Rechargeable batteries are currently the best method to store electrical energy.
The issue is they only store direct current or dc power. Direct current holds it polarity constant. If you supplied direct current to standard electronics you would short circuit them. To power standard household devices you need AC power or alternating current. Alternating current switches polarity at a rate of 60 cycles per second. It would be impossible to manually switch the polarity of the direct current system without a circuit system known as an inverter.
By using a dc to ac power inverter system you can now open and close the positive and negative flow of current at a rate of 60 times per second. Essentially creating an alternating current which will now allow you to use your stored dc power to operate standard household electronics.