The monthly payment is the easy part. What decides whether a solar loan is a good deal is the total interest, the effective annual rate and what your electricity bill looks like while the loan is still running.
A bank quote can hide the rate. If you were given a total repayable amount or a monthly instalment instead of an interest rate, the calculator solves for the rate rather than guessing one.
Cash and credit are compared on the same system. The difference between the two totals is financing cost, not a different quality of solar.
What the EMI calculator answers
“EMI” is an equated monthly instalment: the fixed payment that clears a loan over a set number of months. The calculator goes beyond that single figure and answers four practical questions:
- What leaves my account each month? Principal plus interest, not the system price divided by the term.
- What does the loan add to the price? Total repayable minus the amount borrowed.
- What is the real annual rate? Including the effect of monthly compounding, which is higher than the quoted nominal rate.
- How does the whole 25 years look? Payments, grid bills, maintenance, replacements and export income, kept as separate lines.
This is a planning tool, not a bank offer. It cannot see your bank’s fees, insurance, early-settlement penalty or variable-rate clause, so treat the output as a way to compare offers and ask better questions.
Start with the electricity bill, not the system price
The first input is either your monthly bill or your monthly units (kWh). You only need one; the calculator derives the other. That derived number is the average tariff: bill divided by units.
Your usage pattern — mostly daytime, a bit of both, or mostly evening — decides how much of the solar you consume directly. A household that runs air conditioners at noon can self-consume most of the production. A household that is empty all day and cooks at night will export or waste much of it unless a battery is added. That is why the same system can produce very different bill savings for two neighbours.
The three ways a bank can describe the loan
Any loan is fully described by four quantities: the amount borrowed, the payment, the number of payments and the interest rate. If you know three, the fourth is determined. The calculator accepts whichever three your bank gave you.
| What the bank gave you | What the calculator does |
|---|---|
| An interest rate | Calculates the standard equal-payment instalment from the amount financed, rate and term. |
| A monthly instalment | Multiplies by the number of months to get the total, then solves for the rate that produces that payment. |
| A total repayable amount | Divides by the number of months to get the payment, then solves for the rate. |
| Nothing but the price and term | Reports that it cannot determine a rate rather than inventing one. |
The standard amortisation formula is:
P = amount borrowed after down payment
r = annual rate ÷ 12
n = term in months
When the bank quotes a total instead of a rate, the calculator finds the r that makes the present value of the payments equal the amount borrowed. That is an iterative solve, not a shortcut. If the total is smaller than the amount borrowed, the inputs do not describe a real loan — often because a fee was deducted before the money reached the installer — and the calculator says so instead of producing a fictional payment.
Nominal versus effective rate
A “9%” loan with monthly payments does not cost 9% per year on the declining balance in the way most people read it. The monthly rate is 0.75%, and compounding twelve times gives an effective annual rate of about 9.38%. The calculator reports both, which is why the effective figure is the one to use when comparing a bank loan with a cash discount or a subsidised scheme.
A worked example
Suppose a $12,000 system with a $2,400 down payment, leaving $9,600 financed at 6% over five years:
Payment = 9,600 × 0.005 ÷ [1 − 1.005−60] ≈ $185.59/month
Total repaid = 185.59 × 60 ≈ $11,135
Interest = 11,135 − 9,600 ≈ $1,535
Effective annual rate = 1.00512 − 1 ≈ 6.17%
Now compare that with the bill. If the household pays $180/month and the system covers 90% of its consumption, the solar saving is about $162/month. During the loan the household pays $185.59 to the bank plus $18 to the grid — about $203.59, or $23.59 more than before. After the loan ends, it pays about $18/month instead of $180. The loan is cash-flow negative for five years and strongly positive afterwards; whether that is acceptable is a personal decision, not a mathematical one.
This is the comparison most sales presentations skip. A “bill replacement” pitch that quotes only the post-loan saving hides the years when the household pays both the bank and the grid.
What happens to surplus solar
If the array produces more than the house uses at that moment, the calculator asks what happens to the rest:
- Store it in a battery. Some of the surplus is recovered at night, after round-trip losses and usable-depth limits. The battery’s cost and replacement are added separately.
- Sell it to the grid. Export income is valued at the rate you enter. If you leave it empty, the calculator values exports at zero rather than inventing a tariff.
- Leave it unused. The surplus is curtailed. This is the honest default in markets without export compensation.
Battery savings are reported as the additional benefit over solar alone. That matters: a battery does not get credit for the solar savings that would have happened anyway. If the battery does not pay for itself, the calculator says so rather than folding it into a rosy total.
The 25-year view
The long-run section separates money saved, money earned from export, money paid on day one, money paid to the bank, maintenance and replacements. It deliberately does not collapse them into one “net” figure before you can see the parts. The assumptions behind the long-run numbers are editable:
| Assumption | Default behaviour | Why it matters |
|---|---|---|
| Electricity price rise | 0% unless you change it | A rising tariff makes solar more valuable; the calculator does not assume it for you. |
| Panel degradation | Editable annual loss | Production, and therefore savings, decline slowly each year. |
| Cleaning and repairs | Editable % of system cost | Small annual costs compound over 25 years. |
| Inverter replacement | Year and cost editable | Most inverters are replaced at least once in a 25-year period. |
| Subsidy or rebate | Optional | Reduces the day-one cost and therefore the payback. |
The “no price rise” default is conservative in one direction and optimistic in another: it avoids inventing tariff increases, but it also ignores the real possibility that grid prices rise faster than your loan rate. Run the calculation twice, once at 0% and once at a rise you consider plausible, and see whether the decision changes.
Where the estimate stops
Use it for three things it does well: comparing two loan offers, deciding whether to pay cash, and understanding the cash-flow gap during the loan term. Do not use it to justify borrowing more than you can service in a month when the sun does not shine and the bill still arrives.
Frequently asked questions
How is the monthly solar loan payment calculated?
It uses the standard equal-payment amortisation formula: payment = principal × monthly rate ÷ [1 − (1 + monthly rate)−months]. The principal is the system price minus your down payment. If your bank quoted a total repayable amount instead of a rate, the calculator divides that total by the number of months and solves for the rate that produces it.
My bank gave me the total I will repay. Can I still use the calculator?
Yes. Enter the total under “total amount I repay to the bank”, along with the term and the amount financed. The calculator derives the monthly payment and estimates the implied rate. If the total is less than the amount borrowed, it will flag that the figures do not describe a standard loan rather than producing a misleading result.
Will my electricity bill go down during the loan?
The grid portion usually goes down, but you also have the loan instalment. During the term, your total monthly outgo may be higher than the old bill, roughly equal, or lower depending on the system size, loan rate and how much solar you self-consume. The calculator shows the grid bill and the loan payment separately so you can see the combined cash flow rather than only the saving.
Is paying cash cheaper than taking a loan?
If the system price is the same, yes: a loan adds interest and possibly fees, so the total paid is higher. Financing can still be rational if it preserves cash for emergencies or if the solar saving exceeds the financing cost. The calculator compares the total cost and the monthly cash flow, not just the headline payment.
Does the calculator include a battery?
Yes, as an optional add-on. You enter the battery size, cost, usable share, round-trip loss, life and replacement cost. The battery’s benefit is reported as the extra saving over solar alone, and its cost and replacement are included in the 25-year view. A battery that does not pay back is shown as such.
Why is the effective interest rate higher than the quoted rate?
Because interest is charged monthly and compounds. A nominal 9% annual rate is 0.75% per month; compounding twelve times gives about 9.38% effective annual cost. The gap grows with the nominal rate and is one reason two loans with the same headline rate can cost different amounts if their fees or compounding differ.