Solar panels in cloudy Seattle work better than you might expect
Notes for homeowners thinking about installing solar panels in the Pacific Northwest
Earlier this year I had Solar Panels installed on my roof here in the greater Seattle area. The outcome has exceeded my expectations and I wanted to share what I learnt during the process with others who might be considering a similar project.
Can Solar power work in cloudy climates? (Hint: Yes)
Prior to the install, I had been thinking about installing Solar Panels for over a year. One of the factors that held me back was a lack of clear information on whether Photovoltaic (PV) based systems were viable in Seattle. The city is notorious for overcast weather — Seattle averages 201 cloudy days a year.
While researching this topic, a lot of the information I came across had a marketing bent, other sources abstracted too much of the information leaving me guessing as to the assumptions being made.
Fortunately the National Renewable Energy Laboratory (NREL) has a detailed PVWatts tool to estimate energy production from PV systems throughout the United States. For illustration, I have captured a screenshot of the projected generation capacity for my system over the course of a year below.
PVWatts was one the better calculators I found. It utilized hard data to project expected generation over the course of the year. Since it was developed by NREL it avoids the conflict of interest that comes from promotional material.
With respect to accuracy, the table below compares the real world output of my system in June and July (the first full months of generation since install) with the PVWatts estimates. For both months the system generated more power than the estimates, but was within acceptable variance of what the PVWatts calculator estimates.
One of the big takeaways is that electricity generation occurs on overcast days (though sunny days are better!). In the month of December days are short and overcast yet generation still occurs (albeit at a lower rate). Surprisingly the differential between sunny San Francisco and cloudy Seattle was far smaller than I expected. San Francisco has almost 3x as many clear sunny days than Seattle as well much longer days in winter, but projected generation rates in Seattle are only a third less.
Kilowatts (kW), Kilowatt Hours (kWh) and system sizing
PV systems are sized based on the rate they generate power under ideal conditions. A solar provider will quote an estimate based on the rate the system can generate energy in ideal conditions (kW), while your utility will charge you for power consumed based on the amount of electricity consumed (kWh).
1 kW over 1 hour = 1kWh
As a general rule of thumb, here in the Pacific Northwest the annual generation capacity of a 1kW system is 1000kWh — this makes back of the envelope math simple. As an example, if you had a 4kW system this would generate roughly 4000kWh annually. If your utility charged $0.10 per kWh this would reduce your energy bill $400 annually. Of course for more accurate estimates you can leverage the PVWatts calculator.
Since systems are sold based on generation capacity, solar panel efficiency doesn’t impact your expected output. Said another way — a 1000kW system with commodity panels will generate the same output as a 1000kW system based on the latest high efficiency solar panels.
Rather than pivot your system around the panel (of which there are many), it is better to start with the annual generation capacity you need (reference your utility bill for this), size your system accordingly and obtain proposals for systems that can meet your needs.
One of the reasons to pay a $/watt premium for a high efficiency panel is when you have limited roof real estate to physically place all the panels you need. There are also other considerations such as aesthetics as well as warranties — more on that further on.
Why I went with Tesla Solar
When choosing an solar provider I considered various options, ultimately I end up going with Tesla solar.
It is easy to overlook the fact that Tesla has a Solar Panel offering since so many articles are dedicated to their Electric Vehicles and Solar Roof. It is also easy to confuse the Solar Panel and Solar Roof product offerings from Tesla. Since I had a newer roof, the Solar Panel offering was more applicable to me. There are several reasons why I ultimately went with Tesla.
Turnkey competitive pricing — With the exception of Tesla, pricing is still relatively opaque and requires time and effort to get bids to compare. I had expected Tesla to be the high cost provider, but it turned out to be highly competitive on price. There was no hassles or up-charges, what I saw on their website was exactly what I paid all in. Tesla took care of permits and made sure the process went smoothly.
Tesla adjusts prices regularly. At the time of writing this, their systems work out to approximately $2/Watt before federal tax incentive and only $1.49/Watt when when factoring in the tax credit. EnergySage is another portal you might want to consider to obtain bids. It is interesting to note that Tesla’s prices were far below the average install price for US systems
Tesla will be there to stand behind their product — PV System lifespans are measured in decades. Decade plus warranties are standard in the business. I had a lot more confidence about Tesla being around a decade in the future to honor any warranty than other smaller providers. Based on brand reputation I also had increased confidence that Tesla would make sure any unexpected issues were resolved to my satisfaction.
Superior software — Most solar providers use commodity equipment with the exception of the interface the homeowner uses to consume analytics data. Tesla has the scale to build a world class experience that is intuitive and sets it apart. I already had a Tesla EV so the integrated experience in the Tesla mobile application was a big draw for me.
If you are thinking about going with Tesla Solar, you can use my referral code to get a small credit on your system if you do not already have one.
Closing thoughts
I am extremely happy with the decision to go solar. Frankly it is still a little surreal that our entire household (AC, Fridges, Laundry etc.) as well as our EV have been powered completely by the sun over the last two months. Below is a screenshot of our last bill from our utility Puget Sound Energy.
I’ve been working on calculating the carbon offset in terms of tons of coal burnt. More on that in my next post!
