How the calculator works

The core formula

Calculated for each year over a 10-year projection. "Net cost" is what a vehicle has truly cost you at any point, accounting for what you could sell it for. Both vehicles start at $0 on day one (the car is worth what you paid). Over time, running costs accumulate and the car depreciates, increasing the net cost.

Opportunity cost (the cost of having extra capital tied up in the Tesla) is only charged to the Tesla side. See the opportunity cost section below.

The breakeven point is the year where the Tesla's net cost drops below the current car's net cost.

The two charts

Net Cost of Ownership

Shows the total economic cost over time: purchase price + all running costs + opportunity cost, minus what the car is still worth. This is the main comparison. The dashed lines and shaded bands show the range of possible outcomes based on depreciation uncertainty (25% above and below the base rates).

Annual Cash Running Costs

Shows what you actually pay out of pocket each year: fuel/electricity, maintenance, tyres, registration, WoF, RUC, and insurance. This chart deliberately excludes depreciation (not a cash payment) and opportunity cost (an economic concept, not a bill). It answers "how much more/less will I spend day-to-day with a Tesla?"

Fuel and electricity

You enter the current price per litre at your local station. The default is $3.38/L for petrol, $3.48/L for diesel. When you select a vehicle, L/100km is auto-filled with a typical value for that model. Both can be overridden.

Electricity price defaults to $0.30/kWh but is editable. Tesla efficiency values (kWh/100km):

Maintenance and tyres

The current car's base maintenance ($800) is multiplied by a per-vehicle factor reflecting how expensive that car is to service. A Suzuki Swift (0.7x = $560 base) costs less than a Mercedes C-Class (1.6x = $1,280 base). Both then scale with age and odometer.

Age scaling adds 15% per year above age 3 for the current car (8% above age 4 for Tesla). Km scaling adds a small factor above 100,000 km on the odometer.

Tyre costs are added separately:

Teslas are heavier and have instant torque, leading to faster tyre wear and more expensive EV-rated tyres. See the all data page for maintenance factors per vehicle.

Depreciation

Age-based rates

Each listed vehicle has its own annual depreciation rate table broken into age brackets. For example, a Toyota Corolla depreciates at 14% per year for the first 2 years, dropping to 10% for years 3-5, then 7%, 5%, and eventually 3%. Utes hold value best. European cars depreciate fastest. Japanese cars are in the middle.

Km adjustment

Above 80,000 km on the odometer, an additional ~0.5% is added to the depreciation rate per 10,000 km. A vehicle with 200,000 km gets an extra 6% on top of its age-based rate. Total rate is capped at 25%.

Import adjustment

Japanese imports depreciate at 80% of the NZ-new rate for the same model. They've already taken the biggest depreciation hit before arriving in NZ.

Confidence bands

Depreciation is inherently uncertain. The net cost chart shows three scenarios using dashed lines and shading:

• Optimistic: both vehicles depreciate 25% slower than the base estimate
• Moderate: the solid line, using the base rates
• Pessimistic: both vehicles depreciate 25% faster

The same scenario is applied to both vehicles simultaneously, so the band shows overall uncertainty in depreciation, not a "best case for Tesla" vs "worst case" comparison.

Fallback ("Other / not listed")

If you select "Other / not listed", generic rates are used (15% initial, tapering to 3%).

Tesla-specific assumptions

Energy efficiency

Model 3 at 14.5 kWh/100km and Model Y at 16.0 kWh/100km. Based on WLTP ratings with a ~15% real-world adjustment for NZ conditions (hills, climate control, highway speeds). Actual consumption varies with driving style, temperature, and terrain.

Depreciation

Tesla depreciation is the least certain part of this calculator. The NZ Tesla market is young and volatile. Tesla's own price cuts directly depress used values overnight, and brand sentiment shifts play a role. Our rates (18% for Model 3, 16% for Model Y in the first 2 years, tapering to 3% by age 12+) are estimates. The 25% confidence bands attempt to capture this uncertainty.

Maintenance

Base $400/year (vs $800 for current car) reflects no oil changes, timing belts, spark plugs, or complex transmission servicing. Main costs are brake fluid, cabin filters, and coolant. Out-of-warranty repairs can be expensive due to Tesla's parts pricing and limited NZ mechanic support. The 8% annual age scaling attempts to capture this.

Tyres

$1,500 per set every 40,000 km. Teslas are heavier (battery weight) and have instant torque, both accelerating wear. They also need EV-rated tyres with reinforced sidewalls. At 12,000 km/year: $450/year vs $192 for a current car.

RUC

EVs pay Road User Charges at $76 per 1,000 km (same rate as diesel). Introduced April 2024 after the EV exemption ended. At 12,000 km/year this adds $912 annually, a cost many EV calculators overlook.

What we don't model for Tesla

• Battery degradation and its impact on range and resale value
• Software-locked features that may affect resale (FSD, acceleration boost)
• Supercharger costs vs home charging split
• Potential battery replacement ($15-25k) if needed outside warranty

Registration, WoF, and RUC

Opportunity cost

You need some car regardless, so opportunity cost is only calculated on the difference in capital tied up between the Tesla and your current car. If the Tesla is worth $40k and your car is worth $14k, you have $26k in extra capital that could reduce your mortgage or earn returns.

The rate defaults to 5.5% (typical NZ mortgage rate) and is adjustable. The Tesla's resale value used is the average of start and end-of-year values. The current car has zero opportunity cost since it's the baseline.

Opportunity cost appears in the Net Cost of Ownership chart and the year 1 breakdown, but is excluded from the Annual Cash Running Costs chart since it's not money you physically pay.

What's not included

• Financing - assumes cash purchases. Borrowing adds interest to the Tesla's cost.
• Home vs public charging - electricity price is a single value. Public charging (ChargeNet etc.) costs $0.50-0.80/kWh.
• Price inflation - fuel, electricity, insurance, and maintenance are held constant over the projection.
• ACC levies - included in registration but vary slightly by vehicle type.
• Parking, tolls, ferry costs - assumed equal for both options.

Data sources

• Fuel prices - user-entered (default $3.38/L petrol, $3.48/L diesel)
• Depreciation rates - derived from TradeMe Motors, Turners, and dealer pricing trends
• Maintenance factors - estimated per-vehicle servicing cost ratios
• RUC rates - NZTA schedule
• Tesla efficiency - WLTP ratings with ~15% real-world adjustment
• Registration/WoF - current NZTA and VTNZ rates
• Fuel consumption defaults - typical combined cycle values per model

See the all data page for every hardcoded value.