Is True Green RTC a Reality?

On 18 February 2026, I attended a conference in Mumbai where the CMD of Maharashtra State Electricity Distribution Company Limited (MSEDCL) outlined their proposal to invite bids for Green RTC power. My initial reaction was scepticism about the technical feasibility of such a construct in the present Indian power system. He subsequently clarified that the proposed condition was to ensure minimum 51% green energy in the supply mix.

That clarification triggered a more substantive question in my mind: if one were to design a genuinely round-the-clock green supply architecture—rather than a nominal 51% compliance structure—what would its technical configuration look like in the Indian grid context ignoring the cost aspect? Specifically, what combination of wind, solar, storage, and balancing resources would be required to make it operationally credible and system-compatible? To examine this question rigorously, I relied on Grid Controller of India Limited 15-minute time-block data, using granular, source-wise generation and demand figures to assess the technical feasibility of such a configuration.

I worked on 1 February 2026 grid india data of demand and source wise generation across 96 fifteen-minute time blocks. The actual demand met on that day, as published by Grid-India, provides a granular and unforgiving test for the idea of “Green RTC” — supplying demand round the clock using only wind, solar, and battery energy storage (BESS), without fossil balancing.

This note constructs a conceptual Wind–Solar–BESS system designed to meet the actual demand met on that day in all 96 blocks. The objective is not advocacy but arithmetic: to quantify required capacities, storage sizing, energy balance, and curtailment implications.

1. The data foundation: 96-block demand

The starting point is the actual demand met (MW) for each of the 96-time blocks on 1 February 2026. Let:

D(t) = Demand in MW in block t, where t = 1…96
Each block = 0.25 hour
Daily energy demand = Σ D(t) × 0.25 MWh

This profile contains three structural features:

• Morning ramp
• Midday plateau (winter solar season)
• Evening peak with sharp net-load ramp

Any RTC design must satisfy:

Generation(t) + BESS discharge(t) − BESS charge(t) = D(t)
for every block t.

No deficit is allowed in any time block.

2. Renewable generation modelling

I have used scripts w for Wind, s for Solar, v for VRE, G for generation, D for Demand

Assumed installed capacities:

Ws = Solar capacity (MW)
Ww = Wind capacity (MW)

From time-block CUF profiles for 1 February:

Solar generation:
Gs(t) = Ws × CUFs(t)

Wind generation:
Gw(t) = Ww × CUFw(t)

Total VRE generation:
Gv(t) = Gs(t) + Gw(t)

These CUF profiles reflect actual meteorology of the day, not annual averages. That distinction is critical. A P50 annual CUF is irrelevant for RTC compliance in a specific block.

3. Direct supply first, storage later

For each time block:

If Gv(t) ≥ D(t):
Excess(t) = Gv(t) − D(t) → available for charging or curtailment.

If Gv(t) < D(t):
Deficit(t) = D(t) − Gv(t) → must be supplied by BESS discharge.

This approach minimizes storage cycling by prioritizing direct VRE-to-load supply.

4. BESS specification

Let:

Pb = BESS power capacity (MW)
Eb = BESS energy capacity (MWh)
RTE = 90% (round-trip efficiency assumed)

State of charge evolves as:

SOC(t) = SOC(t−1)

·        [Charge(t) × 0.25 × ηc]
− [Discharge(t) × 0.25 / ηd]

Where ηc × ηd = 0.90.

Constraints:

0 ≤ SOC(t) ≤ Eb
Discharge(t) ≤ Pb
Charge(t) ≤ Pb

To qualify as RTC, SOC must never hit zero during deficit blocks.

5. Evening ramp stress

On 1 February (winter profile), solar output declines sharply after ~17:00, while demand remains elevated. This creates a steep deficit window across roughly 8–16 blocks.

The BESS must therefore:

• Have sufficient power capacity Pb to meet the maximum single-block deficit.
• Have sufficient stored energy Eb to cover cumulative deficit across the evening and early night before wind recovery.

Even if wind contributes partially, its variability forces conservative sizing.

6. Determining required capacities

Step 1: Selected solar and wind ratio.

Because February solar CUF is moderate but evening zero, and wind CUF is typically lower but extends into night, a blended portfolio is required. Pure solar plus storage becomes prohibitively large.

Step 2: Increase Ws and Ww until daily energy surplus ≥ daily demand / RTE.

Daily VRE energy must exceed daily demand because storage losses consume roughly 10%.

Condition:

Σ Gv(t) × 0.25 ≥ Daily Demand / 0.90

Step 3: Size BESS energy (Eb).

Compute cumulative deficit curve:

Cumulative Deficit(k) = Σ (Deficit(t) × 0.25)

The maximum cumulative deficit between two surplus windows determines minimum Eb.

Step 4: Size BESS power (Pb).

Pb ≥ Maximum Deficit(t)

Even if energy suffices, inadequate power rating will fail block-level RTC.

7. Curtailment implications

When Ws and Ww are increased to guarantee evening adequacy, midday surplus grows disproportionately.

Curtailment(t) = Excess(t) − Charge(t)

If BESS is already full, all additional excess becomes curtailment.

In winter conditions:

• Solar peaks around noon.
• Demand plateau may not absorb full solar.
• Once SOC reaches Eb, incremental generation is stranded.

Therefore, to guarantee 96-block reliability, capacity must be sized for worst deficit block, not average block. This inevitably produces:

• High installed MW
• Large Eb
• Significant midday curtailment

RTC compliance is driven by the tail of the deficit distribution, not the mean.

8. Assumptions & Method applied in Excel Model:

The model intentionally isolates Wind + Solar + BESS only.

• 96 time blocks (15-minute resolution)
• Actual solar and wind generation shape (CUF profile of that day)
• Round-trip efficiency (RTE) ~ 90%, Charging / Discharging efficiency = 5% each
• Depth of discharge (DoD) = 90%
• No hydro, no PSP, no thermal backup
• No demand response
• No transmission constraints
• No forecast error
• Zero Loss of Load Probability (LOLP = 0)
• Expected Unserved Energy (EUE = 0)

This is therefore a deterministic adequacy island model.

• Actual 15-minute demand profile used for 1st Feb.2026 (total demand = 4,472 MU).
• Actual wind and solar generation profiles of that day used as shape templates.
• For each time block, wind energy share (10%–90%), wind and solar were proportionally scaled.
• Total VRE energy was forced to equal Demand / 0.90 to compensate storage losses (5% for charging and 5% for discharging.
• Block-wise SOC simulation performed.
• System considered feasible only if SOC never became negative.
• For feasible cases, required RE capacity, BESS energy (MWh) and BESS power (MW) were computed.

b.     The Adequacy Condition

For every block t:

Generation(t) + Discharge(t) − Charge(t) = Demand(t)

State of Charge evolution:

SOC(t) = SOC(t−1)

• Charge(t) × 0.25 × ηc
  − Discharge(t) × 0.25 / ηd

Constraints:

0 ≤ SOC(t) ≤ Eb
Charge(t) ≤ Pb
Discharge(t) ≤ Pb

Feasibility requires SOC(t) never to hit zero in any deficit block.

This is equivalent to enforcing LOLP = 0 for the day.

c.      Daily Energy Requirement

Daily demand = 4,472 GWh

Because storage losses consume ~10%, VRE must produce:

Required VRE energy ≥ 4,472 / 0.90 ≈ 4,969 GWh

Annual CUF assumptions are irrelevant here. Only the actual block profile of 1 February matters.

d.     Shape Mismatch: The Core Structural Driver

The February winter profile exhibits three structural features:

• Moderate solar plateau
• Weak post-sunset wind recovery
• Sustained evening demand

The system stress is driven by:

Maximum single-block deficit (MW)
Maximum cumulative deficit window (GWh)
Evening ramp magnitude (MW/hour)

Sizing is governed by the worst cumulative deficit, not the daily average.

e.      Wind–Solar Mix Sensitivity

Wind share was varied from 50% to 80% of total VRE energy.

Key finding:

Green RTC configurations below ~65% wind share was infeasible.

Solar-heavy portfolios collapse during the evening cumulative deficit window because solar contributes nothing after sunset and storage exhausts before wind recovery.

The minimum total capacity solution with storage occurred at:

Wind energy share ≈ 75%
Solar energy share ≈ 25%

This is winter-specific for a day only but structurally revealing.

f.      Infrastructure Required for 1 February 2026 Green RTC

At optimal mix (~75% wind share):

Wind capacity ≈ 1,010 GW
Solar capacity ≈ 251 GW
Total VRE capacity ≈ 1,261 GW

Peak demand that day ≈ 227 GW.

Thus, installed RE ≈ 5.5–6 times peak demand.

Blended effective CUF (after curtailment and losses) ≈ ~16–18%.

Battery requirements:

BESS energy ≈ 287 GWh (minimum cumulative deficit coverage)
BESS discharge power ≈ 110–120 GW
BESS charge power ≈ ~60 GW

Battery discharge power approaches national peak ramp magnitude.

This is not peak shaving storage. It is adequacy storage.

To accumulate sufficient energy for the evening deficit:

• VRE must overproduce during midday.
• Once SOC reaches Eb, excess generation is curtailed.

Even in the optimal mix case, a significant portion of midday solar becomes stranded once storage saturates.

RTC compliance is driven by the tail of the deficit distribution, not the mean generation.

snapshot of Excel Sheet & load/generation profiles are at the end of the blog.

g.     Why Capacity Explodes

The arithmetic reveals three structural drivers:

1. Energy Losses
   10% storage losses force overgeneration.

2. Intra-Day Shape Mismatch
   Solar-zero evening window requires multi-hour discharge.

3. Cumulative Deficit Window
   Sizing is governed by the longest uninterrupted deficit stretch.

Even if daily energy matches perfectly, block adequacy may fail.

Annual energy sufficiency is irrelevant to 96-block adequacy.

h.     Asset Utilization Implications

This configuration produces:

• Very high installed MW relative to demand
• Low average utilization of VRE fleet
• High capital locked in midday surplus
• Battery cycling concentrated in few stress hours

RTC is therefore a capacity product, not an energy product.

The marginal MW exists to insure against worst deficit blocks, not to serve average load.

Using this framework on the 1 February 2026 profile yields three unavoidable structural outcomes:

First, overcapacity is mandatory. Installed wind + solar capacity must exceed peak demand by a substantial margin to accumulate enough daytime surplus for evening discharge.

Second, storage energy must cover multi-hour deficits, not just peak shaving. In winter, this can extend to 4–6 continuous hours of high discharge.

Third, curtailment becomes structural rather than incidental. Even with optimal charging logic, surplus blocks exceed storage absorption capability once Eb is full.

9. Economic signal

This design reveals a fundamental tension:

RTC is a reliability product, not merely an energy product.

To eliminate fossil balancing:

• VRE capacity must be sized to worst-case deficit.
• BESS must be sized to worst cumulative gap.
• Losses must be overbuilt into the system.

The result is a capital-intensive system with declining marginal utilization of assets.

10. What This Does Not Mean

It does not mean Green RTC is impossible.

It does not mean fossil capacity must remain dominant.

It means:

Reliability is determined by block-level adequacy, not annual energy accounting.

Eliminating fossil balancing shifts the reliability burden entirely to overbuild and storage.

The arithmetic is solvable. The capital intensity is the real question.

11. Single-Day vs Annual RTC

This study examines one winter day under deterministic conditions.

A full 365-day P95 meteorological adequacy test would likely:

• Increase required storage
• Increase wind dominance
• Increase overcapacity
• Increase structural curtailment

A single-day Green RTC is an engineering exercise.
An 8,760-block Green RTC is a planning challenge of an entirely different magnitude.

12. Policy Implication

If Green RTC is procured purely on a ₹/kWh basis, the system’s capacity requirement is obscured. The tariff reflects delivered energy, but not the overbuilt megawatts and storage depth required to guarantee block-level adequacy. When treated correctly as a reliability product, pricing must explicitly account for excess installed capacity, storage sized to worst cumulative deficits, structural curtailment, and low asset utilization. An energy-only framework systematically understates the economics of RTC. Reliability is not an energy commodity; it is a capital-intensive insurance function.

For planners and regulators, three clear implications follow. First, P50 annual energy metrics are inadequate for RTC claims; block-level P90/P95 adequacy is the binding constraint. Second, energy-only tariff structures conceal the true cost of capacity and distort investment signals. Third, curtailment is not a residual inefficiency—it is intrinsic to overbuild strategies and must be internalized in LCOE and procurement design.

A 96-block Green RTC configuration for 1 February 2026 is technically achievable. The arithmetic works. However, it demands substantial over installation of wind and solar, deep battery energy reserves sized to evening cumulative deficits, and acceptance of structural curtailment. The engineering constraint can be solved deterministically. The economic and regulatory challenge is considerably more complex.

Conclusion

A strict 96-block Green RTC configuration for 1 February 2026 is technically achievable using actual demand and meteorological profiles. The simulation, based on the Grid-India 15-minute data and a 90% BESS round-trip efficiency assumption, enforces zero deficit in every block.

However, block-level compliance forces sizing to the worst cumulative deficit window rather than average generation. The system must be engineered against the longest uninterrupted evening shortfall, not the mean CUF.

Under the optimal winter mix:

• Total installed wind and solar exceeds ~1.3 TW
• RE capacity is roughly 5.5–6 times the ~227 GW peak demand and 6.7-7 times the average demand of 186 GW of that day
• Battery discharge power requirement exceeds 100 GW, approaching national ramp magnitudes
• Storage energy requirement lies in the range of ~450–625 GWh
• Structural curtailment becomes unavoidable

Wind dominance is not a preference but a structural necessity in winter conditions. Solar-heavy portfolios fail because storage exhausts during the post-sunset deficit window. Even at the most capacity-efficient mix, wind must contribute roughly two-thirds to four-fifths of total VRE energy.

The binding constraint is cumulative deficit, not annual CUF. Daily energy sufficiency does not ensure block adequacy.

Green RTC, therefore, is not a feasibility problem. The arithmetic works. It is fundamentally a capital allocation problem — how much overbuild is acceptable, how much storage must be carried, how much curtailment can be tolerated, and ultimately who pays for reliability once fossil balancing is excluded from the equation.