The Crystal Mark is awarded for use of everyday English, explanations of technical terms, clear headings, well-sized and clear typefaces.
SSE said it had sought to be awarded the mark so that its customers know that it is committed to “clear and honest” communication and “welcomed feedback” on its letters.
Since its launch in 1990 the mark has been awarded to over 1,600 organisations who have shown a commitment to clear communication to their customers.
SSE’s vulnerability manager Louise Bailey said: “We know our industry can be complex and people can find it confusing, so this is another way SSE is making a real difference for customers.”
The first letters carrying the mark will be sent out to vulnerable customers who may be eligible for help with their energy bills through the Priority Assistance Fund.
The supplier is currently the least complained about energy supplier according to ombudsman figures.
Last month it cut the time it would resolve issues to four weeks, half of the allowed eight weeks, before informing customers of their right to an independent review.