Key facts
- Required departure percentage = 100 × (trip energy + reserve energy) ÷ usable capacity.
- The fictional 180 km example needs 38 kWh for the trip and 6 kWh reserve, producing 73.33% minimum departure.
- At 24 kWh/100 km the same 80% start misses the chosen reserve by 3.2 kWh.
- A 113.33% requirement means the proposed leg cannot fit one full usable battery under the assumptions.
To estimate the EV charge needed at departure, add the trip’s battery-energy demand to your chosen arrival reserve, then divide by the vehicle’s usable battery capacity. Multiply by 100 to express the result as a percentage. A departure target above 100% means the proposed leg does not fit one full usable battery under those assumptions.
This answers a specific question: “What charge should I have before this leg?” It also lets you test whether today’s starting charge meets your chosen reserve. It is a simplified energy budget, with displayed state of charge assumed proportional to current usable energy. It does not guarantee arrival or replace the vehicle’s warnings.
Keep every input on the battery side
Use usable battery capacity, expected battery-side driving consumption and any additional battery energy outside that consumption figure. Do not insert a gross pack specification or a charging receipt as an interchangeable input. Our gross-versus-usable capacity guide explains the distinction.
The U.S. EPA’s testing explanation says its MPGe values include charging losses and reflect electricity at the outlet. That boundary differs from battery energy available for driving. Converting the unit alone does not remove the difference; do not invent a universal loss percentage to turn a label value into a battery measurement.
The departure-charge formula
For distance in kilometres and consumption in kWh/100 km:
- Driving energy = distance × consumption ÷ 100.
- Trip energy = driving energy + extra battery energy.
- Reserve energy = usable capacity × chosen reserve percentage ÷ 100.
- Required departure percentage = 100 × (trip energy + reserve energy) ÷ usable capacity.
Extra energy belongs here only if it is absent from the driving-consumption input: for example, a separately budgeted stationary activity. Do not add normal cabin heating twice when the entered consumption already includes it. DOE/EPA guidance on EV energy use identifies accessories as an energy demand and stresses that vehicle energy-management strategies differ.
Worked example: 180 km with a 60 kWh usable battery
Assume an invented EV has 60 kWh currently usable, starts at 80%, and will travel 180 km at 20 kWh/100 km. The driver chooses a 10% arrival reserve and adds 2 kWh for energy not included in the driving figure. The 10% reserve is an example choice, not a recommended minimum.
| Quantity | Calculation | Result |
|---|---|---|
| Starting energy | 60 × 0.80 | 48 kWh |
| Driving energy | 180 × 20 ÷ 100 | 36 kWh |
| Trip energy including extra use | 36 + 2 | 38 kWh |
| Arrival energy | 48 − 38 | 10 kWh, about 16.7% |
| Chosen arrival reserve | 60 × 0.10 | 6 kWh |
| Margin above that reserve | 10 − 6 | 4 kWh |
| Required departure charge | 100 ×(38 + 6) ÷ 60 | About 73.3% |
The existing 80% start exceeds the calculated 73.3% requirement. The 4 kWh margin is above the chosen reserve; it is not the entire 10 kWh expected to remain. Keeping those two numbers separate makes a deteriorating forecast easier to interpret.
If a charging control accepts only whole percentage points, rounding 73.333… upward gives 74% as the mathematical setting. That rounding is not an allowance for uncertain weather, consumption, capacity or battery-level estimation.
Use our EV charging planning tools for related unit and cost calculations. The departure-charge formulas and examples here work independently.
Test a worse consumption case before committing
Keep every input except consumption unchanged and raise consumption to 24 kWh/100 km. Driving now requires 43.2 kWh; with the extra 2 kWh, the trip needs 45.2 kWh. Starting with 48 kWh leaves just 2.8 kWh, approximately 4.7%.
That is 3.2 kWh below the chosen 6 kWh reserve. The revised departure requirement is 100 ×(45.2 + 6) ÷ 60 = 85.33%. This scenario calls for more departure energy or a different charging plan. The 3.2 kWh shortfall is battery energy; it is not a prediction of the billed kWh needed to add it.
A useful sensitivity test changes an input with a reason: your comparable wet-weather trips, the route forecast, or the vehicle’s updated estimate. An arbitrary multiplier is not a measured winter or wind correction. For the physical distinction, read how headwind and road speed affect EV energy demand.
Recognise a leg that cannot fit one full battery
Now consider 300 km at the original 20 kWh/100 km, with the same 2 kWh extra demand and 6 kWh reserve. The trip needs 62 kWh; departure energy including reserve would be 68 kWh. Dividing by 60 gives a required departure of 113.3%.
Do not round this down to 100% and label the route feasible. Even a full 60 kWh usable battery is 2 kWh short of the trip demand before protecting any reserve. Add a suitable charging stop, shorten the leg or revise an input only when evidence justifies it. A negative calculated arrival is an energy deficit, not a physical dashboard charge level you can drive down to.
What does a calculated maximum distance mean?
With the original 80% start, the distance available above the reserve in this model is (48 − 6 − 2) ÷ 0.20 = 200 km. This holds consumption, extra energy and reserve fixed. It does not discover road gradients, weather, access restrictions or a charger at kilometre 200.
For miles, use matching consumption units. NIST’s conversion table gives one mile as exactly 1.609344 km. Our EV efficiency unit converter handles Wh/km, kWh/100 km, kWh/100 miles and miles/kWh without changing the underlying measurement boundary.
Reconcile the budget with navigation during the trip
Tesla’s Model 3 navigation manual provides one manufacturer example: arrival-energy predictions account for driving and environmental factors and are updated during travel. Some inputs depend on connectivity. Available features differ by region and configuration; check your own vehicle’s manual.
Use the calculation to expose assumptions, then compare it with the vehicle’s route estimate. If they disagree materially, investigate the consumption basis, capacity, distance and extra loads before choosing the more optimistic answer. Reassess early if the predicted arrival falls. This focused budget complements the broader road-trip charging and backup-plan checklist.
Frequently asked questions
Is 10% a suitable arrival reserve everywhere?
No. It is merely the example input. Choose a reserve for the actual route, alternatives, conditions and vehicle guidance; the calculator cannot choose it for you.
Does required departure charge tell me charging time?
No. It estimates the energy target. Charging duration also requires the relevant charging rate and conditions; a higher target can pass through a different part of the charging curve.
Can I use the original brochure capacity for an older EV?
Only if the figure identifies usable capacity, and then only as a clearly labelled assumption about this vehicle’s current usable energy. Do not substitute a gross figure. If current usable capacity is uncertain, the result is uncertain too. This calculation does not measure battery health.
Method: Official sources checked on 12 November 2026. The model assumes charge percentage tracks current usable battery energy. All worked scenarios are original illustrative examples, not first-hand vehicle tests.
1 comment