Can batteries be added to an existing solar system?
Yes, a battery can be added to any grid-connected solar system using an AC-coupled battery . Retrofitting a battery can also be achieved using a special type of hybrid inverter designed to AC-coupled batteries.
What size battery should I get?
Before we get into too much detail, you should first understand the basics. Battery capacity is measured in kilowatt-hours (kWh) and most (lithium) battery systems are modular and scalable, so you can generally get a battery sized to suit your needs. Note, the old sizing terminology used with Lead-acid batteries was Amp-hours (Ah), but this is now mostly superseded with kWh being the industry standard for modern lithium batteries.
The next point you should know is roughly how much electricity your household or business consumes over an average 24 hour period. Another factor to consider is how much electricity you use in the peak evening period (4pm to 9pm) which is often when the cost of electricity is at its highest, depending on your service provider. However, for modern all-electric homes and those with home electrical vehicle chargers, the optimum battery size for maximum self-consumption is increasing. Every household is different and there is no one-size-fits-all solution.