Power System Reliability

SAIDI · SAIFI · CAIDI — Learn the Why and How

Three core IEEE 1366 indices that quantify outage duration, frequency, and restoration time. Clear definitions, realistic use cases, and a live calculator.

Duration

SAIDI — System Average Interruption Duration Index

SAIDI = Σ(Nᵢ × Dᵢ) ÷ Ntotal
Units: minutes per customer · year

Meaning: Average outage minutes each customer experiences in a year.

Use cases:

  • Track annual reliability targets and trend restoration improvements.
  • Communicate typical experience to regulators and customers.
Example: Σ(Nᵢ×Dᵢ)=4,400; Ntotal=1,000 → SAIDI=4.40 min/customer·yr.
Frequency

SAIFI — System Average Interruption Frequency Index

SAIFI = Σ(Nᵢ) ÷ Ntotal
Units: interruptions per customer · year

Meaning: Average number of sustained interruptions per customer per year.

Use cases:

  • Diagnose feeder health: frequent short outages elevate SAIFI.
  • Prioritize vegetation and equipment programs that reduce hits.
Example: ΣNᵢ=140; Ntotal=1,000 → SAIFI=0.14 interruptions/customer·yr.
Restoration

CAIDI — Customer Average Interruption Duration Index

CAIDI = SAIDI ÷ SAIFI
Units: minutes per interruption

Meaning: Average duration of an interruption for those who experience one.

Use cases:

  • Field ops KPI: restoration efficiency when outages occur.
  • Helps isolate whether issues are about how often vs how long.
Example: SAIDI=4.40; SAIFI=0.14 → CAIDI=31.43 min/interruption. If SAIFI=0, CAIDI is undefined.
Quick check

Which denominator is right?

We have three outages: (100 customers, 5 min), (200, 10 min), (300, 11 min). That gives ΣNᵢ=600 and Σ(Nᵢ×Dᵢ)=5,800 customer·minutes.

Method A (IEEE correct)

Use Ntotal = total customers served in the territory (e.g., 10,000).

SAIFI = 600 / 10,000 = 0.06
SAIDI = 5,800 / 10,000 = 0.58
CAIDI = 0.58 / 0.06 = 9.67

Method B (common mistake)

Set Ntotal = ΣNᵢ = 600.

SAIFI = 600 / 600 = 1.00 (always 1!)
SAIDI = 5,800 / 600 = 9.67
CAIDI = 9.67 / 1 = 9.67

Your turn: Which method is right, and why? Post your answer on Coursera under the related discussion post for this module.
Practice

Live calculator — see both methods side by side

Enter outages (Nᵢ customers affected, Dᵢ duration in minutes) and your service territory size Ntotal. We compute both the correct IEEE method and the wrong method (where Ntotal is forced to ΣNᵢ) so you can compare.

Customers affected (Nᵢ) Duration (Dᵢ, minutes)
Leave blank to preview only the wrong method.

Correct (IEEE)

Enter Ntotal to compute correct metrics.

Wrong (sets Ntotal = ΣNᵢ)

Waiting for outages…