In short, any word that is composed entirely of the letters B, E, G, H, I, L, O, S, and Z can be spelled on the calculator, with the possible exceptions of words that begin with "Oh" (as most calculators will truncate a trailing zero) or words longer than eight characters (as some calculators can only display eight digits). Numbers with a leading zero (words that end in "Oh") can be displayed with a decimal place: 0.9 will spell "Go".

1 I
2 Z 7 L
3 E 8 B
4 H 9 G
5 S 0 O

This allows a fairly large number of words, as you have three vowels and five reasonably common letters. And a Z.