diff --git a/libreplan-business/src/main/java/org/libreplan/business/resources/daos/WorkerDAO.java b/libreplan-business/src/main/java/org/libreplan/business/resources/daos/WorkerDAO.java
index 2c0314a78..92079d838 100644
--- a/libreplan-business/src/main/java/org/libreplan/business/resources/daos/WorkerDAO.java
+++ b/libreplan-business/src/main/java/org/libreplan/business/resources/daos/WorkerDAO.java
@@ -148,7 +148,7 @@ public class WorkerDAO extends IntegrationEntityDAO<Worker>
     @Transactional(readOnly = true)
     public List<Object[]> getWorkingHoursGroupedPerWorker(
             List<String> workerNifs, Date startingDate, Date endingDate) {
-        String strQuery = "SELECT worker.nif, SUM(wrl.numHours) "
+        String strQuery = "SELECT worker.nif, SUM(wrl.effort) "
                 + "FROM Worker worker, WorkReportLine wrl "
                 + "LEFT OUTER JOIN wrl.resource resource "
                 + "WHERE resource.id = worker.id ";
diff --git a/libreplan-webapp/src/main/java/org/libreplan/ws/resources/api/ResourceWorkedHoursDTO.java b/libreplan-webapp/src/main/java/org/libreplan/ws/resources/api/ResourceWorkedHoursDTO.java
index ad87c13e5..ff129ebb1 100644
--- a/libreplan-webapp/src/main/java/org/libreplan/ws/resources/api/ResourceWorkedHoursDTO.java
+++ b/libreplan-webapp/src/main/java/org/libreplan/ws/resources/api/ResourceWorkedHoursDTO.java
@@ -36,12 +36,12 @@ public class ResourceWorkedHoursDTO {
     public String resource;
 
     @XmlAttribute
-    public Integer hours;
+    public String hours;
 
     public ResourceWorkedHoursDTO() {
     }
 
-    public ResourceWorkedHoursDTO(String resource, Integer hours) {
+    public ResourceWorkedHoursDTO(String resource, String hours) {
         this.resource = resource;
         this.hours = hours;
     }
diff --git a/libreplan-webapp/src/main/java/org/libreplan/ws/resources/impl/ResourceHoursServiceREST.java b/libreplan-webapp/src/main/java/org/libreplan/ws/resources/impl/ResourceHoursServiceREST.java
index e54101740..3f4a634cd 100644
--- a/libreplan-webapp/src/main/java/org/libreplan/ws/resources/impl/ResourceHoursServiceREST.java
+++ b/libreplan-webapp/src/main/java/org/libreplan/ws/resources/impl/ResourceHoursServiceREST.java
@@ -34,6 +34,7 @@ import javax.ws.rs.PathParam;
 import javax.ws.rs.Produces;
 
 import org.libreplan.business.resources.daos.IWorkerDAO;
+import org.libreplan.business.workingday.EffortDuration;
 import org.libreplan.ws.resources.api.IResourceHoursService;
 import org.libreplan.ws.resources.api.ResourceWorkedHoursDTO;
 import org.libreplan.ws.resources.api.ResourceWorkedHoursListDTO;
@@ -97,7 +98,8 @@ public class ResourceHoursServiceREST implements IResourceHoursService {
 
         for (Object[] pair : hoursPerWorker) {
             ResourceWorkedHoursDTO resourceWorkedHoursDTO = new ResourceWorkedHoursDTO(
-                    (String) pair[0], ((Long) pair[1]).intValue());
+                    (String) pair[0], EffortDuration.seconds(
+                            ((Long) pair[1]).intValue()).toFormattedString());
             result.add(resourceWorkedHoursDTO);
         }