Statistical power is the likelihood that a statistical test will:
Power can range between 0 and 1, with higher values indicating a greater likelihood of detecting an effect.
What is statistical power? 
Statistical power is the probability of correctly rejecting a false H0, i.e., getting a significant result when there is a real difference in the population.
Desirable power 
- Power ≥ .80 generally considered desirable
- Power ≥ .60 is typical of studies published in major psychology journals
Increasing power 
Power will be higher when the:
Estimating power 
Statistical power can be calculated prospectively and retrospectively.
If possible, calculate expected power before conducting a study, based on:
- Estimated N,
- Critical α,
- Expected or minimum ES (e.g., from related research)
Report actual power in the results.
Power calculators 
Try searching using terms such as "statistical power calculator" and maybe also the type of test, and you should turn up links to useful pages such as:
- Statistical power calculators
- One Sample Test Using Average Values
- Post-hoc Statistical Power Calculator for Multiple Regression
- Cohen, J. (1992). Power primer. Psychological Bulletin, 112, 155-159.