A battery can store electric charge so that it can be used to power a craft's systems such as radio transmissions, reaction wheels, and probe cores. While command pods do store some electric charge, most batteries store much more. Every variety of battery has the same charge per unit mass, 20 units of electic charge per kilogram.
Zaltonic Protonic Electronics manufactures all of the available batteries in Kerbal Space Program. (You might say they're a power player in the industry.)
|Image||Part||Radial size|| Cost
|Z-100 Rechargeable Battery Pack||Radial mounted||80||0.005||1 200||8||50||100|
|Z-200 Rechargeable Battery Bank||Tiny||360||0.01||2 000||8||50||200|
|Z-400 Rechargeable Battery||Radial mounted||550||0.02||1 200||8||50||400|
|Z-1k Rechargeable Battery Bank||Small||880||0.05||2 000||8||50||1 000|
|Z-4K Rechargeable Battery Bank||Large||4 500||0.2||2 000||8||50||4 000|
Since the amount of electric charge per unit mass is constant, the best battery to choose is based on craft design and available space. The most cost effective way to store power is to stack instances of the Z-100 Rechargeable Battery Pack since it has the lowest cost per unit charge. That might serve the needs of large rockets (stuff them in service bays) or space stations. For satellites or rovers, where that amount of mounting space is not available, figure out how much juice you want to have available and then pick the battery that fits your mounting need and has the lowest unit cost.