--E-- displayed if unit is three letters long

[WolfgangFahl]

When updating the value every 1 second - each second time the --E-- is displayed.
By experimenting I found out that this only happens if the unit is three letters long.
If i shorten it to two letters "Wh" the issue goes away

[protogenes]

-E- is displayed when there is not enough space to display the value (see LcdSkin#isNoOfDigitsInvalid) but the check seems broken to me, because sometimes I see -E- after the number was shown with a smaller width.