diff --git a/main.py b/main.py
index 50a786b..557a34f 100644
--- a/main.py
+++ b/main.py
@@ -4,15 +4,93 @@ Present observational weather data to Prometheus
 
 import json
 import os
-from typing import Dict, List
+from typing import TypedDict
 
-import requests  # type: ignore
-from flask import Flask, Response  # type: ignore
+import requests
+from flask import Flask, Response
 
 app = Flask(__name__)
 
 
-def fetch_carbon_data(postcode: str) -> Dict:
+class PromData(TypedDict):
+    """
+    Describes the data being returned to Prometheus
+
+    Args:
+        TypedDict ([type]): data to return to Prometheus
+    """
+
+    key: str
+    labels: dict[str, str]
+    type: str
+    value: float
+
+
+class FuelMix(TypedDict):
+    """
+    Generational Fuel Mix
+
+    Args:
+        TypedDict ([type]): the particular fuel's usage
+    """
+
+    fuel: str
+    perc: float
+
+
+class Intensity(TypedDict):
+    """
+    Intensity forecast
+
+    Args:
+        TypedDict ([type]): the intensity forecast
+    """
+
+    forecast: int
+    index: str
+
+
+class PostcodeData(TypedDict):
+    """
+    Data for a postcode
+
+    Args:
+        TypedDict ([type]): Describes the date range, intensity and fuel mix
+    """
+
+    _from: str
+    to: str
+    intensity: Intensity
+    generationmix: list[FuelMix]
+
+
+class RegionData(TypedDict):
+    """
+    Data for a region
+
+    Args:
+        TypedDict ([type]): describes the region and its data
+    """
+
+    regionid: int
+    dnoregion: str
+    shortname: str
+    postcode: str
+    data: list[PostcodeData]
+
+
+class CarbonData(TypedDict):
+    """
+    Overall data for a request
+
+    Args:
+        TypedDict ([type]): describes the returned data
+    """
+
+    data: list[RegionData]
+
+
+def fetch_carbon_data(postcode: str) -> CarbonData:
     """
     Fetch current data for the carbon intensity of the provided region
     """
@@ -21,23 +99,24 @@ def fetch_carbon_data(postcode: str) -> Dict:
         f'https://api.carbonintensity.org.uk/regional/postcode/{postcode}'
     )
 
-    return json.loads(obs_data.content)
+    carbon_data: CarbonData = json.loads(obs_data.content)
+    return carbon_data
 
 
 @app.route('/metrics')
-def metrics():
+def metrics() -> Response:
     """
     Output Prometheus-style metrics
     """
-    postcode = os.environ.get('CARBON_POSTCODE')
+    postcode = os.environ.get('CARBON_POSTCODE') or ''
 
     latest_data = fetch_carbon_data(postcode)['data'][0]
 
-    ret_data: List = list()
+    ret_data: list[PromData] = []
     for generation in latest_data['data'][0]['generationmix']:
         ret_data.append(
             {
-                'key': f'sensor_carbon_generation_perc',
+                'key': 'sensor_carbon_generation_perc',
                 'labels': {
                     'fuel': generation['fuel'],
                     'region': latest_data['dnoregion'],
@@ -58,7 +137,7 @@ def metrics():
         }
     )
 
-    ret_strs = list()
+    ret_strs = []
     for item in ret_data:
         ret_strs.append(f'# HELP {item["key"]} Carbon metric')
         ret_strs.append(f'# TYPE {item["key"]} {item["type"]}')