Methodology: What the calculator does (and what it doesn’t)
This page explains the math behind our EV charging cost estimates, the assumptions we use by default, and how to customize inputs so your results match your real bill.
1) Core charging cost formula
The calculator estimates how many kilowatt-hours (kWh) you consume each month and multiplies that by your electricity price. The key is converting your vehicle efficiency (Wh/mile) into kWh.
- Effective Wh/mile =
WhPerMile × (1 + ChargerLoss%) - kWh per mile =
EffectiveWhPerMile ÷ 1000 - Monthly kWh =
kWhPerMile × MonthlyMiles - Monthly EV cost =
(HomekWh × HomeRate) + (PublickWh × PublicRate)
Why we include “charger loss”: real charging isn’t 100% efficient. Some energy becomes heat or overhead, and public fast chargers can have additional losses.
2) Home vs public fast-charging mix
Many drivers charge mostly at home, but some use a mix of home, workplace, and public fast charging. We model this by splitting your monthly kWh into two buckets:
- Home kWh =
TotalkWh × (1 − PublicMix%) - Public kWh =
TotalkWh × PublicMix%
If you only charge at home, set Public Mix to 0%. If you road trip often, increase it. The calculator treats public charging as a separate (usually higher) price per kWh.
3) EV vs Gas comparison
To estimate gas cost for the same miles, we use your MPG and local gas price:
- Gallons per month =
MonthlyMiles ÷ MPG - Monthly gas cost =
Gallons × GasPrice - Monthly savings =
GasCost − EVCost
This is a fuel-only comparison. Maintenance, insurance, tires, registration fees, and depreciation vary widely and are not included.
4) Break-even estimate for a home charger
If you enter a one-time home charger + installation cost, we estimate how long it takes to “pay back” using monthly fuel savings:
- Net install cost =
InstallCost − Rebates/Credits - Break-even (months) =
NetInstallCost ÷ MonthlySavings(only when savings are positive)
If your EV is not cheaper than gas in your scenario, the calculator shows No payback. That can happen in places with very high electricity prices, low gas prices, low MPG difference, or heavy public fast-charging use.
5) CO₂ estimate (optional)
We show a simple CO₂ difference using “lbs CO₂ per kWh” for electricity and “lbs CO₂ per gallon” for gasoline. This is meant as a rough estimate.
- EV CO₂/month =
TotalkWh × CO₂PerkWh - Gas CO₂/month =
Gallons × CO₂PerGallon - CO₂ savings =
GasCO₂ − EVCO₂
Grid emissions vary by state, hour, and season. If you know your utility’s factor, plug it in for better accuracy.
6) Default assumptions (and why they’re editable)
| Input | Default | Why | How to improve accuracy |
|---|---|---|---|
| Wh/mile | 280 | Common midsize EV range | Use your car’s “energy” screen or recent trip average |
| Charger loss | 10% | Conservative average | Compare kWh added vs kWh billed over multiple sessions |
| Home $/kWh | State typical | Quick starting point | Use your bill’s all-in price (energy + delivery + fees) |
| Public $/kWh | $0.35 | Typical fast charge range | Use your network pricing, memberships, and local taxes |
7) Sources and references
We link to public, reputable sources that explain electricity data, vehicle emissions, and EV fueling concepts.
- U.S. Energy Information Administration (EIA): Electricity data
- U.S. DOE Alternative Fuels Data Center: Electricity as a fuel
- U.S. EPA: Typical passenger vehicle greenhouse gas emissions
Note: This site is informational. Always verify final costs with your utility tariff and charging network terms.
8) Update policy
We periodically refresh defaults, fix UX bugs, and add guides based on user feedback. If you spot a mismatch between the tool and a published reference, contact us and include:
- Your state and utility name
- The $/kWh number you’re seeing on your bill
- A screenshot or the tariff link (optional)