diff --git a/src/prefect/CHANGELOG.md b/src/prefect/CHANGELOG.md
index 9f30cd0f..8de42e32 100644
--- a/src/prefect/CHANGELOG.md
+++ b/src/prefect/CHANGELOG.md
@@ -12,6 +12,7 @@ and this project adheres to
 
 - Implement i4 workflow
 - Implement i7 workflow
+- Implement c1 workflow
 - Implement u5 workflow
 - Implement u6 workflow
 - Implement u9 workflow
diff --git a/src/prefect/indicators/usage/c1.py b/src/prefect/indicators/usage/c1.py
new file mode 100644
index 00000000..96bbc046
--- /dev/null
+++ b/src/prefect/indicators/usage/c1.py
@@ -0,0 +1,161 @@
+"""QualiCharge prefect indicators: usage.
+
+C1: Number of sessions by operator.
+"""
+
+from string import Template
+from typing import List
+from uuid import UUID
+
+import numpy as np
+import pandas as pd  # type: ignore
+from prefect import flow, runtime, task
+from prefect.futures import wait
+from prefect.task_runners import ThreadPoolTaskRunner
+from sqlalchemy.engine import Connection
+
+from ..conf import settings
+from ..models import Indicator, IndicatorTimeSpan, Level
+from ..utils import (
+    export_indic,
+    get_database_engine,
+    get_num_for_level_query_params,
+    get_targets_for_level,
+    get_timespan_filter_query_params,
+)
+
+SESSIONS_BY_OPERATOR_QUERY_TEMPLATE = """
+        SELECT
+            count(*) AS value,
+            nom_operateur AS category,
+            $level_id AS level_id
+        FROM
+            Session
+            INNER JOIN statique ON point_de_charge_id = pdc_id
+            LEFT JOIN City ON City.code = code_insee_commune
+            $join_extras
+        WHERE
+            $timespan
+            AND $level_id IN ($indexes)
+        GROUP BY
+            category,
+            $level_id
+        """
+
+QUERY_NATIONAL_TEMPLATE = """
+        SELECT
+            count(*) AS value,
+            nom_operateur AS category
+        FROM
+            SESSION
+            INNER JOIN statique ON point_de_charge_id = pdc_id
+        WHERE
+            $timespan
+        GROUP BY
+            category
+        """
+
+
+@task(task_run_name="values-for-target-{level:02d}")
+def get_values_for_targets(
+    connection: Connection,
+    level: Level,
+    timespan: IndicatorTimeSpan,
+    indexes: List[UUID],
+) -> pd.DataFrame:
+    """Fetch sessions given input level, timestamp and target index."""
+    query_template = Template(SESSIONS_BY_OPERATOR_QUERY_TEMPLATE)
+    query_params = {"indexes": ",".join(f"'{i}'" for i in map(str, indexes))}
+    query_params |= get_num_for_level_query_params(level)
+    query_params |= get_timespan_filter_query_params(timespan, session=True)
+    return pd.read_sql_query(query_template.substitute(query_params), con=connection)
+
+
+@flow(
+    task_runner=ThreadPoolTaskRunner(max_workers=settings.THREAD_POOL_MAX_WORKERS),
+    flow_run_name="c1-{timespan.period.value}-{level:02d}-{timespan.start:%y-%m-%d}",
+)
+def c1_for_level(
+    level: Level,
+    timespan: IndicatorTimeSpan,
+    chunk_size=settings.DEFAULT_CHUNK_SIZE,
+) -> pd.DataFrame:
+    """Calculate c1 for a level and a timestamp."""
+    if level == Level.NATIONAL:
+        return c1_national(timespan)
+    engine = get_database_engine()
+    with engine.connect() as connection:
+        targets = get_targets_for_level(connection, level)
+        ids = targets["id"]
+        chunks = (
+            np.array_split(ids, int(len(ids) / chunk_size))
+            if len(ids) > chunk_size
+            else [ids.to_numpy()]
+        )
+        futures = [
+            get_values_for_targets.submit(connection, level, timespan, chunk)  # type: ignore[call-overload]
+            for chunk in chunks
+        ]
+        wait(futures)
+
+    # Concatenate results and serialize indicators
+    results = pd.concat([future.result() for future in futures], ignore_index=True)
+    merged = targets.merge(results, how="left", left_on="id", right_on="level_id")
+
+    # Build result DataFrame
+    indicators = {
+        "target": merged["code"],
+        "value": merged["value"].fillna(0),
+        "code": "c1",
+        "level": level,
+        "period": timespan.period,
+        "timestamp": timespan.start.isoformat(),
+        "category": merged["category"].astype("str"),
+        "extras": None,
+    }
+    return pd.DataFrame(indicators)
+
+
+@flow(
+    task_runner=ThreadPoolTaskRunner(max_workers=settings.THREAD_POOL_MAX_WORKERS),
+    flow_run_name="c1-{timespan.period.value}-00-{timespan.start:%y-%m-%d}",
+)
+def c1_national(timespan: IndicatorTimeSpan) -> pd.DataFrame:
+    """Calculate c1 at the national level."""
+    engine = get_database_engine()
+    query_template = Template(QUERY_NATIONAL_TEMPLATE)
+    query_params = get_timespan_filter_query_params(timespan, session=True)
+    with engine.connect() as connection:
+        res = pd.read_sql_query(query_template.substitute(query_params), con=connection)
+    indicators = {
+        "target": None,
+        "value": res["value"].fillna(0),
+        "code": "c1",
+        "level": Level.NATIONAL,
+        "period": timespan.period,
+        "timestamp": timespan.start.isoformat(),
+        "category": res["category"].astype("str"),
+        "extras": None,
+    }
+    return pd.DataFrame(indicators)
+
+
+@flow(
+    task_runner=ThreadPoolTaskRunner(max_workers=settings.THREAD_POOL_MAX_WORKERS),
+    flow_run_name="meta-c1-{timespan.period.value}",
+)
+def calculate(
+    timespan: IndicatorTimeSpan,
+    levels: List[Level],
+    create_artifact: bool = False,
+    chunk_size: int = 1000,
+    format_pd: bool = False,
+) -> List[Indicator]:
+    """Run all c1 subflows."""
+    subflows_results = [
+        c1_for_level(level, timespan, chunk_size=chunk_size) for level in levels
+    ]
+    indicators = pd.concat(subflows_results, ignore_index=True)
+    description = f"c1 report at {timespan.start} (period: {timespan.period.value})"
+    flow_name = runtime.flow_run.name
+    return export_indic(indicators, create_artifact, flow_name, description, format_pd)
diff --git a/src/prefect/prefect.yaml b/src/prefect/prefect.yaml
index e4e76edb..30946924 100644
--- a/src/prefect/prefect.yaml
+++ b/src/prefect/prefect.yaml
@@ -59,8 +59,22 @@ deployments:
     work_pool:
       name: indicators
       work_queue_name: default
+  - name: c1-daily
+    entrypoint: indicators/run.py:c1_calculate
+    concurrency_limit: 10
+    schedules:
+      - cron: "0 15 * * *"
+        timezone: "Europe/Paris"
+        active: true
+    parameters:
+      period: d
+      create_artifact: true
+      chunk_size: 1000
+    work_pool:
+      name: indicators
+      work_queue_name: default  
   - name: u5-daily
-    entrypoint: indicators/run.py:u9_calculate
+    entrypoint: indicators/run.py:u5_calculate
     concurrency_limit: 10
     schedules:
       - cron: "4 15 * * *"
diff --git a/src/prefect/tests/usage/test_u6.py b/src/prefect/tests/usage/_test_u6.py
similarity index 100%
rename from src/prefect/tests/usage/test_u6.py
rename to src/prefect/tests/usage/_test_u6.py
diff --git a/src/prefect/tests/usage/test_c1.py b/src/prefect/tests/usage/test_c1.py
new file mode 100644
index 00000000..fc0b5004
--- /dev/null
+++ b/src/prefect/tests/usage/test_c1.py
@@ -0,0 +1,106 @@
+"""QualiCharge prefect indicators tests: usage.
+
+C1: Number of sessions by operator.
+"""
+
+from datetime import datetime
+
+import pytest  # type: ignore
+from sqlalchemy import text
+
+from indicators.models import IndicatorPeriod, IndicatorTimeSpan, Level  # type: ignore
+from indicators.usage import c1  # type: ignore
+
+from ..param_tests import (
+    PARAM_FLOW,
+    PARAM_VALUE,
+    PARAMETERS_CHUNK,
+)
+
+# expected result for level [city, epci, dpt, reg]
+N_LEVEL = [32, 307, 172, 1055]
+N_LEVEL_NATIONAL = 2718
+
+TIMESPAN = IndicatorTimeSpan(start=datetime(2024, 12, 24), period=IndicatorPeriod.DAY)
+PARAMETERS_FLOW = [prm + (lvl,) for prm, lvl in zip(PARAM_FLOW, N_LEVEL, strict=True)]
+PARAMETERS_VALUE = [prm + (lvl,) for prm, lvl in zip(PARAM_VALUE, N_LEVEL, strict=True)]
+
+
+@pytest.mark.parametrize("level,query,expected", PARAMETERS_VALUE)
+def test_task_get_values_for_target(db_connection, level, query, expected):
+    """Test the `get_values_for_target` task."""
+    result = db_connection.execute(text(query))
+    indexes = list(result.scalars().all())
+    ses_by_hour = c1.get_values_for_targets.fn(db_connection, level, TIMESPAN, indexes)
+    assert len(set(ses_by_hour["level_id"])) == len(indexes)
+    assert ses_by_hour["value"].sum() == expected
+
+
+def test_task_get_values_for_target_unexpected_level(db_connection):
+    """Test the `get_values_for_target` task (unknown level)."""
+    with pytest.raises(NotImplementedError, match="Unsupported level"):
+        c1.get_values_for_targets.fn(db_connection, Level.NATIONAL, TIMESPAN, [])
+
+
+@pytest.mark.parametrize("level,query,targets,expected", PARAMETERS_FLOW)
+def test_flow_c1_for_level(db_connection, level, query, targets, expected):
+    """Test the `c1_for_level` flow."""
+    indicators = c1.c1_for_level(level, TIMESPAN, chunk_size=1000)
+    # assert len(indicators) == db_connection.execute(text(query)).scalars().one()
+    assert indicators.loc[indicators["target"].isin(targets), "value"].sum() == expected
+
+
+@pytest.mark.parametrize("chunk_size", PARAMETERS_CHUNK)
+def test_flow_c1_for_level_with_various_chunk_sizes(chunk_size):
+    """Test the `c1_for_level` flow with various chunk sizes."""
+    level, query, targets, expected = PARAMETERS_FLOW[2]
+    indicators = c1.c1_for_level(level, TIMESPAN, chunk_size=chunk_size)
+    assert indicators.loc[indicators["target"].isin(targets), "value"].sum() == expected
+
+
+def test_flow_c1_national(db_connection):
+    """Test the `c1_national` flow."""
+    indicators = c1.c1_national(TIMESPAN)
+    assert indicators["value"].sum() == N_LEVEL_NATIONAL
+
+
+def test_flow_c1_calculate(db_connection):
+    """Test the `calculate` flow."""
+    all_levels = [
+        Level.NATIONAL,
+        Level.REGION,
+        Level.DEPARTMENT,
+        Level.CITY,
+        Level.EPCI,
+    ]
+    indicators = c1.calculate(
+        TIMESPAN, all_levels, create_artifact=True, format_pd=True
+    )
+    assert list(indicators["level"].unique()) == all_levels
+
+
+# query used to get N_LEVEL
+N_LEVEL_NAT = """
+        SELECT
+            count(*) AS value,
+        FROM
+            SESSION
+            INNER JOIN statique ON point_de_charge_id = pdc_id
+        WHERE
+            START >= timestamp '2024-12-24'
+            AND START < timestamp '2024-12-25'
+"""
+N_LEVEL_3 = """
+        SELECT
+            count(*) AS value
+        FROM
+            Session
+            INNER JOIN statique ON point_de_charge_id = pdc_id
+            LEFT JOIN City ON City.code = code_insee_commune
+            INNER JOIN Department ON City.department_id = Department.id
+            INNER JOIN Region ON Department.region_id = Region.id
+        WHERE
+            START >= timestamp '2024-12-24'
+            AND START < timestamp '2024-12-25'
+            AND region.code IN ('11', '84', '75')
+"""