diff --git a/src/RegistryBuilder.php b/src/RegistryBuilder.php
index 86a20e3..3902117 100644
--- a/src/RegistryBuilder.php
+++ b/src/RegistryBuilder.php
@@ -130,6 +130,20 @@ protected static function initEnergy(Registry $registry): void
                 new UnitPart(1, Dimension::TIME, -2)
             )
         );
+
+        static::registerSiUnit(
+            $registry,
+            'watt-hour',
+            ['Wh'],
+            new Unit(
+                // watt
+                new UnitPart(1, Dimension::MASS, 1),
+                new UnitPart(1, Dimension::LENGTH, 2),
+                new UnitPart(1, Dimension::TIME, -3),
+                // hour
+                new UnitPart(3600, Dimension::TIME, 1)
+            )
+        );
     }
 
     protected static function initLength(Registry $registry): void
diff --git a/tests/EnergyTest.php b/tests/EnergyTest.php
index f2f379b..8f20aaa 100644
--- a/tests/EnergyTest.php
+++ b/tests/EnergyTest.php
@@ -2,41 +2,46 @@
 
 use PHPUnit\Framework\TestCase;
 use Vesper\UnitConversion\Converter;
-use Vesper\UnitConversion\Dimension;
+use Vesper\UnitConversion\Registry;
+use Vesper\UnitConversion\RegistryBuilder;
 use Vesper\UnitConversion\Unit;
-use Vesper\UnitConversion\UnitPart;
 
 final class EnergyTest extends TestCase
 {
     protected Unit $cal;
     protected Unit $joule;
+    protected Unit $watthour;
     protected Converter $converter;
 
     protected function setUp(): void
     {
         parent::setUp();
 
-        $this->cal = new Unit(
-            new UnitPart(4.184, Dimension::MASS, 1),
-            new UnitPart(1, Dimension::LENGTH, 2),
-            new UnitPart(1, Dimension::TIME, -2)
-        );
-        $this->joule = new Unit(
-            new UnitPart(1, Dimension::MASS, 1),
-            new UnitPart(1, Dimension::LENGTH, 2),
-            new UnitPart(1, Dimension::TIME, -2),
-        );
+        $registry = new Registry();
+
+        RegistryBuilder::build($registry);
+
+        $this->cal = $registry->get('calorie');
+
+        $this->joule = $registry->get('joule');
+
+        $this->watthour = $registry->get('watt-hour');
 
         $this->converter = new Converter();
     }
 
     public function test_cal_to_joule(): void
     {
-        $this->assertEqualsWithDelta(4.184, $this->converter->convert($this->cal, $this->joule, 1), 0.0000001);
+        $this->assertEqualsWithDelta(4.1868, $this->converter->convert($this->cal, $this->joule, 1), 0.0000001);
     }
 
     public function test_joule_to_cal(): void
     {
-        $this->assertEqualsWithDelta(1 / 4.184, $this->converter->convert($this->joule, $this->cal, 1), 0.0000001);
+        $this->assertEqualsWithDelta(1, $this->converter->convert($this->joule, $this->cal, 4.1868), 0.0000001);
+    }
+
+    public function test_joule_to_watt_hour(): void
+    {
+        $this->assertEqualsWithDelta(1, $this->converter->convert($this->joule, $this->watthour, 3600), 0.0000001);
     }
 }