How we work out the numbers

Time moves in months. A tool starts at month 0, the day you decide, and steps forward one month at a time to the end of the period you choose.

Prices that rise each year (fuel, electricity, rent) stay flat for twelve months and then step up by the yearly rate. Investment returns and home prices grow a little every month, at the monthly rate that adds up to the yearly figure you enter.

Nothing is rounded until it is displayed.

Loan rates are quoted the way banks quote them: a yearly rate divided by 12 for each month. The EMI is the fixed monthly payment that clears the loan exactly at the end of the tenure. Each month, interest is charged on what is still owed and the rest of the EMI repays the loan.

Break-even is the first month in which the running total of what an option saves catches up with what it cost upfront. If that never happens within the period you chose, the tool says so instead of guessing beyond it.

The pessimistic, base and optimistic results rerun the same calculation with the values in the default scenario table, changing only the inputs the table covers. They are a range of reasonable assumptions, not forecasts.

"What would flip the answer" changes one input at a time and holds everything else. It searches for the value at which the result crosses zero, which is where the other option starts to win.

A lakh in fifteen years buys less than a lakh today. Where a tool shows an amount in today's rupees, it divides the future amount by inflation over the same period.

For each car, the monthly running cost is the fuel or electricity for your distance plus a twelfth of the yearly upkeep. The saving is the petrol car's cost minus the other car's.

The extra purchase price is charged interest at the rate you enter, because that money is either borrowed or not invested. Break-even is the month the running total of savings reaches the extra price with interest added. The savings themselves are not assumed to be invested.

For an EV, the price per unit blends home and public charging by the share you charge at home.

Each month, the tool works out your electricity bill twice through the state's slab tariff: once for your full use, and once for what is left after solar. Because the bill is worked out slab by slab, the units solar saves come off the most expensive slab first.

Units you export beyond your own use earn the export rate. The bill with solar never drops below the fixed charges. Panel output falls each year by the degradation rate, and the tariff rises once a year.

Payback is the first month the savings add up to the system's cost after subsidy. The return (IRR) is the yearly rate at which those savings exactly repay that cost.

The tool follows two households. The buyer pays the down payment, stamp duty and other purchase costs, then each month the EMI, maintenance on the home's value, and property tax that rises with inflation. The renter pays rent that rises once a year, and invests what the buyer spent at purchase, less the security deposit.

Each month, whoever pays less invests the difference at your return. At the end, each side's net worth is compared.

The tool follows two households with the same money until the loan's original end date. One puts the amount into the loan; the other invests it. Both spend the original EMI's worth every month: whatever the loan does not take is invested at your return. With “keep the EMI”, the loan ends sooner and the whole EMI is invested afterwards; with “lower the EMI”, the EMI is recalculated over the same end date and the drop is invested.

A prepayment charge comes out of the amount paid in. If you enter a tax rate and a limit, the tax saved on each year's interest is invested at the year end, on both sides.

The tool follows two buyers with the same money until the loan's last EMI. The cash buyer pays the whole price today and invests the EMI every month. The loan buyer pays the down payment and the processing fee, invests the rest of the price, and pays the EMI. Both own the same car at the end, so its value cancels out.

The return at which the two are level is the loan's true cost: the yearly rate that makes what you receive (loan less fee) equal to the EMIs you pay.

Each AC's units a year come from its star label, scaled by how much you run it compared with the label. The units saved, times your price per unit, is the saving, spread evenly over the months; the price per unit rises once a year. As in the car tool, the extra price is charged interest month by month, and the payback is the first month the savings so far cover it.

The tool works out the tax twice, with the slabs, standard deduction, rebate and cess for the tax year from the Income Tax Department. Taxable income is your salary and other income, less the standard deduction (up to your salary) and the deductions you enter for that regime. The rebate wipes out the tax up to its limit while taxable income is within the rebate's income limit. Under the new regime, marginal relief then caps the tax at the income above that limit. Cess is added on top.

The tool starts with what the plot costs today, price plus stamp duty, registration and other buying costs, and invests the same amount in a fixed deposit and an equity mutual fund for the same years. The plot is sold at the end, less selling costs and long-term capital-gains tax on the gain over what was paid, and its yearly costs are subtracted as paid (without interest, which flatters the plot slightly). FD interest is taxed each year at your rate. The fund is taxed once, on the gain above the yearly exemption. Rates are from the Income Tax Department and PIB.

The same amount goes into each at the start of every month. The SIP grows at the fund return you enter; when sold at the end, instalments held 12 months or less are taxed as short-term gains and the rest as long-term gains above the yearly exemption, with cess. PPF earns its rate monthly on the balance and is credited once a year, tax-free. The recurring deposit grows at its rate after your tax on the interest each year. PPF and tax rules are from the National Savings Institute and the Income Tax Department.

One model runs all three. A lump sum is invested at the start, a SIP at the start of every month (raised each year by any step-up), and withdrawals are taken at the end of every month (raised each year if you choose). Every purchase is kept as its own lot, and withdrawals sell the oldest units first. Gains sold in each 12-month year of the plan are taxed under the equity rules: short-term if held 12 months or less, long-term above the yearly exemption, with cess. The SWP result is how long the withdrawals last and what is left; the most you can withdraw is found by trying amounts until the money just lasts.

Each tool lists the money factors it does not model, and a checklist of the things that are not about money. Where a value is not verified yet, the tool says so and asks you for your own figure instead of filling one in.