Two factories, same rooftop, same DISCOM, same tariff, and they should still choose different solar models. The panels don't change. What changes is how you'd rather pay for them: out of your own capital, out of a per-unit tariff, or out of a plant that isn't even on your roof. Get that decision right and solar becomes one of the cleanest financial moves your plant makes. Get it wrong and you either tie up cash you needed elsewhere, or you sign away years of savings to someone else.
Here's how the four common models actually work, in plain language, with the honest trade-offs your CFO will ask about.
| Model | Upfront | Ownership | You pay | Best for |
|---|---|---|---|---|
| CAPEX | Full project cost | Yours from day one | Nothing after commissioning, you keep every unit | Capital-ready owners who want the highest lifetime return |
| RESCO / PPA | ₹0 | Developer, through the term | A per-unit tariff below your grid rate, for 15–25 years | Plants that want savings without spending capital |
| BOOT | ₹0 | Developer now, you after the term | A per-unit tariff, then the asset transfers to you | Owners who want eventual ownership without the upfront hit |
| Open access | Varies | Remote plant (owned or contracted) | Wheeled power plus state wheeling & banking charges | Multi-MVA loads beyond what the roof can hold |
CAPEX, you buy it, you own it
In the CAPEX model, you fund the plant and it's yours from the day it switches on. Every unit it generates is a unit you don't buy from the grid, so the savings land directly on your books, no middleman, no per-unit tariff, no term that ends. This is the model with the highest lifetime return, because after payback the electricity is effectively free for the rest of the plant's 25-year life.
Payback on a well-sized C&I rooftop plant is typically in the 3–5 year range (indicative, it moves with your tariff, your daytime load and applicable depreciation benefits). The catch is simply the cheque: you're committing capital now. If that capital would earn more inside your core business, or if it's just not available this year, one of the other three models exists precisely for you.
RESCO / PPA, ₹0 upfront, you buy the units
Under a RESCO (Renewable Energy Service Company) model, a developer builds, owns, operates and maintains the plant on your roof at no upfront cost to you. You sign a Power Purchase Agreement (PPA), usually 15 to 25 years, to buy the solar units at a fixed per-unit tariff set below your current grid rate. You pay only for what the plant actually generates.
The appeal is straightforward: savings from month one, zero capital outlay, and the asset sits off your balance sheet with the developer carrying performance and maintenance risk. The trade-off is that you don't own the plant, so you capture a slice of the savings rather than all of it, and you're committing to a long agreement, worth reading the escalation and exit clauses carefully. For a plant that would rather keep its capital in the business, this is often the cleanest way in.
BOOT, the middle path to ownership
BOOT (Build-Own-Operate-Transfer) is a close cousin of RESCO. Again the developer builds and owns the plant with ₹0 upfront from you, and again you buy the units at an agreed tariff for the contract term. The difference sits in that final letter: at the end of the term, ownership transfers to you, usually at little or no cost.
So you get the no-capital start of a PPA during the term, and the free-electricity upside of CAPEX afterwards. It suits owners who genuinely want to own the asset eventually but can't, or would rather not, write the cheque today. As with any PPA-style deal, the tariff, escalation and the transfer terms are what decide whether the numbers work, so those are the clauses to scrutinise.
Open access, solar beyond your roof
The first three models put panels on your roof, and a roof only holds so much. When your load runs into multiple MVA and rooftop capacity simply can't cover it, open access lets you contract power from a larger solar plant somewhere else in the state and have it wheeled to your factory over the grid.
This unlocks a scale rooftop can't reach, but the economics are genuinely state-specific. You pay for the wheeled energy plus the wheeling, banking, cross-subsidy and other charges your state levies, and those charges, along with banking rules and your own load profile, decide whether it pays off. Open access can be attractive depending on state regulations, charges and load profile, and it deserves a proper, state-specific assessment rather than a rule of thumb. Treat every figure here as indicative until your own numbers are run.
How to choose
- You have capital ready and want the best lifetime return, with a plant you own outright? CAPEX.
- You want savings from month one with no capital and no maintenance headache, and you're comfortable not owning the asset? RESCO / PPA.
- You want that same ₹0-upfront start but you do want to own the plant in the end? BOOT.
- Your load is multi-MVA and bigger than the roof, in a state with workable wheeling and banking rules? Open access.
There's no universally cheapest model, there's the one that fits your capital position, your load and your appetite for owning the asset. The honest way to decide is to put your actual tariff, daytime consumption and available roof (or land) against each option and compare like for like. That's what a feasibility assessment is for. You can see how these models map onto real capacity on our solar models overview, and how they've played out on real sites in our case studies.