Why the same mistakes keep happening
Most home solar errors are not arithmetic errors — they are assumption errors. The formula is right, but the input is idealized: a round number, an optimistic rate, a best-case month. Below are the mistakes behind most bad results, grouped by the calculator where they bite.
Solar System Size Calculator
Sizing from a neighbor's system without checking their usage, orientation, or rate — all three move the answer.
Believing panel ratings: a "450W panel" produces ~350W in real conditions after derate.
Sizing to 150% offset in markets without full net metering, then exporting power for pennies.
Solar Payback Calculator
Taking the first quote — price-per-watt spreads of 30–50% exist in the same town for identical hardware.
Counting the 30% credit as an immediate discount — it arrives with your tax filing and only if you owe that much tax.
Installing over an aging roof, then paying double to remove and remount the array for the re-roof.
Solar Production Estimate
Accepting production estimates without a shading analysis — the #1 source of underperforming systems.
Assuming north-facing roofs are fine with "high-efficiency panels" — physics wins; orientation matters more.
Ignoring the seasonal shape: a system sized for winter production is oversized and uneconomic in summer.
Solar Bill Offset Calculator
Expecting a $0 bill — fixed charges survive even perfect offset.
Evaluating a system under full net metering when your utility has already moved to reduced export credits.
Sizing for the roof instead of the usage, then exporting summer surplus for pennies.
The habit that fixes all of them
Write down the assumption you are least sure about every time you run a number. If the answer matters, test it: change that one input by ±20% and see whether the decision flips. If it flips, the assumption — not the math — is your real problem, and it deserves the research time.