Charging loss & efficiency
Not every kWh from the charger ends up in the battery — some is lost as heat.
Conversion and heat loss mean you typically pay for slightly more kWh than the battery actually stores. A 10% charging loss means you need about 33 kWh from the station to add 30 kWh usable.
Loss varies by charger type, temperature, and battery SOC. The calculator's energy billed estimate accounts for the loss percentage you set.