Blog Solar financing
Flat Rate vs Reducing Balance: What Your Solar Loan Really Costs
An 8 percent flat rate is not an 8 percent loan. It costs about the same as 14 percent on a reducing balance, and most solar buyers are never told which one they have been quoted. Here is how to tell the difference and convert between them.
- A flat rate charges interest on the full original amount for the whole term. A reducing balance rate charges interest only on what you still owe. The same headline number means two very different loans.
- An 8 percent flat rate over 7 years costs about the same as 13.7 percent APR. On a 500,000 loan that is roughly 125,000 of extra interest — a quarter of everything you borrowed.
- The common rule that a flat rate is double the real rate overstates it. The true multiplier is about 1.6 to 1.8 over normal solar loan terms.
- You cannot compare two quotes until both are expressed the same way. Convert the flat rate first, then compare on total cost rather than on the monthly instalment.
Two lenders quote you a solar loan. Both say 8 percent. One of them will cost you a quarter of the system price more than the other, and nothing in the quote will tell you which.
This is not a scam and it is not hidden. It is a difference in convention — two ways of expressing interest that have existed in banking for a century — and it catches solar buyers constantly, because rooftop solar is often the first large purchase people finance outside a mortgage or a car.
Here is what separates the two, how to convert between them, and the questions that get you a straight answer from a lender.
The two ways lenders quote interest
A reducing balance rate charges interest only on the money you still owe. You start owing the full amount and pay interest on all of it. A month later you have repaid some principal, so the next month's interest is calculated on the smaller balance. By the final year you owe very little and are paying interest on very little. This is how mortgages work, how car loans work, and how nearly every bank prices a solar loan. When a rate is called an APR, this is what it means.
A flat rate — also called an add-on rate — charges interest on the original amount for every year of the term, regardless of what you have repaid. Borrow 500,000 over 7 years at 8 percent flat, and you are charged 8 percent of 500,000 in year one, and again in year seven, when the balance you actually owe is nearly zero. The lender adds the whole interest bill to the principal at the start and divides the total into equal instalments.
The flat rate is simpler to calculate by hand, which is why it survives: an installer can quote a monthly figure in a few seconds without a spreadsheet. But it prices a loan as though you never repaid a rupee, a peso, or a shilling of it.
The same number, two very different loans
Take a 500,000 solar system financed over 7 years — figures below work in any currency, since the arithmetic does not change.
| 8% reducing balance | 8% flat rate | |
|---|---|---|
| Monthly instalment | 7,793 | 9,286 |
| Total interest | 154,621 | 280,000 |
| Total repaid | 654,621 | 780,000 |
| True annual rate | 8.0% | 13.7% |
The instalment gap looks modest: 9,286 against 7,793, around 1,500 a month. That is exactly why this catches people. Nobody signs a loan because of a 19 percent difference in the monthly payment when the total difference is 125,379.
Notice also what the flat rate does to the shape of the debt. On the reducing balance loan, every payment moves you meaningfully closer to owning the system. On the flat rate loan, a fixed slice of every payment is interest that was decided before you made the first one.
Flat rate to true annual rate: the table
The conversion depends on the term as well as the rate, because a longer term gives the reducing balance more time to actually reduce. This table gives the reducing balance APR that each flat rate is equivalent to.
| Flat rate | 3 years | 5 years | 7 years | 10 years | 15 years | 20 years |
|---|---|---|---|---|---|---|
| 5% | 9.3% | 9.2% | 9.0% | 8.7% | 8.3% | 8.0% |
| 6% | 11.1% | 10.8% | 10.6% | 10.2% | 9.7% | 9.3% |
| 7% | 12.8% | 12.5% | 12.2% | 11.7% | 11.0% | 10.5% |
| 8% | 14.5% | 14.1% | 13.7% | 13.1% | 12.3% | 11.7% |
| 9% | 16.2% | 15.7% | 15.2% | 14.5% | 13.6% | 12.9% |
| 10% | 17.9% | 17.3% | 16.7% | 15.9% | 14.8% | 14.1% |
| 12% | 21.2% | 20.3% | 19.5% | 18.5% | 17.2% | 16.3% |
Read across the 8 percent row and the pattern becomes clear: the shorter the term, the worse a flat rate is in relative terms, because you repay the principal quickly while the interest keeps being charged as though you had not.
You can run your own figures — any currency, any term, both conventions — in our solar loan calculator. Choose “flat / add-on rate” and it reports the equivalent APR next to your monthly payment.
Why “just double it” gets it wrong
The rule of thumb repeated across finance sites is that a flat rate is roughly double the real rate. It is directionally right and numerically wrong, and if you are comparing two live offers the difference matters.
The doubling rule comes from an approximation that assumes your average outstanding balance over the loan is half the original — reasonable as a sketch, but it ignores that interest is compounded monthly and that you hold more than half the balance for more than half the term. In practice:
| Quote | Doubling rule says | Actually |
|---|---|---|
| 8% flat over 5 years | 16% | 14.1% |
| 8% flat over 10 years | 16% | 13.1% |
| 6% flat over 15 years | 12% | 9.7% |
So use doubling as an alarm bell, not as arithmetic. It tells you that a flat quote and an APR quote are not comparable. It will not tell you which of two real offers is cheaper.
How to tell which one you were quoted
Quotes rarely label themselves. These questions do the work:
- “Is that rate flat or reducing balance?” The direct question. In markets where flat rates are normal, the word people use is often “flat”, “add-on”, or “fixed on the sanctioned amount”.
- “What is the total amount repayable, including all fees?” The figure that ends the argument. Compare offers on this and nothing else.
- “What is the APR or effective annual rate?” In several countries a lender must disclose this on request. If a quoted 8 percent comes back as 13.7 percent APR, you were quoted a flat rate.
- “Can I see the amortisation schedule?” On a reducing balance loan the interest column falls every month. On a flat rate loan it is the same figure in every row — that is the tell.
- “What happens to the interest if I settle early?” Flat rate contracts often keep some or all of the scheduled interest. Find out before you sign, not when you try to clear it.
- “Are there processing, insurance, or documentation fees?” A low flat rate with a 2 percent processing fee and mandatory credit insurance can beat or lose to a higher APR. Only the total tells you.
The quickest arithmetic check needs no cooperation from anyone. Multiply the monthly instalment by the number of months and subtract the amount borrowed — that is your total interest. Then divide it by the amount borrowed and by the number of years. If the result is close to the rate you were quoted, it is a flat rate.
Where each convention is used
Reducing balance quoting is standard across Europe, North America, and Australasia, and it is what commercial banks use for solar and equipment lending in most of the world. Consumer credit rules in many of these markets require an APR to be disclosed, which is precisely what makes the flat rate impractical there.
Flat and add-on rates remain common across South Asia, South-East Asia, the Gulf, and parts of Africa. They cluster in a particular place: instalment plans offered directly by the installer or dealer rather than by a bank. That is worth knowing because installer financing is exactly what a solar buyer is most likely to be offered at the point of sale, when a monthly figure sounds reassuring and the total is never mentioned.
Two caveats. First, this is a tendency and not a rule — plenty of lenders in flat rate markets quote reducing balance, and some dealer schemes in reducing balance markets are effectively flat. Second, a subsidised flat rate under a national rooftop programme can genuinely be the cheapest money available. Neither convention is a verdict on its own.
What to do with the answer
Converting the rate is not the goal. Choosing the cheaper loan is, and that means three steps.
Put every offer in the same units. Convert flat quotes to their equivalent APR, or convert everything to total amount repayable. Comparing an 8 percent flat against a 10 percent APR without converting will lead you to the wrong lender.
Then compare on total, not on instalment. A longer term always produces a smaller monthly payment and almost always a larger total. If a lender responds to “that is too expensive” by extending the term, they have made the loan cost more, not less.
Then check the loan against the system. A solar loan is only worth taking if the electricity it buys is worth more than the interest it costs. Work out your monthly bill saving and compare it with the instalment — our solar loan calculator shows this directly as a net monthly cost, and the payback and ROI calculator covers the same question over the system's full life. If the instalment exceeds the saving for the whole term, the system is not paying for itself; you are.
None of this requires you to become a finance expert. It requires one question — flat or reducing? — and one number, the total amount repayable. Ask for both before you sign anything, and the convention stops being a trap and goes back to being what it should have been all along: a detail.
Common questions
What is a flat rate solar loan?
A flat rate, also called an add-on rate, charges interest on the full original loan amount for every year of the term, no matter how much you have already repaid. If you borrow 500,000 at an 8 percent flat rate over 7 years, you are charged 8 percent of 500,000 every year for 7 years — 280,000 in total interest — even in the final year when you owe almost nothing. The interest is added to the principal at the start and the total is divided into equal monthly instalments.
What is a reducing balance interest rate?
A reducing balance rate, the basis of an APR, charges interest only on what you still owe. Each month the interest is calculated on the outstanding balance, so as you repay principal the interest portion of each payment falls and the principal portion rises. This is how mortgages, car loans, and most bank solar loans work, and it is the convention behind the standard amortisation formula.
Is an 8 percent flat rate the same as an 8 percent APR?
No, and the gap is large. An 8 percent flat rate over 7 years works out to about 13.7 percent APR on a reducing balance. Over 5 years it is about 14.1 percent, and over 15 years about 12.3 percent. The flat rate is always the more expensive loan when the headline number is the same, because you keep paying interest on money you have already given back.
How do I convert a flat rate to a reducing balance rate?
There is no simple closed-form formula — the true rate has to be solved numerically. The practical method is to work out the monthly instalment the flat rate produces (principal plus total flat interest, divided by the number of months), then find the reducing balance rate that would produce that same instalment. Our solar loan calculator does this automatically: choose the flat rate option and it reports the equivalent APR alongside your payment.
Is the rule that a flat rate is roughly double the real rate accurate?
It overstates the difference for most solar loan terms. The multiplier is closer to 1.6 to 1.8 times over typical terms of 5 to 20 years. An 8 percent flat rate over 10 years is about 13.1 percent APR, a multiplier of 1.64, not 16 percent. Doubling is a useful warning that the gap is big, but it is not accurate enough to compare two real offers.
Which countries quote solar loans as a flat rate?
Flat or add-on rates are common across South Asia, South-East Asia, the Gulf, and parts of Africa, particularly on instalment plans offered directly by installers and dealers. Reducing balance quoting is standard across Europe, North America, and Australasia, and is also what most commercial banks use for solar and equipment lending everywhere. The convention often varies between lenders within the same country, so always ask rather than assume.
Is a flat rate loan ever the better deal?
Yes, but only when the numbers say so rather than the label. A flat rate loan with a low headline number, no processing fee, and a subsidised rate can still beat a reducing balance loan carrying a high APR plus arrangement fees. The point is not that flat rates are a trap — it is that you cannot compare a flat quote with an APR quote until you convert one into the other.
Does prepaying a flat rate loan save interest?
Usually far less than you would expect, and sometimes nothing at all. Because the interest was calculated upfront on the full principal, many flat rate contracts require the whole scheduled interest regardless of early settlement, or apply a rebate formula that returns only part of it. Ask specifically what happens to the interest if you settle early, and get the answer in writing before signing.