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In some flares the thermal component appears much earlier than the nonthermalcomponent in X-ray range. Using sensitive microwave observations we revisitthis finding made by Battaglia et al. (2009) based on RHESSI data analysis. Wehave found that nonthermal microwave emission produced by accelerated electronswith energy of at least several hundred keV, appears as early as the thermalsoft X-ray emission indicative that the electron acceleration takes place atthe very early flare phase. The non-detection of the hard X-rays at that earlystage of the flares is, thus, an artifact of a limited RHESSI sensitivity. Inall considered events, the microwave emission intensity increases at the earlyflare phase. We found that either thermal or nonthermal gyrosynchrotronemission can dominate the low-frequency part of the microwave spectrum belowthe spectral peak occurring at 3-10 GHz. In contrast, the high-frequencyoptically thin part of the spectrum is always formed by the nonthermal,accelerated electron component, whose power-law energy spectrum can extend upto a few MeV at this early flare stage. This means that even though the totalnumber of accelerated electrons is small at this stage, their nonthermalspectrum is fully developed. This implies that an acceleration process ofavailable seed particles is fully operational. While, creation of this seedpopulation (the process commonly called `injection' of the particles from thethermal pool into acceleration) has a rather low efficiency at this stage,although, the plasma heating efficiency is high. This imbalance between theheating and acceleration (in favor of the heating) is difficult to reconcilewithin most of available flare energization models. Being reminiscent of thetradeoff between the Joule heating and runaway electron acceleration, it putsadditional constraints on the electron injection into the acceleration process.

Thermal to Nonthermal Energy Partition at the Early Rise Phase of Solar Flares