---
title: "Availability: PV systems"
url: "https://ensights.productfruits.help/en/article/availability-pv-systems"
language: "en"
last_updated: "2026-09-09T11:03:44.336Z"
---

# Availability: PV systems

Availability answers: **how much of the plant was actually producing, weighted by how much sunlight it missed?**

An inverter down at noon costs more than the same inverter down at 6pm. PV availability is weighted by irradiance for exactly that reason — it measures lost opportunity, not lost clock time.

## The formula

```
                    Σk [ Δt × (Gk / 1000) × C_down,k ]
Availability = ( 1 − ────────────────────────────────── ) × 100
                     P_total × Σk [ Δt × (Gk / 1000) ]
```

| Term | Meaning | Unit |
| --- | --- | --- |
| C\_down,k | Capacity<br>**not**<br>producing at interval k | kWp |
| P\_total | Sum of installed capacity across all devices | kWp |
| Gk | Irradiance at interval k | W/m² |
| Δt | Interval length in hours (0.25 for 15-minute data) | hours |

Read it as: of all the sunlight-weighted capacity the plant could have offered, what fraction was offline. Subtract from 1 and you have availability.

**Intervals below 50 W/m² are dropped**, so a plant is never marked unavailable for not producing at night.

## What counts as "not producing"

A device is counted in C\_down,k at a given interval when:

-   It reported zero output, or
-   It was not communicating at all, or
-   The **power plant controller was limiting the site** at that interval — a curtailed device is producing, but not producing *freely*, so it lowers availability.

Capacity weighting means the numbers behave the way you would expect. On a 500 kWp site with one 50 kWp inverter down, availability is 90% — not 50% — regardless of how many inverters the site has.

## Two availability figures

| Figure | Curtailment and agreed downtime |
| --- | --- |
| **Availability** | Counted against you. This is the raw measure of what the plant delivered. |
| **Internal Availability** | Excluded, where an availability adjustment event covers the interval. |

Availability adjustment events are logged periods — planned maintenance, grid constraints, owner-instructed curtailment — that are agreed not to count against the plant. Internal Availability is the figure to use when a contract excludes those causes.

Both are calculated over the same intervals from the same data. They differ only in which intervals are excused.

## Availability and PR

They are deliberately separate numbers:

-   **Availability** — how much of the plant was running.
-   **PR** — how well the part that was running performed.

PR already discounts the sunlight that fell on offline equipment, so a day with heavy downtime shows low availability and normal PR. Read them together: low availability with normal PR is a downtime problem; full availability with low PR is a performance problem.