I strongly agree with Scooter68 - sweetness is a perception and this perception will depend on the ABV, on the pH , the TA, tannin levels, and the flavor richness of the wine. okay - so technically, 1 lb of sugar added to 1 gallon of water will raise the gravity of the water by 40 points. One pound = 453.6 g. So 450 g (approximately) will raise the gravity of 1 gallon by 40 points, and so 15 grams being 1/30 (approx of a 450 g) will raise the gravity of 1 gallon by 1/30 of 40 or 1.33 points but you are talking about 15 g /l and so there are 3.78 l in a gallon so you may want to multiply the 1.33 by 3.78 (assuming you will be adding 15 g to each liter) and so the result will be an increase of about 5 points. Bottom line - if you are adding 15g of sugar / liter and your batch is 6 gallons (or 22.7 l) then this will increase the SG by 5 points (approximately).