The calculators are designed to work in any order, but the order below is the one most readers find natural. Each step builds on what you learned in the last — and every result links to a longer methodology page that shows the formulas, sources, and assumptions in plain English.
1. Start with solar — it sets the ceiling on everything else
The Solar Payback Calculator uses NREL's PVWatts model and your local electricity rate to estimate system size, installed cost after state incentives, and how many years until the panels pay themselves off. Even if you don't end up installing, the payback number is the baseline every other electrification decision gets compared against. If your payback is under 10 years, the rest of the WattMath calculators get more interesting; if it's over 15, the math leans toward efficiency upgrades first.
2. Run the EV total cost of ownership next — it's the biggest annual savings
The EV vs. Gas Calculator compares total cost of ownership over 3 to 10 years using your annual mileage, your current car's MPG, and your local gas and electricity prices. It separates fuel savings from maintenance savings — the maintenance number is where most articles get the math wrong, and it's often larger than the fuel delta. The federal EV purchase credit is gone for vehicles bought after September 30, 2025, so the result is honest about what the math looks like at full sticker.
3. Then heat pumps — the most underrated upgrade
The Heat Pump vs. Furnace Calculator estimates your home's heating load, sizes a heat pump in BTU/hr, and compares annual operating cost against your current furnace using your real electricity and fuel rates. Air-source heat pump federal credits expired at the end of 2025, but geothermal still qualifies for the 30% Section 25D credit through 2032. The calculator surfaces state and utility rebates that often matter more than the federal piece.
4. Charging cost — the question every new EV owner asks first
The EV Charging Cost Calculator answers "what does it actually cost to fill up?" for Level 1, Level 2, and DC fast charging. The result includes cost per mile, which is the number to compare against the per-mile cost of gas in your other car. A home Level 2 charger may still qualify for the Section 30C federal credit if installed on or before June 30, 2026.
5. Battery sizing — only after the other four
The Home Battery Calculator sizes backup storage in usable kWh based on the critical loads you actually want to keep running. The usable-versus-nameplate distinction is where the marketing-spec sheets mislead — the calculator uses real usable capacity for Powerwall 3, Enphase IQ Battery 5P, and Franklin aPower 2. Batteries paired with new solar qualify for the 30% ITC; battery-only installs don't.