Why should we focus on the consumed energy?

We must look at the instantaneous power requirements for off grid systems especially. However, that factor is usually reserved for the battery inverter that is providing power when the grid goes out or in a standalone system.

PV inverters generally do not provide the amount of instantaneous power required for loads for a sustained amount of time. We only have so much sun and when you look at the power curve for a typical day 95% of the system power is produced between the hours of 10am and 3pm. Most house load profiles do not match this curve as the loads are usually highest in the morning when people are getting ready for work/school and then at the end of the day when everyone gets home. Both of these time periods are outside the solar window so the PV probably won't be able to sustain the power required by the loads.

We use kw/hr because it is a much more predictable measurement by which to size systems and unless the solar system is extremely large (probably not very practical for small homes with small roofs) the solar cannot sustain the loads.

You are totally right about the mismatch between the Max solar irradiation and the the energy consumption peak hours. But here is a different case: in hot countries, the middle east for example, the max irradiation will give you maximum power to run fans or small AC's. This will be needed at home, at school, at office..
The irradiation might be somehow more stable during a day comparing to Europe or America.
On the other hand, batteries, although indispensable, are something you need to replace very frequently. Even for the deep cycle batteries. Here I am also talking about long hours of electricity cut outs, particularly during the day. And the batteries become the most expensive block in the system and people get more reluctant to use PV system eventually.

- - - -> by: Greg