Fix a calculation bug for total energy

[klaasnicolaas]

I made a miscalculation for total energy for who is using the JS source of the inverter.

[codecov-commenter]

Codecov Report

Merging #19 (ed4805d) into master (2d89da0) will decrease coverage by 0.63%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   87.09%   86.45%   -0.64%     
==========================================
  Files           4        4              
  Lines          93       96       +3     
  Branches       10       12       +2     
==========================================
+ Hits           81       83       +2     
  Misses          9        9              
- Partials        3        4       +1     

Impacted Files	Coverage Δ
omnikinverter/models.py	97.43% <75.00%> (-2.57%)	⬇️

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