Well, I’ve just had a look - there are two ways of showing serious effect
I it causes B to fear, on at least two occasions, that violence will be used against them - s.76 (4)(a); or
If it causes B serious alarm or distress which has a substantial adverse effect on their day-to-day activities - s.76 (4)
Then there are some examples of what might constitute the latter one- non exhaustive-.
Stopping or changing the way someone socialises
Physical or mental health deterioration
A change in routine at home including those associated with mealtimes or household chores
Attendance record at school
Putting in place measures at home to safeguard themselves or their children
Changes to work patterns, employment status or routes to work
Obviously the easier one to go for is the first, as that less open to interpretation.