Methodology

How we calculate EV savings

Every number on this site comes from official government data and documented formulas. Here's exactly what we use, where it comes from, and where we make assumptions.

Data sources

EIA Electricity Retail Sales (monthly)

Residential electricity rates by state in cents/kWh. Updated monthly. We use the most recent published period. All 50 states + DC covered.

EIA Petroleum Retail Prices (weekly)

Retail gasoline prices (all grades, all formulations) by state. Updated weekly. 9 states report directly to EIA; remaining states use their PADD sub-regional price (PADD 1A–1C, 2, 3, 4, 5).

EPA Fuel Economy Guide

Combined city/highway MPG for gas vehicles. Combined mi/kWh and battery capacity for EVs. EPA-rated figures are used for all vehicle efficiency values.

All rates are cached for 24 hours via ISR and refreshed automatically. If EIA is unavailable, we fall back to the most recent static values.

EV charging cost

We calculate what it costs to power an EV for a given number of miles, split between home charging and public charging.

Cost per mile (electric)

cost_per_mile = (electricity_rate_cents / 100) / ev_efficiency_mi_per_kwh

Example: 16.5¢/kWh ÷ 4.0 mi/kWh = $0.041/mile

Annual EV fuel cost

annual_ev_cost = annual_miles × [ (home_pct / 100) × home_rate + (1 − home_pct / 100) × public_rate ] / ev_efficiency

Default: 80% home charging at state rate, 20% public at 2.5× state rate. 15,000 miles/year on state pages; 12,000 miles/year on cost-to-charge pages.

Full charge cost

full_charge_cost = battery_kwh × (home_rate_cents / 100)

Example: 75 kWh × $0.165/kWh = $12.38 to fully charge

Gas fuel cost

Gas cost is calculated from the EPA combined MPG rating and the current state gas price.

Annual gas fuel cost

annual_gas_cost = (annual_miles / gas_mpg) × gas_price_per_gallon

Example: 15,000 mi ÷ 30 MPG × $3.45/gal = $1,725/year

Savings calculation

Annual savings

annual_savings = annual_gas_cost − annual_ev_cost

5-year savings

five_year_savings = annual_savings × 5

Does not account for electricity or gas price changes over time.

Break-even (purchase price difference)

break_even_years = (ev_msrp − gas_msrp) / annual_savings

Only shown when EV costs more upfront and saves on fuel. Does not include insurance, maintenance, or incentives.

Assumptions and limitations

—Charging efficiency loss not modeled. Real-world charging loses 10–15% to heat and AC/DC conversion. Our cost-per-mile figures use EPA rated efficiency, which does not include charger losses. Actual cost is slightly higher.
—No winter degradation. Cold weather reduces EV range and efficiency by 15–30%. Our figures use all-season EPA ratings. If you drive in a cold climate, your real charging costs will be higher in winter.
—Flat electricity rate. We use the state average residential rate. Time-of-use (TOU) plans can reduce home charging cost to 60–75% of the average. States with available TOU rates are noted on relevant pages.
—PADD regional gas prices for 41 states. EIA publishes weekly state-level gas prices for 9 states (CA, CO, FL, MA, MN, NY, OH, TX, WA). All other states use their EIA PADD sub-regional price — a multi-state average. Accuracy is typically ±$0.10–0.30/gallon.
—Gas comparison vehicle is representative, not exact. We pair each EV with a comparable gas vehicle by segment (e.g., Tesla Model Y → Toyota RAV4). The gas price reflects the EPA combined MPG for that vehicle. Your actual gas car may differ.
—No maintenance or insurance difference. EVs typically cost less to maintain (no oil changes, fewer brake jobs) but may cost more to insure. These factors are not included in our fuel-cost comparison.

See something wrong or have a question about the methodology? hello@evchargesavings.com