Reliability & unserved energy
The key idea
A 3 kW shortage for one hour leaves 3 kWh unserved. A 1 kW shortage for three hours also leaves 3 kWh unserved. Their energy totals match, but their durations and effects on customers can differ.
Unmet energy is the energy demand that the system does not supply. Hours with shortage counts time steps in which some demand is unmet. Reserve is spare available power held against a stated requirement. These describe different conditions.
Capacity is not always available power
PV nameplate capacity is not firm output during a cloudy period or at night. A battery can have sufficient discharge power but insufficient stored energy. A generator may have a rating that exceeds the load while maintenance, fuel or operating constraints make it unavailable.
The following exercise keeps firm capacity available throughout. It isolates the accounting before adding those real operating constraints.
Stress the supply
With the defaults, the ninth hour has 12 kW demand, no solar output and 8 kW firm supply. It leaves 4 kWh unmet in that hour. At the assumed 20% reserve requirement, the required spare power is another 2.4 kW.
Increase firm capacity to 12 kW. Predict whether all demand will be served, then check whether reserve also passes. Change the demand multiplier and observe both results.
Load and available supply in the stress period
- Demand
- Available supply
- Unmet load
Read the exact values
| Time / step | Demand | Available supply | Unmet load |
|---|---|---|---|
| Hour 1 | 4.00 | 8.00 | 0.00 |
| Hour 2 | 5.00 | 9.00 | 0.00 |
| Hour 3 | 7.00 | 10.00 | 0.00 |
| Hour 4 | 9.00 | 11.00 | 0.00 |
| Hour 5 | 10.00 | 10.00 | 0.00 |
| Hour 6 | 8.00 | 9.00 | 0.00 |
| Hour 7 | 6.00 | 8.00 | 0.00 |
| Hour 8 | 9.00 | 8.00 | 1.00 |
| Hour 9 | 12.00 | 8.00 | 4.00 |
| Hour 10 | 10.00 | 8.00 | 2.00 |
| Hour 11 | 7.00 | 8.00 | 0.00 |
| Hour 12 | 5.00 | 8.00 | 0.00 |
- Unmet energy
- 7.0 kWh
- Hours with shortage
- 3 / 12 h
- Hours below reserve
- 6 / 12 h
Demand energy: 92.0 kWh. Unmet share: 7.6%. Changing the reserve requirement changes the reserve finding, not the energy already supplied.
Data table: One-hour energy and reserve checks
| Hour | Demand kW | Available kW | Unmet kWh | Reserve shortfall kW |
|---|---|---|---|---|
| 1 | 4.0 | 8.0 | 0.0 | 0.0 |
| 2 | 5.0 | 9.0 | 0.0 | 0.0 |
| 3 | 7.0 | 10.0 | 0.0 | 0.0 |
| 4 | 9.0 | 11.0 | 0.0 | 0.0 |
| 5 | 10.0 | 10.0 | 0.0 | 2.0 |
| 6 | 8.0 | 9.0 | 0.0 | 0.6 |
| 7 | 6.0 | 8.0 | 0.0 | 0.0 |
| 8 | 9.0 | 8.0 | 1.0 | 1.8 |
| 9 | 12.0 | 8.0 | 4.0 | 2.4 |
| 10 | 10.0 | 8.0 | 2.0 | 2.0 |
| 11 | 7.0 | 8.0 | 0.0 | 0.4 |
| 12 | 5.0 | 8.0 | 0.0 | 0.0 |
Model note · Constructed 12-hour cloudy-period stress case; 1-hour steps. Firm capacity is available throughout. Solar follows a fixed teaching trace. No storage, failures, network losses or transient response are modeled.
At 12 kW firm capacity, all demand in this teaching period is served. At the highest load, however, no spare capacity remains. A design can therefore have zero unmet energy and still fail its stated reserve requirement.
Name the period and the denominator
A twelve-hour stress test is not an annual reliability result. For a complete hourly year, Phasor calculates the fraction of hours with unmet load and the ratio of unmet energy to total load energy. Those ratios need their time span and denominator to be meaningful.
Do not rename the shortage-hour count in this diagram as customer SAIDI. Customer interruption indices need the applicable definitions, customer exposure and event data. Likewise, do not choose a universal “acceptable” shortage percentage from this example. A project must declare its own service and reserve requirements.
Energy shortage and reserve use separate equationsOptional — the page reads completely without it
At each one-hour step, with demand D and available supply A:
unmet and served power
Punmet = max( 0 , D − A )
Pserved = D − Punmet
energy-shortage share over the period
This is undefined when demand energy is zero. The fraction of affected hours is the number of shortage hours divided by the twelve modeled hours.
reserve, checked separately
Rrequired = r · D
Rspare = max( 0 , A − D )
Rshortfall = max( 0 , Rrequired − Rspare )
The reserve check does not dispatch equipment or simulate ramp rates, and a reserve shortfall is never added to unmet energy. The fixed solar profile, constant firm availability and absence of storage are teaching simplifications.
See it in Phasor
Review unmet load and the relevant reliability and reserve results with the selected simulation period. Read feasibility findings against the limits you declared, then inspect the hours that caused them.