An appliance manufacturer claims to have developed a compact microwave oven that consumes an average of no more than 250 W. From previous studies, it was determined that power consumption for microwave ovens is normally distributed with a standard deviation of 15 W. A consumer group is suspicious of the claim and believes it is false. They take a sample of 20 microwave ovens and find that they consume an average of 257.3 W.

Referring to the problem statement, the value of the test statistic is ________.