Year-three reports on home solar systems are rare. Most content in this space covers the purchase decision, the install day, and maybe a first-year summary. Zach, a homeowner in Arizona who installed a 10 kW solar array and two Tesla Powerwalls in 2022, has been publishing annual data reports since, and this third-year update is the most useful one yet because the numbers now carry enough history to show trends. In 2025 the system generated 18,030 kWh, about 7 percent above the original projections from PVWatts. Battery degradation sits at 6 percent after three years, up from 4 percent for most of the year. Total household electricity cost for 2025, covering all home consumption plus charging two electric vehicles across roughly 22,000 miles, came to $1,937 for the full year. No gasoline purchases.
The degradation figure is worth putting in context. Tesla's Powerwall warranty guarantees 70 percent capacity retention after 10 years. At 6 percent degradation in three years, Zach is tracking well inside that threshold. Battery degradation is also not linear: cells typically lose capacity faster in early cycling and then slow down. The more worrying trajectory would be if the figure climbed steeply between years two and three, which it did not. The roundtrip efficiency, the ratio of energy stored to energy delivered, came in at 89 percent, consistent with Tesla's published spec for AC-coupled systems. For comparison, the newer Powerwall 3 uses a DC-coupled architecture that can achieve slightly higher roundtrip efficiency by reducing conversion steps, though exact real-world figures from long-term owners are still accumulating. The solar generation itself is interesting: 235 kWh less than 2024, which Zach attributes to normal seasonal variation rather than panel degradation, given the system is still outperforming its original projections by a comfortable margin.
The savings math lands in a place that is honest and a little complicated. Without solar, Zach estimates his annual utility bill would have been approximately $4,320, including the grid electricity equivalent for his EV charging. His actual 2025 bill was $1,937, a savings of about $2,383. His loan payment on the solar system is $200 a month, or $2,400 annually. Net result: he paid $17 more in 2025 than he would have without the system. He frames this as a win, and there is a reasonable argument for that framing: $17 buys backup power capability, protection from future rate increases, and the ability to charge two cars without touching a gas station. His utility, Trico in Arizona, offers a low solar export buyback rate of around 4.5 cents per kWh, which limits the financial upside compared to utilities in states with better net metering terms. In California or Massachusetts, with higher retail rates and stronger buyback programs, the same system would be generating meaningful monthly savings rather than a near-breakeven.
Bottom line: This is the kind of report that actually helps people make decisions. The honest answer on home solar and battery storage is that the financial outcome depends heavily on your utility's rate structure, your buyback rate, and whether you finance the system. In Arizona with a low-buyback utility, a financed 10 kW system with two Powerwalls more or less breaks even while delivering backup power and EV charging. In a high-rate state with good net metering, the same setup generates real monthly savings. Know your utility's numbers before you sign anything.