The slider for the Zappi is for MGL (minimum green level). If you set it to 1%, then it’ll pull from the grid anything up to 1.37kw or thereabouts if there isn’t enough solar. I usually set that to about 80 or 90%, so that it only pulls a small amount from the grid, or stops when there isn’t enough excess.
At 1% it’ll almost always pull from the grid unless you have either zero excess solar, or 1.4kw or more.
If you want to run a scheduled charge and not have it charge from the grid outside of this then have the MGL at 100% or very close to it, so it will only charge when there is enough excess solar, or when the scheduled time arrives.
Edit just to say, using the above I have never had a failed scheduled charge with the Zappi.