Why the same mistakes keep happening
Most EV charging 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.
Home Charging Cost Calculator
Using the headline utility rate while ignoring that delivery charges push the real all-in rate 30–50% higher.
Forgetting charging losses — comparing EVs to gas without them flatters the EV by about a dollar per charge.
Assuming one rate all day; overnight windows on time-of-use plans change the math completely.
Gas vs Electric Cost per Mile
Comparing home charging costs against gas prices without admitting your actual charging mix (road trips count).
Using MPGe directly as MPG — 100 MPGe is not 100 MPG; convert to mi/kWh first.
Ignoring time-of-use plans that would cut the electric side by a third.
EV Road Trip Stop Planner
Planning legs at full EPA range — the first winter highway trip will teach this lesson expensively.
Charging to 100% at every stop; the 80–100% segment is the slowest and adds time, not range per minute.
Ignoring charger spacing in rural sections — a "theoretically fine" 240-mile leg can have no charger at mile 200.
Public Fast Charging Cost Calculator
Comparing the sticker ¢/kWh while ignoring session and idle fees on short sessions.
Using fast charging for routine top-ups when slower, cheaper L2 options exist at the destination.
Forgetting membership pricing you already pay for — the discounted rate is the real rate.
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.