Sources & Assumptions
RetrofitROI's calculations lean on a short list of authoritative references, last reviewed 2026-09-15. Everything below this list is either a hard fact tied to one of these sources, or a planning estimate you can see and change.
Primary references
- IRS Form 5695 instructions and Energy Credit FAQs, for the boundary on federal credits: irs.gov, Form 5695 instructions
- NREL PVWatts technical reference, for solar-generation sanity context: pvwatts.nrel.gov, PVWatts v5 documentation
- DOE Energy Saver, for consumer efficiency context: energy.gov, Energy Saver
- ENERGY STAR heat-pump water heater guidance, for planning context and cross-checks: energystar.gov, heat pump water heater benefits
- U.S. Energy Information Administration residential price data, for optional rate planning presets: eia.gov, electricity data and eia.gov, natural gas data
- ENERGY STAR product specifications, for equipment-performance vocabulary: energystar.gov, product specifications
- DOE Building America climate zones, for climate-zone labeling only: energy.gov, climate-specific guidance
What counts as a hard fact versus a planning estimate
Only the references above back a hard fact, such as a federal credit's documented boundary or a published climate-zone label. Everything else, including a cost class, an equipment lifespan, a local incentive, or a utility rate you have not entered yourself, is a planning estimate: labeled as an estimate, editable, and never presented as a quote, a legal conclusion, or a guarantee.
Where a figure genuinely varies by household, like an electricity rate, a gas rate, or a utility tariff structure, RetrofitROI shows a clearly marked default and asks you to confirm or replace it with your own number. A default federal, state, or utility incentive is never assumed; if a tool needs an incentive amount, it asks you to enter a confirmed figure rather than guessing one on your behalf.
Review schedule
References tied to a fast-changing federal program are reviewed roughly monthly. Everything else is reviewed at least annually, or sooner if the source publishes a material update. No core calculation depends on a live data feed: if a figure has not been reviewed recently enough to trust, RetrofitROI asks you for it directly instead of silently using a stale number.
See the methodology page for how these sources feed into a result, and the disclaimer page for what RetrofitROI deliberately does not attempt to calculate.