diff --git a/src/oemof/solph/_plumbing.py b/src/oemof/solph/_plumbing.py
index 806cf9507..bf18dbdae 100644
--- a/src/oemof/solph/_plumbing.py
+++ b/src/oemof/solph/_plumbing.py
@@ -15,6 +15,8 @@
 from collections import abc
 from itertools import repeat
 
+import numpy as np
+
 
 def sequence(iterable_or_scalar):
     """Tests if an object is iterable (except string) or scalar and returns
@@ -27,8 +29,17 @@ def sequence(iterable_or_scalar):
 
     Examples
     --------
-    >>> sequence([1,2])
-    [1, 2]
+    >>> y = sequence([1,2,3,4,5,6,7,8,9,10,11])
+    >>> y[0]
+    1
+
+    >>> y[10]
+    11
+
+    >>> import pandas as pd
+    >>> s1 = sequence(pd.Series([1,5,9]))
+    >>> s1[2]
+    9
 
     >>> x = sequence(10)
     >>> x[0]
@@ -43,7 +54,7 @@ def sequence(iterable_or_scalar):
     if isinstance(iterable_or_scalar, abc.Iterable) and not isinstance(
         iterable_or_scalar, str
     ):
-        return iterable_or_scalar
+        return np.array(iterable_or_scalar)
     else:
         return _Sequence(default=iterable_or_scalar)
 
diff --git a/src/oemof/solph/components/_offset_converter.py b/src/oemof/solph/components/_offset_converter.py
index c8f742df3..d5a09bcba 100644
--- a/src/oemof/solph/components/_offset_converter.py
+++ b/src/oemof/solph/components/_offset_converter.py
@@ -304,12 +304,12 @@ def normed_offset_and_conversion_factors_from_coefficients(
                 c0, c1 = slope_offset_from_nonconvex_output(
                     flow.max[i], flow.min[i], eta_at_max, eta_at_min
                 )
-                slope += [c0]
-                offset += [c1]
+                slope.append(c0)
+                offset.append(c1)
 
             if max_len == 1:
-                slope = sequence(slope[0])
-                offset = sequence(offset[0])
+                slope = slope[0]
+                offset = offset[0]
 
             conversion_factors = {input_bus: slope}
             normed_offsets = {input_bus: offset}
diff --git a/src/oemof/solph/components/_sink.py b/src/oemof/solph/components/_sink.py
index 1e2676792..7644871fd 100644
--- a/src/oemof/solph/components/_sink.py
+++ b/src/oemof/solph/components/_sink.py
@@ -21,7 +21,7 @@ class Sink(Node):
 
     Parameters
     ----------
-    label : str
+    label : str or tuple
         String holding the label of the Sink object.
         The label of each object must be unique.
     inputs: dict
diff --git a/src/oemof/solph/components/_source.py b/src/oemof/solph/components/_source.py
index fa81fdb65..d775d16d8 100644
--- a/src/oemof/solph/components/_source.py
+++ b/src/oemof/solph/components/_source.py
@@ -21,7 +21,7 @@ class Source(Node):
 
     Parameters
     ----------
-    label : str
+    label : str or tuple
         String holding the label of the Source object.
         The label of each object must be unique.
     outputs: dict