Payback is the number almost every buyer asks for and the one most loosely quoted. The arithmetic is simple. The assumptions underneath it are where quoted figures diverge, and where an optimistic number quietly becomes a wrong one.
What drives the cost
Payback is net cost divided by annual savings. Net cost is the installed price less any subsidy you actually receive. Annual savings is the value of the units the system produces — but not all units are worth the same. Units you consume as they are generated save you the full retail tariff. Units you export are credited at whatever your net metering terms allow, which is typically less. A payback figure that treats all generation as equally valuable is overstating the return.
| Assumption | Effect on payback | Worth checking |
|---|---|---|
| Self-consumption share | Large | What fraction is used vs exported |
| Tariff level and slab | Large | Your effective per-unit rate, not headline |
| Subsidy actually received | Large on small systems | Eligibility confirmed, not assumed |
| Annual generation estimate | Moderate | Assumptions stated and site-specific |
| Future tariff escalation | Moderate | Whether escalation was assumed at all |
| Inverter replacement | Moderate | Usually omitted from quoted figures |
| Panel degradation | Small | Around 0.5% per year |
Directional guidance on sensitivity rather than fixed values. The effect of each assumption depends on system size, tariff, and consumption pattern. Ask for the assumptions behind any payback figure you are quoted.
How we model the numbers
Two omissions flatter quoted payback figures most often. The first is inverter replacement: the inverter will likely need replacing before the modules do, and a payback calculation that runs to year 25 without accounting for it is understating lifetime cost. The second is tariff escalation, which cuts the other way — if grid tariffs rise, your savings rise with them and payback shortens. A serious calculation states both explicitly rather than silently including one and not the other.
Karnataka-specific cost factors
Karnataka's residential tariff is slab-based, which matters more than most buyers realise: solar shaves consumption off your highest slab first, so the value of the first units offset is greater than the average rate suggests. Commercial and industrial consumers face higher tariffs and demand charges, which is why payback is typically shorter for businesses than for households even before depreciation benefits are considered.
Where budgets go wrong
Payback figures go wrong in consistent ways:
- Valuing exported units at the same rate as units consumed directly
- Using the average tariff instead of the top slab that solar actually displaces
- Omitting an inverter replacement from a 25-year calculation
- Treating subsidy as received before eligibility has been confirmed
- Accepting a payback figure with no stated generation assumption behind it
What we recommend
Ask any bidder for the three inputs behind their payback figure: expected annual generation, the tariff rate used, and the assumed split between self-consumption and export. If those three are not stated, the payback number is not comparable to anyone else's and should not be used to choose between quotes.
Common questions
- What is a realistic payback period?
- It depends far more on your tariff and self-consumption than on the equipment. Rather than anchoring on a headline figure, ask for the assumptions and check they match your actual consumption pattern.
- Does exporting more shorten payback?
- Usually the opposite. Exported units are typically credited below retail, so a system whose output is largely exported earns less per unit than one sized to daytime consumption.
- Should inverter replacement be in the calculation?
- Yes, for any calculation running beyond about ten years. Leaving it out makes lifetime returns look better than they are.
- Is payback shorter for businesses?
- Generally, because commercial tariffs are higher and daytime load aligns well with generation. Accelerated depreciation improves it further where applicable.
- Does tariff escalation count?
- It legitimately shortens payback if grid rates rise, but the assumed escalation should be stated rather than buried, since it is doing real work in the number.
Talk to our engineering team for a solar assessment.
We will review your consumption, roof, and site constraints before recommending any capacity.