A resistor is a standard electronic component that has a constant electrical resistance in a circuit. In addition to a resistance value, measured in ohms, it has a maximum power rating, measured in watts. When a circuit does not exceed the power rating, the resistor works reliably and without fuss for decades. Exceed the power rating by more than a few percent, and the resistor burns up within minutes. Generally, the larger the resistor, the more power it handles. You can determine a resistor's wattage with a size chart.
Place the size chart (see reference) on a flat surface.
Find the resistor outline on the chart that most closely matches that of the resistor itself.
Place the resistor on the outline to make sure they match.
Read the wattage value on the chart next to the outline.
Resistors rated for five watts or greater have the wattage rating printed on the body of the part. Simply read the printed rating for these larger resistors. Use the chart for resistors under five watts.