diff --git a/fhem/CHANGED b/fhem/CHANGED
index eebd260c4..3a8aa647d 100644
--- a/fhem/CHANGED
+++ b/fhem/CHANGED
@@ -1,5 +1,6 @@
 # Add changes at the top of the list. Keep it in ASCII, and 80-char wide.
 # Do not insert empty lines here, update check depends on it.
+ - bugfix:  TimeSeries: fix for calculation of standard deviation
  - feature: 76_SMAPortal: new relative time arguments for attr balanceDay,
                           balanceMonth, balanceYear, new attribute 
                           useRelativeNames
diff --git a/fhem/FHEM/TimeSeries.pm b/fhem/FHEM/TimeSeries.pm
index cf304d9a6..ee226fc1f 100644
--- a/fhem/FHEM/TimeSeries.pm
+++ b/fhem/FHEM/TimeSeries.pm
@@ -44,6 +44,9 @@
 #       modified: method elapsed reverted to version from 2015-01-31 to provide downsampling and buffering through fhem.pl
 #       modified: method _housekeeping does not reset time series if hold time is specified
 #
+#     17.10.2020 Boris Neubert
+#       modified: fix for calculation of standard deviation
+#
 ##############################################################################
 
 package TimeSeries;
@@ -240,12 +243,13 @@ sub trimToHoldTime() {
 sub _updatestat($$) {
   my ($self, $V)= @_;
 
-  # see Donald Knuth, The Art of Computer Programming, ch. 4.2.2, formulas 14ff.
+  # see Donald Knuth, The Art of Computer Programming, ch. 4.2.2, p. 232ff, formulas 14ff.
+  # https://doc.lagout.org/science/0_Computer%20Science/2_Algorithms/The%20Art%20of%20Computer%20Programming%20%28vol.%202_%20Seminumerical%20Algorithms%29%20%283rd%20ed.%29%20%5BKnuth%201997-11-14%5D.pdf
   my $n= ++$self->{n};
   if($n> 1) {
     my $M= $self->{_M};
     $self->{_M}= $M + ($V - $M) / $n;
-    $self->{_S}= $self->{_S} + ($V - $M) * ($V - $M);
+    $self->{_S}= $self->{_S} + ($V - $M) * ($V - $self->{_M});
     $self->{integral}+= $V;
     #main::Debug("V= $V M= $M _M= ".$self->{_M}." _S= " .$self->{_S}." int= ".$self->{integral});
   } else {