water in the winter and generate far too much in the summer.  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.

A: There are now three separate subsides for batteries in MA: if you have an array enrolled in SMART you get an “adder” for having a battery. The SMART subsidy in October 2020 is about 8c/kWh (note this deceases with time and varies with the utility company you use so please check what it is today). The adder for the battery is about 4c/kWh. For a typical 10kW array producing 10,000kWh a year the SMART subsidy is worth $800 a year and the battery adder is worth another $400 on top. Both are for 10 years so the adder is worth about $4,000. Both decrease with time – that is the structure of the SMART subsidy scheme. However, once you get your subsidy rate it is locked in for 10 years.
A Powerwall battery costs $10,000 to $15,000 fully installed. So the $4,000 SMART adder cuts the price by a third to a half. The SMART adder is (as is all SMART income) taxable income.  I have yet to see a solar installer admit this.

In addition, Eversource has a subsidy called Connected Solutions where ES pays you to have access to your battery during peak power demand. This payment seems to be about $1,200 a year but the it is not really clear exactly how this is calculated or for how many years it lasts. It is not clear if this is considered taxable income (like SMART) of if it is considered more like a rebate or a credit (the way net metering works, and that is not taxable income). If the Connected Solutions program lasts 10 years it would pay for the entire cost of the battery which seems excessively generous and therefore unlikely. However, it will be something else to reduce the cost to you. 

Finally there is a brand new subsidy, called CPEC (which was just announced in August 2020) or Clean Peak Energy Certificates. Power stations are now required to buy these CPECs to reduce their carbon emissions during peak demand on the grid. They are a bit like SRECs which is what SMART replaced. Peak demand (say on a hot summer afternoon when everyone has their AC on) brings the fossil fuel “peaker” plants into operation. These are not only the most expensive power-generating plants on the grid but they are often also the dirtiest, i.e., cause the most pollution and CO2 emissions. Hence, the state has a strong interest in reducing the use of these peaker plants through encouraging batteries. CPECs are brand new and no-one can even tell me how much I can earn from them or whether they are taxable income. However, it is sure to be favorable to you. So, overall, and despite all the uncertainty, batteries are looking like a better and better bet. I have ordered battery systems for both our home and our rental property.

My final point is not about batteries but about generators. A diesel or propane generator costs about $500 a year to set serviced so it will work when you actually need it. This is $5,000 over 10 years. The generator itself will probably cost you $3,000 up front. So, the 10-year cost of a generator is $8,000. The 10-year cost of a battery is between $5,000 and $10,000 with the current SMART subsidy alone. Add in Connected Solutions and CPEC and batteries are looking like a really good investment. Oh, and did I mention that they have a zero carbon footprint and emit no asthma-inducing soot, nitric oxide or sulfur dioxide?

Note this post was current as of October 2020. The subsidies for batteries are a moving target and one that changes by state, by utility, over time and even by zone you are in within a utility’s service area. Please check the latest information for your home before deciding to install a battery.

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 power your entire house electrical load. However, this is not how most people use a battery. Most people use them as an alternative to a diesel or propane back-up generator. So, the battery or generator is powering an emergency panel which usually runs just 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 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.

A: HDD will not have much influence on the payback period for solar panels. The biggest determinants of the payback on solar panels are what you are currently paying for utility electricity and the state-level subsidies. 

A: Metal standing-seam roofs are very good roofs that last well beyond the life of a shingle roof. Solar panels install easily on standing-seam roofs because the panels are clamped to the standing seams and no penetration of the roof is required. If I were designing a new house, this is how I would do it. I would also design it so that the panels fit exactly edge-to-edge on the roof, so the entire roof is just solar panels with no shingles or membrane showing. Keeping the sun off the waterproofing will make the waterproofing last much longer. It will also make the roof look much nicer because the solar panels will look like they are the roof rather than them looking like they are on the roof. If you use the all-black panels (most manufacturers offer them, but they are about ½ % less efficient than the ones with the grid pattern on the front) then it will be hard to tell there are solar panels on the roof. I think this is a better solution today than solar tiles.

A:  You cannot walk on solar panels. They need to be installed on a flat roof with some walking paths between them for maintenance and snow shoveling. In the first winter with solar panels I shoveled the snow off, but in subsequent years I did not. I calculated that I was only making about $2 worth of electricity on those winter days and it was taking me over an hour to shovel the snow off. $2 an hour is well below minimum wage!

A:  In most cases, yes. It will be expensive to remove the solar panels in order to replace the roof. However, this is not true is the roof is flat and the solar panels are held on to the roof with weights (called a ballasted system).

A: The answer to this question is very dependent on the subsidies available in your area and, in MA, it is also highly dependent on the size of the array you pair with the battery. 

Where we live in Massachusetts the subsidies for batteries are now very generous. When a battery is paired with a large array the battery can now be free. I have recently bought a pair of Generac batteries to go with each of my three arrays. I did not buy the Tesla battery because, even though the Powerwall battery is cheaper per kWh of storage than other batteries, Tesla cannot deliver batteries for several months. Since we are heading into winter, which is when the power outages are around us, the subsidies decline with time, and with today’s subsides the new batteries either save me money or work out cheaper than replacing my ancient generators, I decided to buy the batteries now. The economics of these three pairs of batteries are explained below.

When paired with a large array (20,000kWh/year in my case) the batteries (after the federal tax credit, the MA tax credit, the state subsidy (called SMART) and the Connected Solutions subsidy from our utility), are more than free – I am actually paid about $3,000 to own them.

When paired with a medium sized array (10,000kWh/year) the IRR on the battery and the array is 6.6% compared to 10% with no battery. So the battery is not free but it still makes an acceptable IRR and I will have a reliable back up which I do not have today with my old generator.

When paired with a small array (6,000kWh/year) the battery ends up costing my about $4,500. But my alternative is to buy a new generator (the old one is broken beyond repair) and pay $500 a year to get it serviced. So I think it is worth it to have back up power again.

A: In general batteries have two uses. 

The first is to take advantage of time-of-day (TOD) 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 purpose. 

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. In December 2020 I bought batteries to replace my back up generators at three houses.

The Tesla Powerwall is considerably cheaper per kilowatt-hour of electricity stored than other batteries from companies like Sonnen, LG Chem, Generac and Simpliphi. 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 failed to come on when we needed it – during a power outage. Over 10 years this maintenance cost is $5,000. So, the full cost of a back-up generator over its lifetime 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 battery to replace my defunct back-up generator in the near future. The regulations on qualifying for this adder are very complicated!