The kW measure power: how quickly energy is generated, consumed, or transferred at a given moment. The kWh measure the amount of energy: how much was generated, consumed, or stored over a period of time.
For panels, you will also come across kWp, which indicates the rated power under specified test conditions. A 5 kWp solar system does not consistently generate 5 kW throughout the day.
A simple example: a constant load of 2 kW operating for 3 hours consumes 6 kWh. A battery with 8 kWh of usable energy available could theoretically supply a load of 2 kW for 4 hours, before taking losses and operating limitations into account.
Capacity alone is not enough to know which appliances can be powered at the same time. The battery and inverter also need sufficient output power, with provision for startup surges. That is why you should ask for both figures in the quote: available kWh and permitted kW.