The dust storm is quite large to the point it is classified as a regional dust storm. Instead of reflecting sunlight, it is absorbing it and causing the entire planet to heat up 45 degrees Fahrenheit.
If the dust storm keeps expanding, there is slight chance Opportunity’s solar panels can be caked with dust, reducing its energy supply. Curiosity on the other hand has radioisotope thermoelectric generator that won’t be affected by the dust storms. More here.