When buying a battery for PV modules you first need to make sure it fits your solar system and meets the needs of your house. The voltage of the battery better be equal or lower than the voltage of your panels. In any case, you should always use a solar charge controller
to keep your battery safe. It lowers the voltage of panels down to the battery level, prevents the overcharge and disconnects the battery when discharged below a certain point. At night batteries without a regulator tend to give some of the accumulated energy back to panels — the charge controller doesn't let that happen.
To size a battery for your home, at first you would need to calculate your daily energy consumption. Lead-acid batteries are measured in Amp-hour. Lithium-acid batteries capacity is measured either in Amp-hour or in kWh (kilowatt-hours). While you're connected to the grid, your energy consumption can be calculated using energy bills for the last year. Or you can take the current draw (measured in Amps) of appliances, multiply by the amount of hours of runtime and you get Amp-hours. There are also special devices that can tell you the amount of energy you're spending everyday: plug-in power meters, submeters, special inverters. Kilowatt-hours can be measured through Amp-hours and vice-versa: Watt-hour = Amp-hour x Voltage of the battery.