diff --git a/sense_energy/plug_instance.py b/sense_energy/plug_instance.py
index 23b6efc..761ed1f 100644
--- a/sense_energy/plug_instance.py
+++ b/sense_energy/plug_instance.py
@@ -1,87 +1,107 @@
-# Copyright 2020, Charles Powell
-
-import hashlib
-from time import time
-
-
-class PlugInstance:
-    def __init__(self, id, start_time=None, alias=None, power=0, current=0, voltage=120, mac=None, device_id=None):
-        self.id = id
-
-        self.voltage = voltage
-        self.power = power
-        self.current = current
-
-        if alias:
-            self.alias = alias
-        else:
-            self.alias = id
-
-        if start_time:
-            self.start_time = start_time
-        else:
-            self.start_time = time() - 1
-
-        if not self.power:
-            self.power = self.voltage * self.current
-        if not self.current:
-            self.current = self.power / self.voltage
-
-        if device_id:
-            self.device_id = device_id
-        else:
-            self.device_id = self.generate_deviceid()
-
-        if mac:
-            self.mac = mac
-        else:
-            self.mac = self.generate_mac()
-
-    def generate_mac(self):
-        end = [int(self.device_id[i : i + 2], 16) for i in range(0, len(self.device_id), 2)]
-        mac = [53, 75, 31] + end[:3]
-        return ":".join("%02x" % b for b in mac)
-
-    def generate_deviceid(self):
-        return hashlib.sha1(self.id.encode("utf-8")).hexdigest()
-
-    def generate_response(self):
-        # Response dict
-        response = {
-            "emeter": {
-                "get_realtime": {
-                    "current": self.current,
-                    "voltage": self.voltage,
-                    "power": self.power,
-                    "total": 0,  # Unsure if this needs a value, appears not to
-                    "err_code": 0,  # No errors here!
-                }
-            },
-            "system": {
-                "get_sysinfo": {
-                    "err_code": 0,
-                    "sw_ver": "1.2.5 Build 171206 Rel.085954",
-                    "hw_ver": "1.0",
-                    "type": "IOT.SMARTPLUGSWITCH",
-                    "model": "HS110(US)",
-                    "mac": self.mac.upper(),
-                    "deviceId": self.device_id.upper(),
-                    "hwId": "60FF6B258734EA6880E186F8C96DDC61",
-                    "fwId": "00000000000000000000000000000000",
-                    "oemId": "FFF22CFF774A0B89F7624BFC6F50D5DE",
-                    "alias": self.alias,
-                    "dev_name": "Wi-Fi Smart Plug With Energy Monitoring",
-                    "icon_hash": "",
-                    "relay_state": 1 if self.power > 0 else 0,
-                    "on_time": time() - self.start_time,
-                    "active_mode": "none",
-                    "feature": "TIM:ENE",
-                    "updating": 0,
-                    "rssi": -60,  # Great wifi signal
-                    "led_off": 0,  # Probably not important
-                    "latitude": 39.8283,  # Center of the US
-                    "longitude": -98.5795,  # Center of the US
-                }
-            },
-        }
-        return response
+# Copyright 2020, Charles Powell
+
+import hashlib
+from time import time
+from typing import Optional, Dict, Any
+from functools import lru_cache
+
+
+@lru_cache(maxsize=256)
+def _generate_mac(device_id: str) -> str:
+    """Generate a MAC address from the device ID only once."""
+    end = [int(device_id[i : i + 2], 16) for i in range(0, len(device_id), 2)]
+    mac = [53, 75, 31] + end[:3]
+    return ":".join("%02X" % b for b in mac)
+
+
+@lru_cache(maxsize=256)
+def _generate_device_id(id: str) -> str:
+    """Generate a device ID from the ID only once."""
+    return hashlib.sha1(id.encode("utf-8")).hexdigest().upper()
+
+
+class PlugInstance:
+    """Class to represent a single plug instance."""
+
+    __slots__ = (
+        "id",
+        "voltage",
+        "power",
+        "current",
+        "alias",
+        "start_time",
+        "device_id",
+        "mac",
+    )
+
+    def __init__(
+        self,
+        id: str,
+        start_time: Optional[float] = None,
+        alias: Optional[str] = None,
+        power=0.0,
+        current=0.0,
+        voltage=120.0,
+        mac: Optional[str] = None,
+        device_id: Optional[str] = None,
+    ) -> None:
+        """Initialize a plug instance."""
+        self.id = id
+        self.voltage = voltage
+        self.power = power
+        self.current = current
+        if not self.power:
+            self.power = self.voltage * self.current
+        if not self.current:
+            self.current = self.power / self.voltage
+        self.alias = alias or id
+        self.start_time = start_time or time() - 1
+        if device_id:
+            self.device_id = device_id.upper()
+        else:
+            self.device_id = _generate_device_id(id)
+        if mac:
+            self.mac = mac.upper()
+        else:
+            self.mac = _generate_mac(self.device_id)
+
+    def generate_response(self) -> Dict[str, Dict[str, Any]]:
+        """Generate a response dict for the plug."""
+        # Response dict
+        return {
+            "emeter": {
+                "get_realtime": {
+                    "current": self.current,
+                    "voltage": self.voltage,
+                    "power": self.power,
+                    "total": 0,  # Unsure if this needs a value, appears not to
+                    "err_code": 0,  # No errors here!
+                }
+            },
+            "system": {
+                "get_sysinfo": {
+                    "err_code": 0,
+                    "sw_ver": "1.2.5 Build 171206 Rel.085954",
+                    "hw_ver": "1.0",
+                    "type": "IOT.SMARTPLUGSWITCH",
+                    "model": "HS110(US)",
+                    "mac": self.mac,
+                    "deviceId": self.device_id,
+                    "hwId": "60FF6B258734EA6880E186F8C96DDC61",
+                    "fwId": "00000000000000000000000000000000",
+                    "oemId": "FFF22CFF774A0B89F7624BFC6F50D5DE",
+                    "alias": self.alias,
+                    "dev_name": "Wi-Fi Smart Plug With Energy Monitoring",
+                    "icon_hash": "",
+                    "relay_state": 1 if self.power > 0 else 0,
+                    "on_time": time() - self.start_time,
+                    "active_mode": "none",
+                    "feature": "TIM:ENE",
+                    "updating": 0,
+                    "rssi": -60,  # Great wifi signal
+                    "led_off": 0,  # Probably not important
+                    "latitude": 39.8283,  # Center of the US
+                    "longitude": -98.5795,  # Center of the US
+                }
+            },
+        }