The monthly electricity bills make it easy to estimate the average daily usage (KWh/day) - but those are only averages, with electric usage likely fluctuating from day-to-day. We just went through planning for our pending solar panel/PowerWall configuration. Because of efficiency loss in charging/discharging, it probably takes more like 15kW/PW to go from totally-empty to fully-charged. And if your solar doesn't produce enough over what the house draws then it may not reach full-charge over the course of a day. Each PW stores around 13kWh, so if the full 5kW is available it should recharge in around 3 hours (and if the PW wasn't close to empty it will charge in less time), but in the morning while the sun is still rising you probably won't have the full power available, so it will charge more slowly.
If the solar is producing 10kW and the house is drawing 1kW, then there's 9kW available that 2 or more PW's will easily soak-up until they get close to fully-charged (at which point the PWs will shut down the solar and the the house will draw from the PWs until they're ready to start drawing full-power from the solar again).
So if your solar is producing 1kW but the house is drawing 1kW, you won't be charging the PWs at all. As the PW nears 100% its draw starts to ramp-down (a requirement in Lithium battery charging), this is why the PWs start to throttle/disable solar when the grid is down and the battery is almost full, because they can no longer draw the full 5kW/PW and there might end up being too much current sourced by the solar inverters and nowhere for it to go. The rest will go into the PW to charge it up, up to the maximum charge rate. So it's really a matter of how much your solar produces, subtracting how much the house is consuming. Each Powerwall 2 will charge at a maximum rate of 5kW.
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