Explainer/ Renewable energy transition · the arithmetic
Contracting more solar does not keep raising your renewable share. Generation only counts in the block you consume it, which caps open access at roughly 60% for a round-the-clock load and makes spill, storage and wind the real levers.
By jouleWise · · about 5 minutes
Most industrial renewable procurement starts from one figure: annual consumption. Divide it by a plant's capacity utilisation factor and you get the megawatts that would, on paper, generate every unit the plant uses in a year. On paper, that is 100% renewable. In practice, a plant that contracts that much solar will find its renewable share stalls far lower and its blended cost rising. The annual figure hides the only thing that matters, which is when.
Open access electricity is scheduled and settled in fifteen-minute blocks. A unit of solar generated at noon can only displace a unit the plant is drawing at noon. If the plant is drawing less than the portfolio generates in that block, the surplus is spilled, or banked on terms the state regulator sets and routinely tightens. The plant has paid for the unit either way. Meanwhile, at 2 a.m., the plant is importing at tariff because the sun is down.
For a round-the-clock industrial load, this caps what open access solar and wind can reach at roughly 60% of consumption before storage or a different balance between solar and wind moves it further. Beyond that point, more of the same capacity spills rather than substitutes.
REplanner reports the share of generation that never reaches the plant because it arrived in blocks the plant was not consuming. In its default case — 63 million units a year, 12 MW of open access solar, 8 MW of wind and 3.6 MWp of rooftop with no storage — spill is small. Push the solar slider up and watch spill climb and blended cost with it, while the renewable share creeps. That is the point of the tool: it is as willing to say no as yes.
Wind generates through the night and the monsoon, so a solar–wind mix matches a flat load far better than solar alone. Rooftop solar sits behind the meter and so carries no open access charges, but it is limited by roof area rather than capital. Battery storage moves generation into the blocks the plant is consuming; the modelling assumes 330 cycles a year, and the figure that decides it is the cost per kWh discharged over the asset's life, not the capital cost per kWh installed. Thermal storage does the same job for heat, often more cheaply per unit shifted.
The headline tariff from a developer is ex-bus. What the plant pays is landed: ex-bus plus wheeling, banking, cross-subsidy surcharge and additional surcharge, each set by the state regulator and each changing year to year. A case that works in one state can be marginal in the next. The regulatory work — open access approvals, connection agreement, scheduling registration and ABT metering — is the project; the panels are the easy part.
In short/ Renewable share is set by matching, not capacity
Most industrial renewable procurement starts from one figure: annual consumption. Divide it by a plant's capacity utilisation factor and you get the megawatts that would, on paper, generate every unit the plant uses in a year. On paper, that is 100% renewable. In practice, a plant that contracts that much solar will find its renewable share stalls far lower and its blended cost rising. The annual figure hides the only thing that matters, which is when.
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