diff --git a/.gitignore b/.gitignore
index 338e667..825b83a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,3 @@
 /src/.env
 .env
-*.pyc
+*.pyc
\ No newline at end of file
diff --git a/pv_site_api/main.py b/pv_site_api/main.py
index 48111f9..198c694 100644
--- a/pv_site_api/main.py
+++ b/pv_site_api/main.py
@@ -16,6 +16,7 @@
 from fastapi.responses import FileResponse, Response
 from pvlib import irradiance, location, pvsystem
 from pvsite_datamodel.pydantic_models import GenerationSum, PVSiteEditMetadata
+from pvsite_datamodel.read.site import get_site_by_uuid
 from pvsite_datamodel.read.status import get_latest_status
 from pvsite_datamodel.read.user import get_user_by_email
 from pvsite_datamodel.write.generation import insert_generation_values
@@ -276,7 +277,26 @@ def post_pv_actual(
             }
         )
 
+    # Set the error generation capacity factor from environment variable
+    capacity_factor = float(os.getenv("ERROR_GENERATION_CAPACITY_FACTOR", 1.1))
+
     generation_values_df = pd.DataFrame(generations)
+    site = get_site_by_uuid(session=session, site_uuid=site_uuid)
+    site_capacity_kw = site.capacity_kw
+    exceeded_capacity = generation_values_df[
+        generation_values_df["power_kw"] > site_capacity_kw * capacity_factor
+    ]
+    if len(exceeded_capacity) > 0:
+        raise HTTPException(
+            status_code=422,
+            detail=(
+                f"Error processing generation values. "
+                f"One (or more) values are larger than {capacity_factor} "
+                f"times the site capacity of {site_capacity_kw} kWp. "
+                "Please adjust this generation value, the site capacity, "
+                "or contact quartz.support@openclimatefix.org."
+            ),
+        )
 
     logger.debug(f"Adding {len(generation_values_df)} generation values")
 
diff --git a/tests/test_generation.py b/tests/test_generation.py
index a85f4e2..5a089e3 100644
--- a/tests/test_generation.py
+++ b/tests/test_generation.py
@@ -1,5 +1,4 @@
 """ Test for main app """
-
 import json
 import uuid
 from datetime import datetime, timedelta, timezone
@@ -146,25 +145,52 @@ def test_post_pv_actual(db_session, client, sites):
     db_session.query(GenerationSQL).delete()
 
     site_uuid = sites[0].site_uuid
+    site_capacity_kw = sites[0].capacity_kw
 
-    pv_actual_value = PVActualValue(
-        datetime_utc=datetime.now(timezone.utc), actual_generation_kw=73.3
+    # below capacity testcase
+    pv_actual_below_capacity = PVActualValue(
+        datetime_utc=datetime.now(timezone.utc), actual_generation_kw=site_capacity_kw - 1
     )
 
     # make iteration of pv values for one day at a specific site
-    pv_actual_iteration = MultiplePVActual(
-        site_uuid=str(site_uuid), pv_actual_values=[pv_actual_value]
+    pv_actual_iteration_below = MultiplePVActual(
+        site_uuid=str(site_uuid), pv_actual_values=[pv_actual_below_capacity]
     )
 
     # this makes sure the datetimes are iso strings
-    pv_actual_dict = json.loads(pv_actual_iteration.json())
+    pv_actual_dict_below = json.loads(pv_actual_iteration_below.json())
 
-    response = client.post(f"/sites/{site_uuid}/pv_actual", json=pv_actual_dict)
+    response = client.post(f"/sites/{site_uuid}/pv_actual", json=pv_actual_dict_below)
     assert response.status_code == 200, response.text
 
     generations = db_session.query(GenerationSQL).all()
     assert len(generations) == 1
-    assert str(generations[0].site_uuid) == str(pv_actual_iteration.site_uuid)
+    assert str(generations[0].site_uuid) == str(pv_actual_iteration_below.site_uuid)
+
+
+def test_post_pv_actual_above_capacity(db_session, client, sites):
+    db_session.query(GenerationSQL).delete()
+
+    site_uuid = sites[0].site_uuid
+    site_capacity_kw = sites[0].capacity_kw
+    capacity_factor = 1.1
+
+    # above capacity testcase
+    pv_actual_above_capacity = PVActualValue(
+        datetime_utc=datetime.now(timezone.utc),
+        actual_generation_kw=(site_capacity_kw * capacity_factor) + 1,
+    )
+
+    # make iteration of pv values for one day at a specific site
+    pv_actual_iteration_above = MultiplePVActual(
+        site_uuid=str(site_uuid), pv_actual_values=[pv_actual_above_capacity]
+    )
+
+    # this makes sure the datetimes are iso strings
+    pv_actual_dict_above = json.loads(pv_actual_iteration_above.json())
+    response_above = client.post(f"/sites/{site_uuid}/pv_actual", json=pv_actual_dict_above)
+
+    assert response_above.status_code == 422, response_above.text
 
 
 def test_pv_actual_no_data(db_session, client, sites):