Begin with your distance

A useful monthly estimate starts with how far you travel, not the smallest amount in an advertisement. Record commuting, weekends and longer journeys. If you lack records, begin with an explicit scenario and revise it as evidence becomes available. Keep energy separate from total ownership cost. Purchase price, depreciation, insurance, maintenance, parking and a charging installation remain other categories, even when they all affect the same household budget.

Choose a suitable consumption assumption

Use a value relevant to the car and route, then test a less favourable case. A standardized figure or particularly economical trip cannot promise performance in every season. Speed, weather, load and use can change the result. Establish what the figure measures: energy used by the car or energy billed during charging. If you already use billed energy, adding another charging-loss allowance may count the same expenditure twice. Explain which approach your spreadsheet uses.

Calculate the energy quantity first

The basic formula is distance divided by 100, multiplied by consumption per 100 km. In a wholly fictional example, 12,000 annual kilometres at 18 kWh/100 km gives 2,160 kWh. Adding an illustrative 10% overhead produces 2,376 kWh. That percentage is an arithmetic assumption, not a measurement applying to every electric car. When your own charged-energy and mileage records become available, use them to review the assumption instead of preserving it simply because it was the starting value.

Use the full price you pay

For home charging, check the supplier's documents and what the chosen unit price includes. For public charging, inspect the operator's current app or terms before starting. Subscription, time or occupancy conditions may not be represented by one per-kWh number. One operator's rate is not a national average. In the fictional example, 2,376 kWh at RON1.50/kWh would cost RON3,564 annually. This is a calculation example, not a current electricity offer.

Weight your charging locations

Allocate energy to each location before adding costs. For arithmetic illustration, 80% at RON1.50/kWh and 20% at RON2.50/kWh produces a weighted unit price of RON1.70/kWh. Applied to the example energy quantity, that means RON4,039.20. Both rates and proportions are fictional teaching assumptions. Eighty per cent of charging sessions does not necessarily mean eighty per cent of energy: a long and a short session can deliver very different quantities.

Keep fixed costs separate

An installation, electrical assessment, subscription or parking charge cannot be compared directly with a per-kWh rate. Give each a separate line and state the period over which it is spread. Do not assume that any equipment can be installed or operated in your parking space; obtain the relevant technical assessment and permissions. Also consider time, travel to a charger and a backup location. A headline fast-charging rate alone does not describe the convenience of a repeatable routine.

Make a fair fuel comparison

Use the same distance and a suitable consumption assumption for the petrol or diesel alternative, then multiply litres by the chosen fuel price. Call the difference an energy comparison, not guaranteed total savings. Purchase price, finance and depreciation belong elsewhere in the ownership calculator. With a plug-in hybrid, assess electric and fuel mileage separately. One favourable consumption figure cannot describe every mixed-use routine. Check that no expenditure is also included under another category.

Test changing assumptions

Build an ordinary-routine scenario and one with more public charging or higher consumption. Notice which variables change the outcome most. Carletta's public ownership calculator helps organize these assumptions without an account; its initial values remain editable examples. Save your calculations and update them using your actual bills and mileage. A useful result is a number whose energy quantity, tariff and limitations you can explain, rather than simply the lowest number the inputs permit.

Checking the first month's result

Record billed energy, cost and distance over comparable periods. Charging at the end of one month for travel in the next can distort a simple monthly consumption calculation. Follow several periods and explain major variations before replacing every assumption. Compare the average unit price you actually paid with the weighted calculation, not with the cheapest advertised tariff you could theoretically access. This helps distinguish a change in distance from a change in consumption or charging location. Keep fixed charges visible as well; averaging them into energy can hide why one month appears unusually expensive.