Fast charging, long-lasting, heavy duty batteries. Just two of them is enough to keep my house running all night (~200 Wh) without dropping below about 60%. And charging them up the next day at 2 kWh or so only takes about 3 hours. The occasional draw-surge from a refrigerator kicking in or such will dim the output voltage a bit for a third of a second or so, but won't show a noticeable hit on the battery's voltage.
For the most part, these batteries will hang around the 52/53 volt range under normal minor draws. This can definitely confuse battery monitors that are designed for non-lithium batteries, as the curve is practically flat. Make sure to read all the manuals SimpliPhi provides, put a breaker on the connection between the batteries and the rest of your system to prevent amperage in AND out from being too high (the batteries are rated at C/2 for their maximum both in and out, so 37.5A per battery max). For two batteries, I went with a 60A breaker with a steep trip curve (B, or C at worst) so that its 125% "must-trip" value would be 75, then limited my charge controllers and inverter to around 55 maximum DC output. Read the warranty PDF for details about sizing battery banks to match your inverter and charge controller, but definitely think about keeping a breaker on the wire just to be absolutely safe.