Renewable fraction & curtailment
The key idea
Curtailment is available generation that is not used. Unmet energy is demand that is not supplied. They can occur in the same day, at different hours, if the plant cannot move enough energy through time.
A large PV array can therefore have high production and limited useful contribution. Read a renewable percentage together with its accounting boundary and denominator.
Follow one hour first
At noon in the example, an 8 kW PV scale produces 8 kW while demand is 4 kW. With no battery or export path, 4 kW serves the load and 4 kW is curtailed. Over that one-hour step, the curtailed energy is 4 kWh.
At midnight, PV output is zero. The diesel supplies all 4 kW demand. Increasing the PV scale alone does not change that hour.
Compare plants that serve the same load
Increase the PV scale from 8 to 16 kW. Predict whether curtailed energy or useful PV energy will increase faster. Compare the alternatives in the table, including their fuel and assumed daily cost.
Available PV and the portion that serves demand
- Available PV
- PV to load
- Load
Read the exact values
| Time / step | Available PV | PV to load | Load |
|---|---|---|---|
| 0:00 | 0.00 | 0.00 | 4.00 |
| 1:00 | 0.00 | 0.00 | 4.00 |
| 2:00 | 0.00 | 0.00 | 4.00 |
| 3:00 | 0.00 | 0.00 | 4.00 |
| 4:00 | 0.00 | 0.00 | 4.00 |
| 5:00 | 0.00 | 0.00 | 4.00 |
| 6:00 | 0.00 | 0.00 | 4.00 |
| 7:00 | 2.07 | 2.07 | 4.00 |
| 8:00 | 4.00 | 4.00 | 4.00 |
| 9:00 | 5.66 | 4.00 | 4.00 |
| 10:00 | 6.93 | 4.00 | 4.00 |
| 11:00 | 7.73 | 4.00 | 4.00 |
| 12:00 | 8.00 | 4.00 | 4.00 |
| 13:00 | 7.73 | 4.00 | 4.00 |
| 14:00 | 6.93 | 4.00 | 4.00 |
| 15:00 | 5.66 | 4.00 | 4.00 |
| 16:00 | 4.00 | 4.00 | 4.00 |
| 17:00 | 2.07 | 2.07 | 4.00 |
| 18:00 | 0.00 | 0.00 | 4.00 |
| 19:00 | 0.00 | 0.00 | 4.00 |
| 20:00 | 0.00 | 0.00 | 4.00 |
| 21:00 | 0.00 | 0.00 | 4.00 |
| 22:00 | 0.00 | 0.00 | 4.00 |
| 23:00 | 0.00 | 0.00 | 4.00 |
- Renewable share of service
- 41.8%
- Curtailed PV
- 20.6 kWh/day
- Diesel energy
- 55.9 kWh/day
Available PV 60.8 = used PV 40.1 + curtailed PV 20.6 kWh/day. Used PV + diesel = 96 kWh/day served. With no storage, more noon output cannot supply the night.
Same service, different energy use
- Curtailed PV
- Diesel energy
Read the exact values
| Time / step | Curtailed PV | Diesel energy |
|---|---|---|
| 0 kW PV | 0.00 | 96.00 |
| 4 kW PV | 0.00 | 65.62 |
| 8 kW PV | 20.62 | 55.86 |
| 12 kW PV | 48.94 | 53.79 |
| 16 kW PV | 77.53 | 52.00 |
Fuel and cost assumptions: 0.30 L per diesel kWh, USD 1/L fuel, and USD 0.30 per installed PV kW per day. Constant 2026 USD; invented accounting allowances, not market prices. No starts, idling, degradation, replacement or financing.
Data table: Equal-service PV alternatives
| PV kW | Served kWh/d | Renewable % | Curtailed kWh/d | Fuel L/d | Cost USD/d |
|---|---|---|---|---|---|
| 0 | 96 | 0.0 | 0.0 | 28.8 | 28.80 |
| 4 | 96 | 31.6 | 0.0 | 19.7 | 20.89 |
| 8 | 96 | 41.8 | 20.6 | 16.8 | 19.16 |
| 12 | 96 | 44.0 | 48.9 | 16.1 | 19.74 |
| 16 | 96 | 45.8 | 77.5 | 15.6 | 20.40 |
Model note · Constructed 24-hour case: constant 4 kW demand, a sinusoidal daytime PV shape, unlimited diesel backup, no battery, no exports and no conversion losses. Every size serves the same 96 kWh/day. Values are illustrative, not plant simulation results.
All five alternatives serve 96 kWh/day. This makes the comparison clear: a lower fuel total is not produced by quietly serving less demand. The changing cost is only the result of the explicitly stated fuel and PV allowances.
A battery, flexible demand or an export connection can change how much surplus becomes useful. Each introduces its own capacity, loss, operating or commercial constraints. This exercise omits those paths so that the timing effect remains visible.
Do not relabel stored diesel energy
Battery discharge is not automatically renewable. Its origin depends on what charged it. If a generator charges the battery, calling all later discharge renewable would overstate the renewable contribution.
Phasor’s annual summary uses a defined accounting convention: one minus nonrenewable production divided by served load energy plus grid sales, bounded between zero and one. Diesel production and grid purchases are counted as nonrenewable in that convention. It is not a renewable-attribute certification method.
The accounting boundaryOptional — the page reads completely without it
In this no-storage, no-export, lossless case, the available PV splits into the part that served the load and the part that was curtailed, and diesel fills the rest of the load:
available PV splits
EPV,available = EPV,used + Ecurtailed
served load is PV plus diesel
EPV,used + Ediesel = Eserved
renewable fraction, this lesson
RF = 1 − EdieselEserved = EPV,usedEserved
The denominator is always the 96 kWh/day served, never the available PV production.
Phasor's annual-summary convention
RF = clamp( 1 − Ediesel + Egrid purchasesEload − Eunmet + Egrid sales , 0 , 1 )
Storage losses and generator charging mean a battery-discharge total must not be substituted for renewable energy. The no-storage widget avoids that ambiguity rather than inventing source tracking. The linear 0.30 L/kWh fuel factor and daily PV cost allowance are authored assumptions.
See it in Phasor
Compare renewable fraction, curtailment, unmet demand and cost for saved design alternatives. Keep the same demand and declared accounting basis when explaining a change in performance.