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A quantitative description of dark energy is assembled from observations of supernovae Ia at different redshifts. The fluxes of supernovae are compared with nearby supernovae in the rest frame comma thus accurate relative flux with wavelength calibration is critical to understand the nature of dark energy. The dominant systematic error in measuring relative fluxes is currently from uncertainties in the flux of stellar standards. Our proposal will directly address this critical barrier. We propose STIS and WFC3 low resolution observations that will lead to a network of 16 cool (Teff < 20 comma 000 K) white dwarfs. This will provide a new set of primary flux standards that is expected to have higher accuracy at near-IR and IR wavelengths compared to the currently most precise set of fluxes which comes from HST spectrophotometry of three hot white dwarfs (Teff > 30 comma 000 K). Our program will provide the legacy for flux calibration of near-IR and IR instruments onboard future missions such as JWST comma Euclid and WFIRST. We also expect that our network will lead to an accuracy of <0.5% on the wavelength-dependent flux calibration in the optical (ground or space) comma increasing by a factor of two the precision on the luminosity of the tens of thousands of supernovae Ia to be detected from LSST.