Does the effect size mean the expected difference between the means I am designing an experiment and want to define the sample size. To identify this, I am setting my significance level to 0.05 and the power to 0.8. My alternative hypotheses says the two means should be at least 
15% different from each other. Does this 15% correspond to the effect size? 
I had a look at this  and this, but I am still not sure how to interpret this 15% I am setting my alternative hypothesis in the context of power analysis. 
I tried to calculate the sample size based on the assumption that the 15
% is the effect size:
from statsmodels.stats.power import TTestIndPower
# parameters for power analysis
effect = 0.15
alpha = 0.05
power = 0.9
# perform power analysis
analysis = TTestIndPower()
result = analysis.solve_power(effect, power=power, nobs1=None, ratio=1.0, alpha=alpha)
print('Sample Size: %.3f' % result)

The output is Sample Size: 934.954
This does not seem reasonable. I am not sure if I a doing it the right way.
Can someone help here?
 A: I think you're referring to measuring effect size in Cohen's D. If so, the effect size would be the difference in means, divided by the pooled standard deviation. 
So, let's say you're looking at groups' comprehension scores on a test. Let's say your control group scores an average of 50, standard deviation 10. And let's say your treatment group scores an average of 57.5, standard deviation 5. 
Then the Cohen's D would be = (57.5-50) / ((10+5)/2) = 1 
(Note that this assumes equal sized control and treatment groups. Deriving the pooled standard deviation is actually a bit more complicated than averaging the two groups' standard deviations, if the two groups have different sample sizes. See wikipedia page on Cohen's d, for example.) 
In your code, 0.15 refers to the minimum effect size (in Cohen's D) that you want to be powered to detect. What this means for your data depends on your samples' standard deviation(s). You haven't collected the data yet, so you'll have to make assumptions here based on similar past experiments, the literature, your judgement, etc. Researchers often run multiple power calculations using different assumptions and create a table that shows the various sample sizes required given these different assumptions. For example, you could look at the sample sizes needed given various effect sizes, and/or given various standard deviation(s). 
A: Different power analysis programs will allow different measures of effect size for different types of statistical analyses. "Means will very by 15%" is certainly an effect size, but it may not be usable by your software, in which case you might need to convert it to something else. 
EDIT In your initial sentence you say you set power to 0.8 but your code has power at 0.9.  
Also, you have effect = 0.15.  What does that mean? You need to figure that out from the documentation for TTestIndPower. But it cannot be a % difference. 
