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 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.

Q:Tips on planning new construction for solar panels on the roof?;Metal roofing?

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.

Q:If you have a flat roof and have solar panels, can one walk on them if one needs to remove tons of snow in a bad winter?

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!

Q: What is the ROI for a battery to go with solar panels?

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.

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

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!

Q: battery disposal;a big problem?

A: This is very unlikely. Batteries contain a lot of valuable metals like lithium that is expensive to mine. Hence, just like aluminum cans, batteries will get recycled. However, the infrastructure to recycle batteries is not yet widespread.

Q: Can the battery back up really run the heat pumps and for how long?

A: A 10kWh battery can only power a heat pump for an hour or less. The better way to add a battery is to keep your fossil-fuel furnace and have the battery run only the emergency circuits on your house like the fridge, lights, burner circuit, circulating fans, water pump and a few outlets. Together these might draw about 10 amps, 1kW or about 1 kWh per hour so a 10kWh battery would last you 10 hours.

Q: My solar panels have been installed 5 years; would it be financially smart to get a battery when I am on the SREC program? I have 28 panels and have not paid an electric bil since they were installed even with my pool & EV.

A: Great news on your house, pool and EV – you are well ahead of the pack! Arrays that are in the SREC program cannot be later enrolled in SMART. Don’t be too sad though, the SREC subsidy is quite a bit more generous than the SMART subsidy. However, the Connected Solutions subsidy is not dependent on enrolling the array in SMART. The Connected Solutions subsidy is not well documented but, in my opinion, once an incentive is in the electrical tariff it is quite hard to get it out again. Since the Governor just signed a major climate change bill in MA that commits the state to substantial increases in renewable power, I am guessing that the Connected Solutions subsidy will be there for at least 10 years. This was what I assumed when recently I ordered batteries to go with my arrays. One of those batteries is not eligible for the SMART battery adder because the array is enrolled in SREC. The NPV on that battery is about     -$3,000 (the battery purchase price is close to $20,000) but I am doing it because my existing back-up generator is broken beyond repair and buying a new one would cost me about $3,000 plus $500 a year to get it serviced. There is no service cost on the battery.