Solar complete

Solar and battery and generator.

Finally, more than six months after we received the initial proposal from a local solar-power company (Solaflect), we have a complete system installed: last week, they completed the installation of a battery back-up solution from a German company called Sonnen. Our house can now be supplied by solar power, grid power, battery power, or propane-generator power, depending on the situation. Although I won’t dig into all the details, it’s pretty cool, so read on…

diagram of power flowing
A screenshot from the Sonnen battery portal showing power flowing from the solar tracker into the house (supporting its consumption) and into the battery (keeping it charged) and into the grid (selling power back to the electric company). The actual graphic is animated, showing power ‘flowing’ from the top to the other three destinations.

Although our solar panels, mounted on a two-axis tracker, were installed back in November, and the tracker base was installed in early September, we’ve been eager for the final phase to be installed. (The pandemic has created an intense boom in regional real estate, as city people seek to move to rural areas like ours, buying up old homes and having them renovated… leading to a severe shortage of electricians and other contractors. As a result, our local vendor (Solaflect) fell far behind on the installation of solar trackers and, in particular, the battery-backup systems they source from Sonnen.)

We were able to take delivery of the 10 kWh battery-backup hardware at the end of December, but it was February before it could be installed, configured, and tested. It sat in boxes in our basement for a couple of months…

Once installed, it is a compact floor-to-ceiling box within the mechanical room, with analog and digital electronics at the top and a set of large battery packs at the bottom.

Top of the Sonnen cabinet, showing the analog electronics at top-left, digital controls at top-right and on the open door at right. Battery packs are in black at bottom.
Close-up of the four black battery packs, together comprising 10kWh. The bottom (closed) compartment can hold another 10kWh if we decide to expand.

The coolest thing about this system is that it has the smarts to manage our power consumption, generation, and storage. When the sun is shining, it directs the power to support the demands of the house; if there is excess power and the battery is low, it charges the battery; if there is excess power and the battery is fully charged, it pushes power back into the grid (selling power to the electric company).

And, when the sun is not shining, it powers the house off the battery until the battery is low, then pulls power from the grid. Even cooler, when the grid is offline – sadly, all too common around here – it will power the house off solar and battery while possible, and if that’s not enough, it will automatically start the propane generator to keep the house supplied with power and recharge the battery.

Our second full day of operation (Friday 26 February) provides an excellent example; as it happens, this day was our as-yet record-setting day of solar production, 40.22 kWh according to the Fronius inverter mounted on the tracker itself:

A record-setting production on a perfectly sunny day.

Our tracker has some trees and a hill to the east, so its production starts off slowly; later, when the sun is setting, it loses power as the sun sets behind the trees. Most of the day, however, the tracker maxed out at 5.77 kW (not 6.0 kW, long story) and resulted in a total production of 40.22 kWh. The Sonnen battery system receives this power and computes its own metrics about production as well as consumption, below; differences in the measurement technique, and maybe a tiny bit of power lost in transmission from tracker to house, led Sonnen to report that we produced only 36.4 kWh:

What I like about this diagram, though, is that it compares production with consumption; overall we produced more than consumed, meaning we ran the house fully off solar power and pushed excess power back into the grid for others to use. That said, we can get a more detailed view:

Graph of power consumed and produced, and battery state of charge.

Here we can see the home power usage in dark blue, roughly 1-2 kW as a base load, with some spikes caused by our activities; the support of that consumption by solar production in light blue; the total solar generation in yellow; and the battery level as a green line. We can see that, when the sun was up, the home’s power needs were more than met by the sun, and the excess power was used to charge the battery and then supply the grid; after sunset, the home drew down the battery, which leveled off at its configured minimum charge (15%) by approx 10pm; overnight, then, the home was supplied by grid power.

Cool! Our home was powered by the sun, from approx 10am-10pm. Unfortunately, the following day (Saturday 27 February) was cloudy and snowing:

We generated almost no solar power, supplying only a fraction of the home’s needs (light blue in this diagram). This sad state is reflected in the stats for the day:

Anyway, it’s all pretty interesting. As we head toward the equinox, and then solstice, I’m optimistic we’ll see days with far more production. Meanwhile, this experience already has changed my way of thinking. A glance out the window makes me think “huh, the sun is shining, now is a good time to run the dishwasher” or “hmm, it’s snowing, maybe I’ll run my laundry tomorrow when it’s supposed to be sunny.”

All of these musings don’t reflect the presence of an electric car, my Tesla; it is sadly still in the shop due to the accident two weeks ago. When it returns, we have the system configured to charge the Tesla from the sun, but not from the battery… the Tesla charger pulls so much current it would overwhelm and shut off the battery system. This is one downside to our design: if the grid is down, and the sun is down, we can’t charge the car off the battery or generator. (I’m not too worried; we can drive it to a supercharger less than 20mi away if needed.)

Ok, enough geeking out on solar power. I still have a lot of questions about the nuances of operation, so I may be back in a few months with new observations once we have a better handle on system operation and performance.

Author: dfkotz

David Kotz is an outdoor enthusiast, traveller, husband, and father of three. He is also a Professor of Computer Science at Dartmouth College.

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