How to use this calculator
Record the energy supplied to charging over a defined start and end time. Enter the increase in stored battery energy over that same interval. If you only have a capacity and state-of-charge estimate, read the limitations below. The tool divides battery energy gained by wall energy and reports the remainder. It does not assume a standard loss percentage.
The difference includes everything between your chosen measurement points and may include conversion losses, vehicle auxiliaries, thermal management and measurement error. It is not a direct measurement of heat or an isolated charger efficiency rating. This model assumes wall charging is the only energy source during the interval. Do not combine a whole-house meter reading with vehicle-only battery gain unless unrelated loads have been excluded.
The formula
A worked example
A hypothetical session supplies 40 kWh at the wall and increases stored battery energy by 36 kWh. Efficiency = 36 ÷ 40 × 100 = 90%. The difference is 4 kWh, or 10% of the wall input. This is not the same denominator as 4 ÷ 36 = 11.11% additional wall energy relative to the battery gain. To plan for 36 kWh stored at an assumed 90% efficiency, divide by 0.90: 40 kWh is required at the wall.
Example sessions at the same wall input
Hypothetical arithmetic examples, not typical efficiency ratings for vehicles or chargers.
| Wall input (kWh) | Battery gain (kWh) | Efficiency | Difference (kWh) |
|---|---|---|---|
| 40 | 32 | 80% | 8 |
| 40 | 36 | 90% | 4 |
| 40 | 38 | 95% | 2 |
Common questions
Can I estimate battery gain from state of charge?
As an approximation, battery gain = current usable capacity × (ending SOC − starting SOC) ÷ 100. For example, 60 kWh usable capacity from 20% to 80% gives an estimated 36 kWh. Rounded SOC, capacity uncertainty and how the vehicle estimates stored energy can materially change the result. Use current usable capacity, not a gross nameplate capacity, and label the result an estimate.
Is DC energy delivered to the pack the same as stored energy gained?
No. Energy entering the battery terminals can also include internal battery losses. If you enter DC energy delivered, the ratio describes wall-to-battery-input efficiency instead of stored-energy efficiency. Keep that boundary explicit when comparing reports; this page labels its input as stored energy gained.
Why does the tool reject a result above 100%?
Under the stated single-source model, stored energy cannot increase by more than the wall input. A larger recorded gain points to inconsistent intervals, units, another energy source, or uncertain measurements. The tool rejects the inputs rather than clipping the result to 100%.
How do I combine multiple sessions?
Sum wall energy for complete sessions and sum their matching battery gains, then divide the totals. For 20 kWh in and 16 kWh gained, followed by 60 kWh in and 54 kWh gained, the total is 70 ÷ 80 = 87.5%. The unweighted average of 80% and 90% would be 85% and is wrong here.
Can I use kW readings instead of kWh?
No. kW measures power at a moment; kWh measures energy accumulated over time. Charging power changes during a session. Use energy totals or an appropriately integrated power record.
Should I add these losses again to wall-based driving consumption?
No. If your cost estimate already uses electricity drawn from the wall, charging losses are already included in that measurement. Applying the same correction again would double count them.
Does a low value diagnose a bad charger or battery?
No. These two totals cannot identify the cause. Compare the same measurement boundaries and conditions, check the records, and consult the vehicle or charger documentation before interpreting a difference as a fault.
Sources & calculation method
The U.S. Department of Energy discussion of EV energy use distinguishes wall-to-battery charging losses from energy used to move the car. That historical illustration does not establish the efficiency of your session. This calculator uses only your entered energy totals and the output/input ratio stated above. Its examples and table are original hypothetical calculations, not vehicle tests.
Use the EV Charging Cost Calculator to estimate a session bill from a battery target and an assumed efficiency, or the EV Charging Time Calculator for a constant-average-power time estimate. This tool instead calculates efficiency from two energy records.
Reference accessed September 18, 2026. The source does not endorse this site or provide independent professional review.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.