Power: or “authority” (Good News Translation), or “the right.” This is given them by God, and it may be that in some languages it will be better to translate “God has given them the authority (or, power).”
To shut the sky: where this figure of speech makes no sense or makes the wrong sense, it may be necessary to say simply “to stop the rain from falling”; in English the normal way to say this is “to keep it from raining.” This is the power that the prophet Elijah had (see 1 Kgs 17.1; 18.1; Luke 4.25; James 5.17).
During the days of their prophesying: the drought caused by Elijah lasted into the third year (1 Kgs 18.1, 43-45), that is, it lasted not quite three years. But Luke 4.25 and James 5.17 show that it had become three and a half years. Days of their prophesying may also be rendered as “the time when they proclaimed God’s message.”
Power over the waters to turn them into blood: this is the power that Moses exercised (Exo 7.14-21). Instead of “the springs of water” (Good News Translation), the meaning of the Greek the waters may be better expressed by “all bodies of water” or “to turn water into blood.”
To smite the earth with every plague: this is the power that Moses had to bring the plagues down on Egypt (see 9.18). The verb to smite is used also in 19.15; it means “to strike,” “to hit,” “to injure,” “to wound.” For plague see 9.18. Translators in some languages will express this sentence as “to cause every kind of calamity (or, terrible trouble) to injure those living on the earth.”
An alternative translation model for this verse is:
• God has given them authority to stop the rain from falling during the time they proclaim his message. They have also received the authority to turn water into blood, and to cause all kinds of terrible calamities to hurt (or, strike) the earth. They can do this as often as they want to.
Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on The Revelation to John. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .