The Monkey House

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Major Milestone: 50MWh

It's been almost five years since the last solar update and it's been smooth sailing. The trees keep growing and encroaching a little bit more every year, but not much. All the equipment has kept working without a hitch, so it was just a matter of time before we reached 50MWh. 50,000kWh. Over $10,000 worth of energy.

Retrospective: new panels

When I expanded the system half a decade ago, I expected that "the new panels will provide 2400/5700 as much electricity as my existing array, or slightly under 3MWh per year." How did that pan out? I'm going to score that prediction as 50% correct. The new panels have yielded just a tiny bit over 2.5MWh per year which I think is more than "slightly under" the 3MWh target, for a miss on that half of my prediction. On the other hand, I also predicted they'd yield 2400/5700, or 42.1%, as much as the rest of the array; in reality, they have yielded 44.6% as much as the rest of the array, slightly exceeding my prediction.

Retrospective: overall solar project

I now have almost 7 years of real-world data, so let's revisit some of the questions I asked way back in part three when I only had about a year's worth of data.

Show me the money: payback progress and rate structures

Including the cost of the expansion, my 8.1kWh system cost just a hair under $15,000. At the 50MWh mark, six and a half years in, I've saved $10,070 in electricity costs. That puts me on track for a simple payback period of about 9.8 years. But that ignores the fact that the first two years, before the expansion, were yielding less power and less value and including them in the analysis underestimates the current payback rate by about 6%. Extrapolating based on just the past four and a half years leads to an estimated payback period of 9.5 years. In reality, it will probably be closer to ten years, given the likelihood that the trees will keep growing and the solar panels will continue to age at something like 1% per year. That's right in line with the optimistic end of the initial estimate, and a bit behind the revised estimate I made when the new panels got installed. The first couple years were, I think, slightly sunnier than average and they skewed my projections slightly.

I'm on time-of-use rates for electricity, betting that the price for my purchased electricity will go up only slightly, while the price paid for the electricity I sell to my local utility will go up quite a bit. Through the six years of rate changes, this has remained true: every month of the year, including the winter months with little solar generation, has seen the combination of solar panels and ToU billing save me more money than the solar panels on their own. On average, I'm currently being paid 19 cents per kWh I sell, as compared to 15 cents per kWh that I buy. This compares favorably to the 16.6 cents per kWh flat rate. Though note that the 15 cents per kWh is NOT what I would be paying for electricity that I purchase if I did not have solar panels: without panels, I would be buying more on-peak electricity, which would drive up my average cost. Due to the changing ToU rate structure, the cost I pay for electricity I purchase has risen by 0.16 cents per kWh per year, while the price I'm paid for electricity I sell has only risen by 0.12 cents per kWh per year. This effect reduces the value of my solar panels by approximately $12/yr, a bit less than losses due to shading or panel degradation.

Speaking of rate structures, the anticipated decline in net metering's generosity has begun: in 2023 both electric utilities serving my area have opened new rate cases with the public utility commission seeking to, in very broad strokes, halve the value of net metering for new customers starting next year. For the moment, existing customers like me can remain on the old rate. There's considerable local opposition to this plan, among residential customers and larger commercial users as well as both the city and county governments. I expect a reduction in the value of net metering for new customers this year, though less than the utilities have asked for. If I remain grandfathered into the old rates for at least three more years, my payback period will be unaffected.

Wear-out: solar panel and inverter degradation

There is too much weather variation from year to year in the amount of sunlight my panels receive to estimate the actual rate of photon-induced degradation of the panels, and reduction in efficiency of the inverter. If I had several integrating light meters gathering real-time solar irradiation data, I could subtract out the weather and foliage variation to isolate the contribution of changes to the system, but I do not have that data available.


Based on the first year's worth of data, my self-consumption rate was 37% and that has remained surprisingly steady, despite the addition of more solar panels and a heat pump for home space heating. Over the full period, I am now at 38% self-consumption, with the remaining 62% of the panels' production sold to my electric utility and purchased back later. ToU rates encourage me to minimize daytime electricity use and thus self-consumption, so I continue to charge my car off-peak.

Over the life of the panels so far, generation has covered 71% of my household demand, or 81% excluding the electric car. But recently my air conditioner broke and needed replacement; I replaced it with a heat pump which now provides the majority of our home's heating and that has significantly increased our household demand. Over the past 12 months, the solar panels are only providing 54% of household demand, or 59% excluding the EV.

What about batteries?

Still no, not yet. Battery prices have risen sharply in the past few years, offsetting some of the long-term price drop. The downward trajectory is expected to continue, but prices remain too high to be economically justifiable where I live.

Looking Ahead

My inverter has some spare capacity in one string, which theoretically would allow me to expand the system by about 25%, though I lack the roof space to accommodate those panels. Given the new heat pump demand, I will likely do one last expansion to add as much additional capacity as I can, probably in the 15-20% department, which might push up my yields as high as 2/3 of my total household consumption. That will be the end of the PV solar project, and it will go into maintenance mode which should consist of removing and replacing the panels when the roof needs to be replaced, and replacing the inverter when it dies in approximately 15 years.

There may be a brief update covering a second expansion; otherwise I anticipate a next update in 5-6 years when I hit 100MWh.

Version 0.3     |     Originally written: 05 July, 2023     |     Latest revision: September 10, 2023     |     Page last generated: 2023-09-10 10:11 CDT