Rooftop Solar Savings & Payback Analyzer

About the Rooftop Solar Savings & Payback Analyzer

Rooftop solar pays back through the bills it cuts, and how fast depends on more than the panels: the sun where you live, the slabs of your tariff, the way your utility treats the units you send out (net metering, net billing or gross metering), the subsidy, a loan, maintenance, an inverter replaced halfway and panels that make a little less every year.

This analyzer works all of it out month by month for every year. It estimates what your system makes from NASA POWER’s sun data for 237 cities (or your own figure), builds your bill before and after solar from your tariff, banks and settles surplus units the way net metering does, and runs the money year by year: your own money at the start, the savings, EMIs and running costs. You get the payback, the payback with the savings discounted, the gain over 25 years, NPV, IRR, the cost of a solar unit and the CO2 avoided, with the three metering arrangements side by side.

For homes in India it shows PM Surya Ghar: Muft Bijli Yojana’s published assistance as a reference you can apply with one press; it never assumes a subsidy you have not entered. Without a pass the free preview shows the payback in full with the first rows of each table; every other figure, the full 25-year report as a PDF and the CSV come with a Pro pass. Nothing you enter or open leaves your browser.

Common uses

• Deciding whether rooftop solar is worth it before you ask for quotes.
• Seeing what net metering is worth against gross metering or net billing for your home.
• Checking whether a solar loan pays for itself from the bill savings.
• Taking a 25-year report to your family, an installer or a bank.

How the savings are worked out

• Performance ratio: PR = (1 − system losses) × inverter efficiency × (1 − heat loss) × (1 − extra shading). With the PVWatts defaults of 14% losses and a 96% inverter, and 7% for hot panels, PR is 0.768 (IEC 61724-1 defines PR as the final yield ÷ the reference yield).
• Generation: each month, kW × the peak sun hours on the panels × the days × PR; each later year × (1 − degradation × (year − 1)).
• The bill: energy charge (one rate, or each slab’s units at its own rate) + per-unit surcharges, with the duty, + the fixed charge, which solar never removes.
• Savings: each month, the bill before solar − the bill after it, + what the utility pays for exported units.
• Money: today, −price + subsidy + loan; each year, savings − EMIs − maintenance after the years included − insurance − the inverter in its year. Tariffs and costs grow by the rises you enter.

Net metering, net billing and gross metering

• Net metering: one two-way meter. Each month the units exported come off the units imported; a surplus is banked and carried to later months, and what is still banked at the end of the settlement year is paid at the export rate, or every month, or lapses where the rules say so. Solar then saves your top slab’s rate on every unit it replaces, which is why it usually pays back fastest.
• Net billing: the solar you use at once saves the tariff, and every unit you export earns the export rate. The share used at once depends on when you use power; a smart-meter file lets the analyzer work it out hour by hour.
• Gross metering: every unit the panels make is sold at the feed-in tariff and your bill stays as it was.

Which arrangement you get, up to what size and at what export rate is set by your utility’s regulator. The comparison table shows the same system under all three, so you see what the rules are worth to you.

PM Surya Ghar: Muft Bijli Yojana (India)

The scheme gives homes central financial assistance (CFA) of 60% of the system cost for the first 2 kW and 40% of the additional cost from 2 to 3 kW, capped at 3 kW: at the benchmark prices ₹30,000 for 1 kW, ₹60,000 for 2 kW and ₹78,000 for 3 kW or more (PIB). The analyzer pro-rates sizes in between per kW. The announcement also mentions collateral-free loans at around 7% for home systems up to 3 kW. Households apply on the National Portal, which decides what is sanctioned; the analyzer only uses the subsidy you enter.

Sun data, losses and ageing

City values are NASA POWER’s 20-year climatology of the sun on a surface facing the equator, flat, tilted at the latitude or at latitude + 15°, for the grid cell around each city (about 100 km across); for another place, the NASA POWER Data Access Viewer gives the peak sun hours for any coordinates. These data were obtained from the NASA Langley Research Center (LaRC) POWER Project funded through the NASA Earth Science/Applied Science Program. The 14% system losses and the 96% inverter are the defaults of the PVWatts Version 5 Manual. Panels lose output slowly: nearly 2,000 measured degradation rates have a median of 0.5% a year (NREL review); your panel’s output warranty gives its own figure.

Payback, NPV, IRR and the cost of a unit

• Payback: the time the running total of your money gets back to zero, the year’s flow spread evenly over it. With the savings discounted, later savings count for less.
• NPV: the gain in today’s money at your discount rate (what your money would earn elsewhere, or your loan’s rate).
• IRR: the yearly return on your own money that makes the NPV zero; with no money of your own at the start it is not defined.
• Cost of a solar unit (levelised): everything you pay over the life, discounted, ÷ the units made, discounted. When it is below your tariff, solar is cheaper than the grid.

Sources

• PIB: Cabinet approves PM-Surya Ghar: Muft Bijli Yojana — CFA, loans
• PM Surya Ghar National Portal
• NASA POWER Data Access Viewer — sun data
• PVWatts Version 5 Manual — default losses
• Photovoltaic Degradation Rates — An Analytical Review — degradation
• IEC 61724-1, Photovoltaic system performance — Part 1: Monitoring (performance ratio)

Limitations

• An estimate to plan and compare, not financial advice: shade, the roof’s direction, the inverter and the weather change the real output, and a site survey settles it.
• Metering rules, export rates and size limits differ by utility and change over time; enter yours from your utility’s or regulator’s current order.
• The subsidy is what you enter: the scheme’s amounts are shown as a reference, and the National Portal decides what is sanctioned.
• City sun data are averages for an area about 100 km across; a hazy city or a shaded roof makes less.

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