Honest solar maths: what a Bengaluru home really saves, using your own bill

Every solar quotation you will be shown contains a savings number, and almost every one of them is built the same way: take a flattering per-unit tariff, multiply it by an optimistic generation figure, project it over 25 years, and print the total in bold. The number is large. It is also, usually, somebody else's number.

Your roof does not run on averages. It runs on your consumption, your sanctioned load, your shading, and the tariff you actually pay. This article shows you how to do the maths on your own bill instead of on a brochure's — and it is honest about the one input nobody can warrant.

Why is your real tariff higher than the headline tariff?

Open your BESCOM bill and find two numbers: units consumed and the amount you actually paid. Divide the second by the first. That figure — your effective per-unit rate — is the only tariff that matters, and it is almost always higher than the slab rate printed on the tariff order.

The reason is that your bill is not just energy charges. It carries fixed charges tied to your sanctioned load, FPPCA (the fuel and power purchase cost adjustment, which moves), and electricity duty. Once those are folded in, a household quoting the headline slab rate can understate what it genuinely pays per unit by a meaningful margin.

This cuts in your favour, incidentally. A higher effective rate means every unit your roof produces displaces a more expensive unit — so the honest arithmetic tends to make solar look better, not worse. Which is exactly why it is worth doing properly rather than borrowing an average.

A caution in the other direction: if you use very little electricity, your bill is dominated by fixed charges that solar does not remove. Below a monthly bill of roughly ₹1,500, the maths gets thin. We say so plainly rather than sell you a system that will disappoint you.

How should you size the system — from your bill, not from an average home?

The second place brochures cheat is sizing. "A typical Bengaluru home needs 3 kW" is a marketing sentence, not an engineering one.

Take twelve months of units consumed, if you can — not one month. Bengaluru consumption swings: summer air-conditioning, monsoon lighting loads, a geyser that only earns its keep in December. A system sized on your April bill will be oversized for August; one sized on your July bill will leave you buying grid units every summer.

Then check your sanctioned load on the bill. BESCOM requires your sanctioned load to be at least the size of the system you propose to connect. Many older Bengaluru homes are sanctioned well below what their roof warrants — which is a real statutory line in your costing, charged extra and not optional:

• +1 kW — ₹10,000 · +2 kW — ₹15,000 · +3 kW and above — ₹7,000 per kW · all plus 18% GST
• Nil if your existing sanction already suffices, or if you arrange the increase yourself

Shown only when you need it. Never buried inside another line.

You can get your own sized figure, against your own bill and sanctioned load, in under a minute: helira.in/start.

What can nobody promise you — and why we are saying it out loud

Here is the sentence most solar marketing will not print: sunlight is not warrantable.

Generation depends on irradiance, and irradiance varies. It varies by season — Bengaluru's monsoon months are simply less productive than its clear pre-summer ones. It varies year to year, because some monsoons are longer than others. And it varies with your specific roof: a water tank, a neighbour's parapet, or a tree that will be taller in three years all take units off the table that no contract can restore.

We can be accountable for design, for components, for workmanship and for service. We are. Those things are ours, and they are written down: Warranted by Helira.

We cannot be accountable for the weather, and any company that tells you a fixed savings figure is a promise is either not being careful with language or is hoping you will not notice the difference. So: nothing in this article is a guarantee of savings, generation or payback. Everything below is an estimate, with its assumptions stated where you can see them.

What does a 3 kW system actually cost — and what does payback follow from?

The third sleight of hand is comparing payback against the gross price. It should follow from your net outlay — what leaves your hands after the subsidy arrives.

Take the base case, no sanctioned-load increase needed. For a 3 kW system on a Bengaluru independent house the all-in cost is ₹2,62,725 including GST, and the net outlay after the ₹78,000 PM Surya Ghar subsidy is ₹1,84,725 — the number your payback actually runs from, broken out line by line in our worked example.

That is an illustration, not a quote. Your own figure depends on your roof, your bill and your sanctioned load.

Three honest footnotes on those numbers. First, the ₹78,000 subsidy is credited to your bank account after commissioning and net metering — it is not a discount booked against the invoice, so the full amount leaves your hands first. Second, the subsidy requires DCR panels, which cost more; the true incremental benefit is closer to ₹59,000 than to ₹78,000, and that is an approximation, not a precise figure. Third, regulatory handling is one line at ₹15,000 — the PM Surya Ghar application where you opt in, your society or RWA NOC, civil-work permissions (BBMP/BWSSB where applicable), the net-metering paperwork and BESCOM coordination. One fee, one point of accountability. You sign the forms we prepare; we run the queues.

How do you project 25 years without fooling yourself?

A rooftop system's benefit accumulates slowly and for a long time. Which means a 25-year projection is unavoidable — and also the easiest place in the whole exercise to lie by omission.

The honest way is to state the assumptions on the same page as the number. There are two that matter:

Tariff growth. Your savings grow only if grid tariffs rise. They historically have, but nobody knows by how much. An estimate assuming 5% per year should say "assuming 5% per year" beside the total — because at 3% the number is materially smaller, and you are entitled to know which dial was turned.

Panel degradation. Modules produce slightly less each year. An estimate assuming 0.5% per year should say so. A projection that quietly holds year-25 output equal to year-1 output is not optimistic; it is wrong.

Do the arithmetic yourself if you like: your effective per-unit rate, times your estimated annual units, escalated at a tariff-growth assumption you choose, degraded at roughly half a percent a year, against a net outlay of ₹1,84,725 in the example above. You will get a range, not a number. The range is the truth. Anyone handing you a single confident figure has hidden the assumptions rather than resolved them.

Who does the work, and who answers for it

Helira is a brand and technology company for residential rooftop solar. The physical installation is carried out by installers certified to Helira’s standard — but your contract is with Helira Energy Private Limited, and the warranty and accountability stay with us, not with whoever held the drill. You never chase a manufacturer or an installer.

That is the part we can put in writing, and do: Warranted by Helira.

Run the numbers on your own bill

The closing argument of this whole series is one line: a decision that runs for 25 years deserves your own arithmetic, not an average home's. Read your effective tariff off your own bill. Size from your own twelve months. Check your own sanctioned load. Judge payback against your net outlay, not the gross price. And insist that every projection you are shown carries its assumptions in plain sight.

Get a transparent estimate for your own roof in minutes at helira.in/start, or send us your bill on WhatsApp and we will walk through the maths with you: wa.me/918073291022. We work with owner-occupied independent houses in Bengaluru with a monthly bill above about ₹1,500.