With the rapid development of portable/wearable electronic devices, the demand for highly efficient and sustainable self-powered systems to support their off-grid operation has risen considerably. Recently, metal halide perovskites (MHPs) have been suggested as promising materials for energy harvesting and storage devices because of their excellent optoelectronic, ferro/piezoelectric, and ion migration properties. However, the high toxicity and poor physicochemical stability of MHPs hamper their practical application. Herein, for the first time, the potential of methylammonium tin halide (MA2SnX6; X = Cl, Br, and I) double perovskite thin films for high efficiency self-charging power units (SPUs) is elucidated by virtue of their high stability in air. To this end, the integration of an MA2SnCl6-based piezoelectric nanogenerator (PENG) with a lithium metal battery (LMB), which also uses an MA2SnCl6-based thin film as the cathode, is demonstrated. The MA2SnCl6-based PENG exhibited a high output power density of 7.33 mu W cm-2 and an excellent mechanical durability. Furthermore, MA2SnCl6 as a cathode in LMB recorded the highest specific capacity of 589.98 mAh g-1. A 1 nm-thick Al2O3 coating on the MA2SnCl6 surface improved the capacity retention from 16.9 % to 75.1 % for 100 cycles, indicating its stable performance. The MA2SnCl6-based LMB is charged by the MA2SnCl6-based PENG and utilized as an SPU for operating various small-scale electronics (humidity meter, calculator, and LED). This work not only demonstrates the feasibility of MHPs toward high-capacity batteries and energy harvesters but also provides insights into the general design principles of MHP-based devices.