Year

Projected solar is NREL's PVWatts typical-year estimate for a 23.31 kW DC array at 20° tilt facing 180°, 35.9 MWh a year. Each bill's projection covers its billing period, scaled day by day to the array in service: 17.85 kW from Oct 15, 2023, 23.31 kW from Jul 3, 2026; months not billed yet use the calendar month at today's size. The line adds them up from January 1.

The typical weather range is where the running total lands in 8 of 10 synthetic years built from the 19 measured bills since install: each bill's actual ÷ projected solar (on average 4.5% above the projection, σ = 8.7% a bill), strung together in blocks of 3 consecutive bills from random starting points, 2,000 times. Blocks keep whatever runs of good or poor months the bills have, so the range doesn't assume months are independent; the bills' month-to-month correlation is −0.15 (a bill boundary moves a few days of sun between neighbouring bills, so a strong month tends to sit next to a weaker one). The range is centred on how the bills have actually run, not on the projection, and by December it spans −1.8% to +9.1% of the projected year. 19 bills are under two years, so a whole year better or worse than any on file isn't in it yet. Estimated and partly metered bills are left out.

Comparisons with 2022 use the same bill months, and only months both years had solar for the whole bill: the system went into service on October 15, 2023, so earlier bills and the install bill itself don't count. When only some months qualify the answer and the Used tile name them, and with none there is no comparison. The 2022 line on the solar chart stops before its first bill without solar. House use against 2022 compares each such bill with the one a year earlier, hatched when either is estimated. Self-powered is 1 − Σ grid_import_kwh ÷ Σ home_kwh over the year's bills. What you'd have paid is the bill plus that bill's savings (realized plus credit banked, at its rate; a bill that drew on banked credit saves less than zero). The best day ignores any daily total above the array's size × 12 hours, which only a broken meter read can produce, and any estimated day. Hatched marks are estimated or partly metered bills; they count in the totals.

2023's solar avoided about 1.2 t of CO₂, roughly what 20 trees take up in ten years. CO₂ at 0.355 kg CO₂ per kWh, EPA eGRID2023 FRCC; trees from the EPA Greenhouse Gas Equivalencies Calculator (16.5 per tonne).