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Calculator Methodology

Formulas, assumptions, and data sources — published for every calculator. No black-box answers.

Last reviewed: May 2026 · Tax-credit rules reflect OBBBA Public Law 119-21

WattMath publishes the formulas, assumptions, and data sources behind every calculator. If you find an error, please contact us.

Solar Payback Calculator

Route: /solar-payback-calculator

System sizing

System size (kW) = (monthly bill / electricity rate) × 12 / annual production (kWh/kW). Annual production comes from the NREL PVWatts API for the user's ZIP code, adjusted by orientation and shading factors (south = 1.0; east/west = 0.85; north = 0.65; no shading = 1.0; partial = 0.85; heavy = 0.65).

System cost

National average installed cost: $3.00/watt, adjusted by a state-specific factor. Includes panels, inverter, racking, wiring, permits, and installation labor.

Federal tax credit

The Section 25D residential clean energy credit (solar ITC) was terminated for residential systems placed in service after December 31, 2025 by Public Law 119-21 (OBBBA). This calculator applies a $0 federal credit for all 2026+ installations.

State incentives

Curated from the DSIRE database and state energy offices. Verified quarterly. Last-verified dates shown in results.

Savings projection

Annual savings = system production (kWh) × electricity rate. 25-year cumulative savings assumes 2.5% annual rate inflation, consistent with the historical US average. Payback = net cost / first-year savings.

Data sources

EV vs. Gas Total Cost of Ownership

Route: /ev-vs-gas-calculator

Fuel cost

Gas cost = (annual miles ÷ MPG) × gas price.
EV cost = (annual miles ÷ 100) × kWh/100mi × electricity rate. Efficiency figures (kWh/100mi) from EPA fueleconomy.gov ratings.

Maintenance cost

Gas: $0.101/mile (AAA/DOE average, includes oil changes, filters, spark plugs, timing belt, transmission, brake work).
EV: $0.061/mile (excludes oil changes, spark plugs, timing belt; reduced brake wear from regenerative braking).

Federal EV credit

The Section 30D new clean vehicle credit was terminated for vehicles acquired after September 30, 2025 by Public Law 119-21 (OBBBA). No credit is applied.

OBBBA Auto Loan Interest Deduction

OBBBA introduced an above-the-line deduction of up to $10,000/year of auto loan interest on new US-assembled vehicles, through December 31, 2028. This deduction is not factored into calculator results as its value depends on individual marginal tax rates and loan terms.

CO2 calculation

Gasoline: 8.89 kg CO2/gallon (EPA).
Grid electricity: 0.386 kg CO2/kWh (EPA eGRID national average). Actual emissions vary by local grid mix.

Data sources

Heat Pump vs. Gas Furnace ROI

Route: /heat-pump-vs-furnace-calculator

Equipment sizing

Required BTU/hr = square footage × BTU/sqft factor (30 BTU/sqft for climate zones 1–3; 40 for zones 4–5; 50 for zones 6–7). Rounded to the nearest 6,000 BTU. Converted to tons (1 ton = 12,000 BTU/hr) and rounded up to nearest half-ton.

Installed cost

Standard air-source: $800/ton installed. Cold-climate (zones 5+): $1,100/ton installed. Minimum floor: $5,000.

Coefficient of Performance (COP)

Seasonal COP by climate zone: zones 1–3 = 3.5; zones 4–5 = 2.8; zones 6–7 = 2.2 (cold-climate heat pump assumed for zones 5+).

Federal credits — post-OBBBA

Air-source (25C): Expired December 31, 2025. $0 federal credit for 2026+ installations.
Geothermal (25D): 30% of installed cost, no cap, through 2032. Still active under OBBBA.

Data sources

EV Charging Cost Calculator

Route: /ev-charging-cost-calculator

Charger power output

Level 1 (120V): 1.4 kW. Level 2 (240V home): 7.2 kW. DC Fast (public): 50 kW average.

Cost calculation

kWh per session = (miles per session ÷ 100) × kWh/100mi.
Cost per session = kWh × effective rate.
DC fast rate = home rate × 2.5 (network premium multiplier).
Monthly cost = monthly miles × (kWh/100mi ÷ 100) × effective rate.

Section 30C Home Charger Credit

30% of home Level 2 charger cost, capped at $1,000. Available for installations placed in service through June 30, 2026 (Public Law 119-21). After that date, the credit is $0 for residential property.

Data sources

Home Battery Sizing Calculator

Route: /home-battery-calculator

Energy requirement formula

For each load: Wh = watts × hours/day × (backup duration / 24).
Total required usable kWh = sum(Wh) ÷ 1,000 ÷ (inverter efficiency × depth-of-discharge allowance) = total ÷ (0.92 × 0.90) = total ÷ 0.828.

Inverter efficiency

92% — consistent with modern string and hybrid inverters (DC to AC conversion loss).

Depth of discharge

90% usable capacity assumed (10% state-of-charge floor to preserve long-term cycle life).

Federal ITC

Batteries installed alongside solar qualify for the Section 25D Residential Clean Energy Credit (30%, through 2032). Standalone batteries without solar do not currently qualify.

Data sources