StraightWatt

Corrections

Every figure on this site is computed from data, and data can be wrong. When a number a reader could have acted on changes for any reason other than a routine refresh, it is written down here: what the site showed, what it shows now, and what changed it.

A solar calculator that never publishes a correction is not one that has never been wrong. These entries are the ones we have found so far — 3 of them, newest first.

  1. · figures were wrong in the reader's favour

    Hawaii was published with a 4.2-year payback and has been withdrawn

    What it showed
    Hawaii showed a 4.2-year payback, the shortest on the site, on a $2.60/W national average installed cost and an assumed full-retail export credit.
    What it shows now
    No Hawaii page. The state is not served until both its installed cost and its current export compensation come from primary sources.
    Why it was wrong
    Both inputs were fallbacks, and both pointed the same way. Hawaii is among the most expensive states in which to install, so the national cost figure was too low; and Hawaii closed retail net metering to new customers in 2015, so the full-retail assumption was simply wrong for anyone signing today. A page resting on two fallbacks is worse than no page.
    Who it affected
    Hawaii only. Colorado, Georgia, North Carolina and Nevada were withdrawn at the same time under the same rule, before they had been published with figures.

    What changed so it cannot happen again

    • A state is now published only when it has its own record for all four of rate, production, installed cost and export compensation - a fallback on any of them disqualifies it.
    • A missing export rule now raises an error flag and uses a conservative 0.30 default instead of full retail, so a gap can never again read as the most favourable case.
    • The build refuses to publish any page that carries an error flag.

  2. · figures were wrong in the reader's favour

    Illinois was published at full-retail net metering

    What it showed
    Illinois exports were credited at 100% of the retail rate.
    What it shows now
    Exports are credited at 0.49 of the retail rate: the supply portion only.
    Why it was wrong
    The Illinois Power Agency states that full retail rate net metering is no longer available to new customers and that they receive only energy supply credits for excess energy sent to the grid. Retail-rate netting closed to new customers after 31 December 2024 under Public Act 102-0662. Electricity generated and used on site is still worth the full retail rate; only exports changed.
    Who it affected
    Every Illinois figure. At a $200-a-month bill the 25-year net fell from about $40,800 to about $18,700.

    What changed so it cannot happen again

    • All eleven published states now carry an export rule read from the regulation, statute, tariff sheet or state agency publication that governs it, each linked with the date it was read.
    • Arizona moved from 0.50 to 0.37 and California from 0.25 to 0.20 in the same pass; Texas moved from 0.90 to 0.70 and remains flagged, because no source states a single Texas number.

  3. · figures were wrong against the reader

    Illinois and New Jersey figures left out incentives the site had already recorded

    What it showed
    The Illinois Shines REC contract and the New Jersey SuSI SREC-II were recorded, sourced and shown on the page as 'not modelled'. Neither entered the payback, the 25-year net or the charts, so both states' returns were understated - Illinois materially, immediately after its export correction.
    What it shows now
    Both are paid out by the engine on their own contract term and payment schedule, and appear in the cash flow, the payback and the 25-year net like any other credit.
    Why it was wrong
    Understating a return is the same failure as overstating one. The rule that removed Hawaii has to run in both directions.
    Who it affected
    Illinois and New Jersey. On a $200-a-month bill, Illinois goes from a 16.4-year payback and a $18,680 25-year net to 8.2 years and $31,952 - the REC contract is worth about $13,271, half of it paid at switch-on. New Jersey goes from 7.9 years and $48,720 to 6.1 years and $58,806, worth about $10,086 paid as the system generates.

    What changed so it cannot happen again

    • The engine now pays out a performance incentive on its own contract term and payment schedule: paid as the system generates, or part at switch-on with the rest spread over later years.
    • An incentive on file that the model cannot pay out now raises an error flag, so a state cannot be published while carrying one.

Corrections are never removed or quietly edited. If a correction is later itself corrected, a new entry is added and the old one stays.

Routine data refreshes — a new month of EIA rates, a new production run — are not corrections; each data file carries its own verification date, and the methodology page lists them.