Know what the calculators assume

Simple payback

Net cost divided by positive annual savings gives simple payback. A zero or negative saving has no positive payback. This calculation excludes financing interest, discount rates, maintenance and replacement timing unless explicitly included. Compare a full replacement cost with a full alternative, or an upgrade premium with the cheaper replacement you would otherwise buy.

Electricity rates

Use the variable charges that change with consumption. Total bill divided by kWh is only a starting average because fixed charges may remain. Time-of-use tariffs need time-specific consumption. The original 15.2-cent example is not a verified current Dominion tariff.

Solar

Production is system kW × daily peak-sun-hours × 365 × 0.78. It assumes 40% self-consumption, the selected export-credit fraction on the remaining 60%, 0.5% annual degradation, and the entered rate escalation. The before-bill chart assumes production covers 90% of implied consumption; it does not use your measured household load. Use a roof-specific design for a purchase decision. SREC and escalation defaults now start at zero.

Heating, controls and insulation

The heat-pump calculator uses entered seasonal COP and simplified existing-system efficiencies. Its old heat-pump COP is 2.2, oil cost is fixed at $4/gallon, and the cooling comparison uses an approximate SEER/SEER2 conversion. These are coarse assumptions. Thermostat and insulation estimates multiply the relevant spending by an assumed saving percentage. The percentage must come from an appropriate assessment or conservative scenario.

Water heating

The original demand model uses people × 875 × 0.92 kWh of delivered heat annually, then divides by UEF. It assumes a gas tank when old UEF is below 0.75, using $1.60/therm. That fuel inference is imperfect and can misclassify equipment. For gas replacements, use explicit fuel-cost arithmetic and your bill rather than relying on this model. The new guide shows a separate illustrative electric-tank comparison.

Lighting, storage and EVs

LED savings are lamp count × watt reduction × hours × 365 ÷ 1,000 × rate. Battery sizing sums average loads for the same runtime and divides by an illustrative 85% delivered-energy factor; it still requires motor-starting, voltage and reserve checks. EV cost uses entered kWh per mile as electricity purchased at the wall. If the source reports battery energy, add losses before entering it. Charger cost divided by fuel savings is not an incremental Level 2-versus-Level 1 return.

The headline sum is not a combined forecast

Solar, weatherization, heat-pump and thermostat savings overlap. The calculator sum adds independent scenarios and does not resolve those interactions. Carbon totals were removed because the original fixed emissions factor and mixed-fuel approximations were unsuitable for a dependable whole-home claim. Use the audit guide to build a sequential plan.

Content and commercial approach

Guides combine primary technical references with original hypothetical arithmetic. No hands-on testing, product star ratings or guaranteed savings are claimed. Affiliate payments do not change calculator formulas. Sources and review dates are shown on the relevant pages.