  Determining the energy scale of inflation is crucial to understand the nature
of inflation in the early Universe. We place observational constraints on the
energy scale of the observable part of the inflaton potential by combining the
7-year Wilkinson Microwave Anisotropy Probe data with distance measurements
from the baryon acoustic oscillations in the distribution of galaxies and the
Hubble constant measurement. Our analysis provides an upper limit on this
energy scale, 2.3 \times 10^{16} GeV at 95% confidence level. Moreover, we
forecast the sensitivity and constraints achievable by the Planck experiment by
performing Monte Carlo studies on simulated data. Planck could significantly
improve the constraints on the energy scale of inflation and on the shape of
the inflaton potential.
