I got zero in the file size of PE.xvg which is the output of potential energy as I used this command
gmx energy -f tprout.edr -s tprout.tpr -o PE.xvg
that show
Select the terms you want from the following list by
selecting either (part of) the name or the number or a combination.
End your selection with an empty line or a zero.
-------------------------------------------------------------------
1 Angle 2 Proper-Dih. 3 Improper-Dih. 4 LJ-14
5 Coulomb-14 6 LJ-(SR) 7 Disper.-corr. 8 Coulomb-(SR)
9 Coul.-recip. 10 Potential 11 Kinetic-En. 12 Total-Energy
13 Conserved-En. 14 Temperature 15 Pres.-DC 16 Pressure
17 Constr.-rmsd 18 Vir-XX 19 Vir-XY 20 Vir-XZ
21 Vir-YX 22 Vir-YY 23 Vir-YZ 24 Vir-ZX
25 Vir-ZY 26 Vir-ZZ 27 Pres-XX 28 Pres-XY
29 Pres-XZ 30 Pres-YX 31 Pres-YY 32 Pres-YZ
33 Pres-ZX 34 Pres-ZY 35 Pres-ZZ 36 #Surf*SurfTen
37 T-Protein 38 T-non-Protein
39 Lamb-Protein 40 Lamb-non-Protein
10
Last energy frame read 200000 time 200000.000
Statistics over 100000001 steps [ 0.0000 through 200000.0000 ps ], 1 data sets
All statistics are over 1000001 points
Energy Average Err.Est. RMSD Tot-Drift
-------------------------------------------------------------------------------
Potential -87747.3 4.1 290.237 -25.9832 (kJ/mol)
GROMACS reminds you: "People are DNA's way of making more DNA." (Edward O. Wilson)