diff --git a/qctools/hydrogen.py b/qctools/hydrogen.py
index e3ba1a0..261ea2d 100644
--- a/qctools/hydrogen.py
+++ b/qctools/hydrogen.py
@@ -86,6 +86,6 @@ def calc_energy(n, Z=1, units='hartree'):
     elif units == 'eV':
         return E*27.2114
     elif units == 'SI':
-        return E*4.3597482e-18
+        return E*4.35974e-18
     else:
         raise ValueError("Invalid units. Must be 'hartree', 'eV', or 'SI'.")
\ No newline at end of file
diff --git a/tests/test_hydrogen.py b/tests/test_hydrogen.py
index 9e903c2..c79191f 100644
--- a/tests/test_hydrogen.py
+++ b/tests/test_hydrogen.py
@@ -47,4 +47,20 @@ def test_hydrogenic_wavefunction():
 
     true_answer = np.array([0.+0.j, 0.+0.j, 0.+0.j, -0.+0.j, 0.00661248+0.j, 0.02022248+0.j, 0.+0.j, 0.0049428+0.j, 0.00040025+0.j, 0.01440961+0.j])
     
-    np.testing.assert_allclose(res, true_answer, atol=1e-5, rtol=1e-3)
\ No newline at end of file
+    np.testing.assert_allclose(res, true_answer, atol=1e-5, rtol=1e-3)
+
+def test_calc_energy():
+    """
+    testing calc_energy function
+    """
+    E_h = calc_energy(1, Z=1, units='hartree')
+    E_h_true = -0.5
+    np.testing.assert_allclose(E_h, E_h_true, rtol=1e-5, atol=1e-5)
+
+    E_ev = calc_energy(1, Z=1, units='eV')
+    E_ev_true = -13.605693009
+    np.testing.assert_allclose(E_ev, E_ev_true, rtol=1e-5, atol=1e-5)
+
+    E_h = calc_energy(2, Z=1, units='SI')
+    E_h_true = -2.18e-18
+    np.testing.assert_allclose(E_h, E_h_true, rtol=1e-5, atol=1e-5)
\ No newline at end of file