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SolarRoof.Cool: Grid Tied Solar System

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The Grid

Grid Connection

Our system is grid-tied. This means we're still hooked up to a traditional utility (PG&E, in our case). We have our system configured to use it as little as possible—it’s a backup for us.

Because of the Powerwall—and contrary to most solar energy customers—we don’t care about grid import/export on a daily scale.

In the winter, we rely on the grid to make up for the winter loss of production, mostly influenced by the low Sun angle and cloudy and rainy weather. For us—based on my rough estimates when I publish this in January of 2020—we get about 40% of our forecasted typical solar production during the winter months, which results in a self-powered percentage of about 60% most days. (This is due to the low Sun angle not casting as much light on our northwest facing array.) In the summer, we’ll feed our excess power to the grid, which offsets our winter energy use.

Net Energy Metering

Net Energy Metering lets you sell your excess energy to your utility company at retail price for credit. You can then use this credit to buy power at times when your energy installation does not meet your needs. Our utility, PG&E, gives us a monetary credit for excess energy (rather than a kWh credit), and charges more for electricity after sunset then for when we’re selling it back to them.

Energy Plans + Time-of-Use Pricing

PG&E is our energy utility, and they force us into a time-of-use energy plan as a solar customer; energy prices change based on time of day. We’ve selected PG&E’s EV2-A pricing plan intended for customers with EVs or home battery storage—of which, we have both. With this plan, energy costs are tripled in the evening hours, but quite cheap during off-peak hours.


Pricing Time of Day Price Delta
Off-Peak 12am - 3pm 17¢ 1.0x
Shoulder 3pm-4pm, 11pm - 12am 37¢ 2.1x
Peak 4pm - 11pm 48¢ 2.8x

If we didn’t have Powerwall, this would be pretty non-economical for us—the excess energy generated during the day would be totally negated by the higher nighttime pricing. For the sake of fairness, there are other time-of-use energy plans available for solar customers, where peak and off-peak pricing are quite closer. Take PG&E’s E-TOU-3C rate plan, with a mouthful of a marketing name:

PG&E Residential Time-of-Use (Peak Pricing 4 - 9 p.m. Every Day)

Pricing Time of Day Price Delta Delta over EV2-A Base
Off-Peak 9pm - 3pm 33¢ 1.0x 1.9x
Peak 3pm - 9pm 40¢ 1.2x 2.5x

Even though the difference between peak and off-peak is just 7 cents on this plan, for the average solar customer, that would mean you’d lose about 1/5th of the power you back-feed into the grid. That’s a lot to lose—roughly equivalent to keeping my house powered for two hours—but, if you think about it, that’s the “tax” of paying the grid to be a “battery” for you.

If you install a home battery, then you get to unlock that 17 cent rate for all of your power, because you can optimize your battery use to never pull from the grid during Peak hours. (And, if the conditions are right, you can use the Tesla app's Time-Based Control option to feed into the excess power into the grid rather than your Powerwalls during Shoulder and Peak times—my system does still generate energy past 4 p.m.)

The Duck Curve of Energy Demand

(or, why energy is so expensive after sunset)

The rapid adoption of solar energy in California hasn’t been all that kind to its grid. Most solar installations back feed into the grid when their excess energy isn’t needed, using it as a short-term “battery” to use when the Sun goes down. This creates a substantial drop in the net demand of energy, colloquially called the duck curve.

Even though the total demand for electricity is still a smooth curve throughout the day, renewables reduce the net demand, creating an artificial demand spike in the evenings. By the pure nature of supply and demand, energy is at its most expensive at night.

(I haven’t seen this discussed enough: in my opinion, the duck curve is the greatest threat to the continued adoption of clean, renewable energy, at least in California—putting the continued existence of Net Energy Metering and solar investment tax incentives at risk. The reality of climate change and the environmental crisis is important enough that there should be no threat to these incentives, and there should be better plans and options for energy storage installations, as well as renewable energy access for low-income families.)

Installing Powerwall to avoid the grid: Money isn't everything

Money isn’t everything

You can argue that I still won’t save tons of money by unlocking such a low off-peak price by having a Powerwall, that power outages are infrequent, and we should get over it, or that it will take too long to pay itself off. The justification isn’t just that.

Powerwall also shrouds me and my investment from PG&E, my local utility company...and their general inability to run a local utility company.

I have no reason to believe they will be financially solvent or continue to operate in five or ten years, or won’t just dramatically hike energy prices. And, I think that’s an incredibly realistic assumption!

It’s also a logical conclusion that the amplitude of the duck curve will only continue to grow as more solar comes online in California. Installing Powerwall now insulates me long-term from higher and higher peak pricing or a world where advantageous net energy metering ceases to exist.

But what about going off-grid?

Grid-tied vs. Off-grid installations

It would be more “fun” to have an off-grid install, but this isn’t entirely realistic.

If we were to consider going entirely off-grid, we would need substantially more solar (to generate enough to make it through winter days) and considerably more storage (to store tons more energy or risk losing it if the batteries fill). As a quick estimation, it would more than double the cost of the system, and we plain don’t have the roof space.

Instead, the Tesla “Max Solar” configuration we chose pragmatically meets most of our energy needs. We’ll only need to rely on the grid more as a buffer to supplement energy during bad weather and to feed excess power during streaks of great weather and sunny days.

The only reason why I include this bit is that I get this question all the time. Also, the legality of going entirely off-grid in a typical residential installation in California is...dubious. An off-grid install wasn’t even a consideration for us because we don’t need it. Grid-tied works just fine.

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