MuneebK

With only 10kWh of battery (1 x 10 or 2 x 5), I would suggest that you change the geyser element to 2kW - one hour of use (say) early morning before sunrise would drain ~20% of your battery capacity, whilst with a 3kW element you would drain ~30%.
My only two reasons for preferring the 10kWh units over the 5kWh ones were space (two larger units take up considerably less wall space than four 5kWh units...), and cost (again, 2 x 10kWh batteries were cheaper than 4 x 5kWh batteries, taking into account the additional (thick) wiring, high-amperage fuses, etc.)