Methodology

1. Panel Capacity — NEC 220.82 Optional Method

The Panel Capacity Checker implements the NEC 220.82 Optional Method for dwelling unit load calculations (National Electrical Code, Article 220, Part IV). This is the standard method US licensed electricians use to determine whether a residential electrical panel can safely support an added continuous load such as an EV charger.

Step 1 — General load

• General lighting and receptacles: 3 VA per square foot of floor area (NEC 220.82(B)(1)).
• Small appliance and laundry circuits: Three 1,500 VA circuits = 4,500 VA (NEC 220.52).
• Nameplate appliances: Actual nameplate VA for range/oven, clothes dryer, water heater, electric dryer, and any other 240 V loads the user declares. Where nameplate ratings are unknown, we use standard NEC Table 220.55 and common nameplate values: range 8,000 VA, dryer 5,000 VA, electric water heater 4,500 VA.

Step 2 — Demand factor

The total general load (lighting + small appliance circuits + nameplate appliances) is reduced by a demand factor per NEC 220.82(B):

• First 10,000 VA: counted at 100%
• Remainder above 10,000 VA: counted at 40%

This demand factor reflects the statistical reality that not all loads in a home run at full draw simultaneously.

Step 3 — HVAC load

Per NEC 220.82(C), the HVAC load is added at 100% of the larger of the heating or cooling nameplate load — not both. Heat pumps are treated as the combined heating/cooling load. Central air conditioning nameplate is used for cooling. Electric resistance heat and baseboard electric heat are used for heating. Gas, oil, and propane heat are zero VA.

Step 4 — EV charger load (NEC 625.41)

EV chargers are continuous loads (operating for 3 hours or more). Per NEC 625.41, continuous loads must be sized at 125% of the nameplate current. A 40 A charger is therefore calculated as 40 × 1.25 × 240 V = 12,000 VA.

Safe capacity threshold

The calculated total load (in amps at 240 V) is compared against 80% of the panel’s rated ampacity. For a 200 A panel, the safe continuous capacity is 160 A. This 80% rule is required by NEC 210.20(A) for circuits with continuous loads.

2. Charging Cost Formula

The Charging Cost Calculator estimates the annual and monthly cost to charge an EV at home using the following formula:

The 12% loss factor accounts for AC-to-DC conversion losses in the charger and battery management system. Real-world charging efficiency typically falls between 85–92%; 12% is a conservative mid-range assumption consistent with Department of Energy guidance.

Vehicle efficiency figures come from EPA MPGe ratings converted to miles per kWh. Electricity rates default to the most recent annual average for the user’s selected state, sourced from the EIA (see Data Sources below). Users can override with their actual utility rate.

3. Charging Time Formula

Charging time from a given state of charge (SoC) to a target SoC is calculated as:

The effective charge rate is capped at the lower of the charger’s output and the vehicle’s on-board charger (OBC) limit. For example, a 48 A charger outputting 11.5 kW connected to a car whose OBC maxes at 7.2 kW will charge at 7.2 kW — the car is the bottleneck.

The 0.88 efficiency factor accounts for real-world charging losses (heat, conversion inefficiency, battery management).

Taper above 80% SoC: Lithium-ion batteries reduce charge acceptance above 80% SoC to protect battery health. The calculator applies a simplified taper: the portion of charging above 80% SoC is estimated at 50% of the nominal charge rate, adding additional time for that segment. This is an approximation; actual taper curves vary by vehicle and battery chemistry.

4. Data Sources

Electricity rates — EIA Form EIA-861

Residential average retail electricity prices by state, sourced from the US Energy Information Administration (EIA) Electric Power Annual, 2025 edition. Rates are annual averages in ¢/kWh and are updated annually when new EIA data is published.

Solar production — NREL PVWatts

Annual solar production estimates use kWh per kW of installed capacity by solar resource zone, derived from the National Renewable Energy Laboratory (NREL) PVWatts calculator data. Zone assignments are based on approximate US geographic regions.

Vehicle efficiency — EPA fuel economy data

EV efficiency ratings (miles per kWh) and on-board charger limits are sourced from EPA fuel economy data (fueleconomy.gov) and supplemented with manufacturer specifications for vehicles not yet included in EPA data. Battery capacity figures are manufacturer-published usable kWh.

Panel upgrade costs — RSMeans regional cost data

Panel upgrade cost estimates use RSMeans residential construction cost data adjusted by US census region. Costs reflect national averages and typical contractor rates; actual quotes vary significantly by market and specific project conditions.