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The main challenges in designing the power reserve control (PRC) lie in the rapid estimation of the maximum available power ($P_\mathrm{avi}$) in real time and effective drift avoidance under the condition of fast-changing irradiation. Conventional PRC strategies utilize direct measurement or curve-fitting-based estimation to determine the $P_\mathrm{avi}$, which shows obvious limitations, such as extra hardware, implementation complexity, and slow estimation speed. Meanwhile, the drifted reserved power ($\Delta P$) may deteriorate the dynamic response performance, bring high dc-link overvoltage risk, and affect the system operation stability due to the voltage-step-$\Delta P$ regulation mechanism in conventional PRC methods. To address these issues, an improved PRC strategy with a fast $\Delta P$ transient mechanism is proposed with the rapid estimation of $P_\mathrm{avi}$ through just one pair of photovoltaic (PV) voltage and current sampling. Thus, the proposed algorithm is cost-effective, easy to implement, and compatible with existing PV systems since no additional hardware components are required. Moreover, the proposed PRC breaks through the inherent limitation of the voltage-step-$\Delta P$ mechanism, which can ensure the $\Delta P$ drift mitigation even under fast-changing weather conditions. Main experimental comparisons with other advanced PRC strategies were conducted to verify the advantages of the proposed algorithm in terms of rapid real-time available power estimation and drift avoidance.