What's wrong with my return statement?

Why this code is wrong ? When I change "else if(this.power >= p)" into only "else", it's right. Can anybody give me a hint?

public boolean useBattery(double p) {
    if(this.power < p) {
        this.power = 0.0;
        return false;
    } else if(this.power >= p) {
        this.power = this.power - p;
        return true;
    }
}