So you purchased a battery, congratulations, you can now capture your excess solar energy and also some grid excess energy in the middle of the day.

But one question still remains, you keep getting a large power bill and left wondering why, when the new battery isn't empty?

The answer lies in a few things:

1. Smart meter is too slow to capture you usage in time as you turn appliances on. When you turn an appliance on, the battery pushes back against the grid covering your usage, but there is a millisecond delay between when you flip the switch to when the battery responds to the demand, this will cause a small amount of power from the grid to flow, resulting in an energy bill.
2. Your energy usage exceeds the output capacity of your battery. For example, some batteries are labelled 5kWh but only output 2.5kW at any given time. This is evident in the istore (Huawei) battery where each module will produce only 2.5kw and a maximum of 5kW with a large stack of three modules.
3. Your energy consumption at any given moment exceeds the maximum output of the inverter. Simply put you have turned on too many appliances at once and it exceeds the output limit of the battery inverter resulting in power being imported from the grid. See the below image, any thing above the "Inverter Maximum" line exceeds the 5kw limit of the inverter, while the energy consumption below the line is covered by the inverter output.