A: SunPower panels are about 22% efficient. Solaria, Panasonic, LG, REC are about 20-21% efficient and there are many companies (mostly cheaper brands made in China) that are in the 15-19% range. However, most efficient electrically is not the same as most efficient economically. For this, calculate the cost per kilowatt-hour generated by the array over the period for which the electricity output from the panels is guaranteed by the manufacturer. Today this is almost always 25 years. So if you have a 10kW array generating 10,000kWh per year (both fairly typical in MA) and your array cost $30,000 before the subsides and $15,000 after the federal and state subsidies then your cost of electricity (sometimes called the levelized cost of electricity or LCOE) is $15,000 / (10,000 x 25) or about 6c per kWh. To me this is the better way to look at efficiency and not the stated electrical efficiency of the panels.
Q: What about snow collection on a flat-mounted array?
A: The panels can easily bear the snow load, including during that winter a few years ago when we had 6’ of snow.
Q: are there solar installers that will install on slate roofs?
A: Not that I know of. It is very difficult to seal penetrations through a slate roof.
Q: Is it true that panels in the future are to look like a traditional roof with tiles?
A: Tesla currently sells a roof tile system like this. However, in a recent interview with a Tesla employee who had installed them on his roof he stated that they cost 35% more than traditional panels plus the cost of the new roof. That sounds like terrible value for money to me.
Q: What happens if there is an issue with the roof, like a leak?
A: The panels need to be removed before the roof can be repaired. This is not as bad as it sounds as only one or two panels usually need to be removed and they just bolt onto the rails.
Q: Can you tell us the actual DC KW size of your PV array and the inverter(s)?
A: On the roof of our house is a 15kW array that produces about 15,000kWh a year with a 10kW and a 5kW inverter. On the roof of our garage (which is partly shaded) is a 13kW array that produces about 7,000kWh a year with a single 10kW inverter. On the roof of our rental property is a 17kW array that produces about 15,000kWh a year through a 10kW and a 5kW inverters.
Q: Will leasing panels still be financially beneficial?
A: Paying cash is the cheapest way to buy solar panels. Getting a loan is the second cheapest and leasing them is the most expensive. In this sense it is just like leasing a car. Unlike leasing a car, you cannot return the panels at the end of the lease. Solar leases can be very expensive compared to getting a loan but if your credit score is not high enough to get a cheap loan then leasing can still make sense.
Q: Completely disagree with your re solar hot water panels;We’ve now had it for 12 months and have never ran out of hot water in a 2-family house with 6 residents.
A: If your hot water is entirely coming from your solar thermal panels then please let me know. Most solar thermal panel systems come with back-up electric heating element so that you never have cold showers even on bitterly-cold, sunless days in winter. So, it is possible that your year-round hot showers are created by using electricity in winter and not by using the sun’s heat.
Q: If power from the utility is cut (storm, trees, down lines), the utility will NOT accept power from your panels. To survive an outage a battery or generator will then be needed; savings and eco-impact are then significantly different.
A: By law, solar panels must disconnect from the grid during a grid outage. This prevents the linemen from getting electrocuted by your solar electricity. If you have a battery you can, during a grid outage, reconnect that battery and the solar panels to your house “behind the meter” (meaning still disconnected from the grid). This is what I am currently installing at three properties. Now the battery and solar panels are acting like a back up generator.
Q: What percentage window to wall area do you recommend and should overhangs be utilized to prevent summer overheating?
A: The less the window area the better the thermal envelope will be. Often houses have about 15-20% window area compared to wall area (without the windows) but our house has 42% and we still have a zero carbon footprint. This is one of the main reasons we need so many solar panels to get to a zero-carbon footprint. Overhangs are great for allowing the sun in in winter and keeping it out in summer. The optimal width of the overhand is about 3’ at 9’ height and 6’ at 18’ height.