Methodology

1. What Watt2Buy computes

For every comparison we compute the Total Cost of Ownership (TCO) over the user’s chosen ownership horizon. TCO is the sum of (a) on-road capital cost net of applicable EV incentives, (b) annual fuel or energy cost, (c) annual maintenance, (d) annual insurance, and (e) at end-of-life, residual resale value. The headline number on each card is a lifecycle rupee figure — not just sticker price.

The calculation engine is deterministic. Given the same inputs (vehicles, city, driving profile, financing knobs) it will always return the same numbers. There are no machine-learning models in the pricing path.

2. Cost basis — the “Ranked by” selector

The same comparison can be viewed under four cost bases, switchable from the sidebar of every comparison page and preserved in the share URL:

• Cash — pay everything in today’s rupees, no inflation, no financing.
• Inflation-adjusted — future fuel, electricity, and maintenance inflated year-by-year using the inflation sliders you set.
• Financed — cash total plus the lifetime interest you’d pay on the loan terms you choose (loan %, APR, tenure).
• Financed + Inflation — both adjustments stacked.

The “Lowest TCO” badge and the vehicle ranking always respect the basis you have selected, so the answer stays consistent with your financing assumptions.

3. Fuel and electricity prices — city-level

Petrol, diesel, CNG, and home/public-DC electricity rates are stored per city, not as national averages. Every fuel slider on the profile and comparison pages shows the source name and the effective date of the rate it is using. Sources include:

• Petrol & diesel — central retail-price revisions (most recently the 15 May 2026 PIB order) applied to city baselines from IOCL / BPCL / HPCL.
• CNG — city gas distributor portals (MNGL Pune, MGL Mumbai, IGL Delhi, Adani Total Gas Ahmedabad, Bhagyanagar Gas Hyderabad, GAIL / BGL Bengaluru, IOAGPL Chennai, BPCL / GAIL Kolkata).
• Electricity — latest notified DISCOM domestic-slab tariffs per city, including time-of-day where applicable.

If a city has no verified row, we fall back to the nearest state-proxy or national-average rate and label the slider accordingly so you know the confidence level.

4. State EV-policy treatment

EV road tax, registration fee, central / state purchase subsidies, and toll waivers come from a structured policy table maintained per state, vehicle category, and registration class. Highlights of what the engine handles today:

• Maharashtra — state purchase subsidy is restricted to commercial registrations per the current notification; personal buyers do not receive it.
• Karnataka — EV road tax follows the tiered 5% / 8% / 10% slab introduced by the KA Motor Vehicles Tax Amendment Bill 2026, not a blanket waiver.
• Gujarat — EV road tax applies at 1% pre-GST per the current state policy.
• All other states — engine falls back to the legacy state-policy blob (registration waiver, blanket road-tax waiver) until structured rows are added.

Final eligibility, exact subsidy amount, and disbursal timing are confirmed by the dealer and RTO at the time of registration. Caps and fund availability can change without notice. Please confirm the applicable benefit with your dealer at the time of booking.

5. Vehicle data — specs, prices, range

Ex-showroom prices, ARAI mileage, claimed range, battery capacity, segment, and safety ratings are sourced from OEM portals and cross-referenced with public listings (CarDekho, BikeDekho, official PR releases). Each comparison card stamps the data refresh date.

Vehicle images are sourced from Wikimedia Commons (CC BY-SA) and a self-hosted Cloudflare R2 mirror. Missing photos fall back to a brand-initial placeholder; the “Suggest photo” link on each card lets users contribute.

6. Maintenance & insurance

Per-vehicle annual maintenance is the average of OEM-published service schedules across the first five years of ownership, plus a wear-parts allowance (tyres, brakes, fluids). For EVs we additionally model battery degradation and an end-of-warranty battery-replacement event when the ownership horizon exceeds the OEM warranty period.

Year-1 insurance is bundled into the on-road price; renewal premiums use IRDAI third-party rates plus a standard comprehensive multiplier per body type. Comparison fairness: identical NCB assumption applied across all vehicles in the same comparison.

7. CO₂ — tank-to-wheel + upstream

Operational CO₂ for ICE / hybrid is tank-to-wheel from real-world consumption; for EVs we use the wall-to-wheel grid emission factor for the city / state grid mix. Upstream (refining, T&D losses) is reported as a separate band on the CO₂ tab so you can see both the direct and lifecycle picture.

8. What we explicitly do not model (yet)

• Battery resale value separate from vehicle resale (handled implicitly via residual).
• Time-of-use electricity pricing for fleet / commercial users.
• Vehicle-to-grid revenue.
• Insurance NCB carry-over from a previous vehicle.
• State-specific scrappage benefits beyond the published purchase-subsidy line.
• Carbon-credit revenue from corporate fleets.

We mark every assumption with a tooltip on the comparison page. If you spot a number that doesn’t match a published policy or a real-world quote you’ve received, please flag it using the floating “Spot an error?” button on the comparison page, or via /report-an-issue.

9. Update cadence & changelog

Fuel prices are re-checked monthly; state policies are re-checked on every notification we become aware of and at minimum quarterly; vehicle specs are refreshed at every OEM model-year rollout.

10. Independent review

The methodology has been reviewed under NETRA’s Sustainable Mobility and C&I Decarbonization Councils. The calculation engine is covered by a deterministic golden-scenario test suite that re-runs on every code change to prevent silent drift in the reference numbers.