How accurate are the numbers

The bill calculation is checked against real utility bills — LADWP bills within ±3 cents, most SCE bills to the cent. The utility is found right for 99% of tested homes. Production hasn’t yet been checked against monitoring data of installed systems.

A forecast is only useful if you know how far to trust it. Here is what has been checked, and what hasn’t yet.

Checked against real data

Part	How it was checked	Result
LADWP bills	64 real LADWP bills, 2023–2026, recalculated from their usage	All within ±3 cents; 23 to the cent
LADWP solar credit	Credit use and surplus credit on real bills of homes with solar	Reproduced to the cent and within 2 cents
SCE bills without solar	19 real SCE bills	15 to the cent; the rest explained
Which utility serves the home	156 homes with a known utility	154 right (99%); the two misses sit on a utility border
Rate tables	SCE and Clean Power Alliance rates checked against the printed tariff sheets	No wrong numbers found

Not checked yet

Production against real systems. The model uses well-tested public engines and data (see How production is calculated), but it hasn’t yet been compared with monitoring data of installed systems. That work is in progress.
The SCE bill with solar over a full year against real Net Billing bills.

What always adds uncertainty

Weather: the calculation uses a typical year, real years vary.
How the household uses power by the hour — see Why the first-year forecast is a range.
Future rates and the rate increase you assume.
The roof as traced and the age of the aerial image.

Present the numbers as estimates — the proposal says so too.

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Updated October 6, 2026