diff --git a/1ano/2semestre/poo/src/aula11/ex3/Customer.java b/1ano/2semestre/poo/src/aula11/ex3/Customer.java
new file mode 100644
index 0000000..40e56b5
--- /dev/null
+++ b/1ano/2semestre/poo/src/aula11/ex3/Customer.java
@@ -0,0 +1,29 @@
+package aula11.ex3;
+
+import java.util.LinkedList;
+
+public class Customer {
+    private int customerId;
+    private LinkedList<Double> meterReadings;
+
+    public Customer(int customerId, LinkedList<Double> meterReadings) {
+        this.customerId = customerId;
+        this.meterReadings = meterReadings;
+    }
+
+    public int getCustomerId() {
+        return customerId;
+    }
+
+    public void setCustomerId(int customerId) {
+        this.customerId = customerId;
+    }
+
+    public LinkedList<Double> getMeterReadings() {
+        return meterReadings;
+    }
+
+    public void setMeterReadings(LinkedList<Double> meterReadings) {
+        this.meterReadings = meterReadings;
+    }
+}
diff --git a/1ano/2semestre/poo/src/aula11/ex3/EnergyUsageReport.java b/1ano/2semestre/poo/src/aula11/ex3/EnergyUsageReport.java
new file mode 100644
index 0000000..f99fd2c
--- /dev/null
+++ b/1ano/2semestre/poo/src/aula11/ex3/EnergyUsageReport.java
@@ -0,0 +1,60 @@
+package aula11.ex3;
+
+import utils.MathTools;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.LinkedList;
+
+public class EnergyUsageReport {
+    private final LinkedList<Customer> customers = new LinkedList<Customer>();
+
+    public void load(String file) {
+        LinkedList<String> lines = new LinkedList<>();
+        Path path = Paths.get(file);
+        try {
+            lines = new LinkedList<>(Files.readAllLines(path));
+		} catch (IOException e) {
+			System.out.printf("Certifique-se que o ficheiro \"%s\" está na raiz da pasta do projeto", path);
+            System.exit(1);
+		}
+
+        for (String line : lines) {
+            String[] data = line.split("\\|");
+            int customerId = Integer.parseInt(data[0]);
+            LinkedList<Double> meterReadings = new LinkedList<>(Arrays.stream(data).toList().subList(1, data.length).stream().map(Double::parseDouble).toList());
+            customers.add(new Customer(customerId, meterReadings));
+        }
+    }
+
+    public void addCustomer(Customer newCustomer) {
+        customers.add(newCustomer);
+    }
+
+    public void removeCustomer(int id) {
+        customers.removeIf(customer -> customer.getCustomerId() == id);
+    }
+
+    public Customer getCustomer(int id) {
+        return customers.stream().filter(customer -> customer.getCustomerId() == id).findFirst().orElse(null);
+    }
+
+    public double calculateTotalUsage(int id) {
+        return MathTools.sum(getCustomer(id).getMeterReadings());
+    }
+
+    public void generateReport(String path) {
+        for (Customer customer : customers) {
+            try {
+                Files.writeString(Path.of(path), String.format("Customer ID: %d%nTotal usage: %.1f%n%n", customer.getCustomerId(), calculateTotalUsage(customer.getCustomerId())), (new File(path)).exists() ? java.nio.file.StandardOpenOption.APPEND : java.nio.file.StandardOpenOption.CREATE);
+            } catch (IOException e) {
+                throw new RuntimeException(e);
+            }
+        }
+    }
+}
diff --git a/1ano/2semestre/poo/src/aula11/ex3/EnergyUsageReportTester.java b/1ano/2semestre/poo/src/aula11/ex3/EnergyUsageReportTester.java
new file mode 100644
index 0000000..6669324
--- /dev/null
+++ b/1ano/2semestre/poo/src/aula11/ex3/EnergyUsageReportTester.java
@@ -0,0 +1,34 @@
+package aula11.ex3;
+
+import java.io.IOException;
+import java.util.LinkedList;
+import java.util.List;
+
+public class EnergyUsageReportTester {
+
+    public static void main(String[] args) throws IOException {
+        
+        // Create an instance of the EnergyUsageReport class
+        EnergyUsageReport energyReport = new EnergyUsageReport();
+        
+        // Load the customer data from a text file using the load() method
+        energyReport.load("datafiles/aula11/ex3/clients.txt");
+        
+        // Add one or more customers to the collection using the addCustomer() method
+        Customer newCustomer = new Customer(999, new LinkedList<>(List.of(1500.0, 2000.0, 2500.0, 3000.0)));
+        energyReport.addCustomer(newCustomer);
+        
+        // Remove one or more customers from the collection using the removeCustomer() method
+        energyReport.removeCustomer(1015);
+        
+        // Retrieve a customer from the collection using the getCustomer() method
+        Customer retrievedCustomer = energyReport.getCustomer(1025);
+        
+        // Calculate the total energy usage for a specific customer using the calculateTotalUsage() method
+        double totalEnergyUsage = energyReport.calculateTotalUsage(1003);
+        System.out.println("Total energy usage for customer 1003: " + totalEnergyUsage);
+        
+        // Generate a report of all customers and their total energy usage using the generateReport() method
+        energyReport.generateReport("datafiles/aula11/ex3/energy_report.txt");
+    }
+}