Reply #12: Some things to consider during your comparison, hope this helps

[txlibdem]

It seems like you are trying to compare an on grid power backup system with a hybrid solar on+off grid system. There are a lot of variables to consider, such as:
1. Does your utility give a discount for power used during "off peak" hours?
2. Does your state/local gov't or your utility offer any rebates for solar?
3. How efficient is your charge controller (you call it DC-DC converter)?
4. How efficient is your DC-AC converter?
5. What percentage of your total load will be used to power DC equipment?
6. How many hours of peak sunlight do you get at your location?

In short, if #1 is yes then you would set the charge controller to charge your batteries during the lowest cost period. Use that cost per kWh (8c/kWh becomes .08) divided by the efficiency of your charge controller (95% becomes .95). In that example it works out to 8.4 cents per kWh.

For the purposes of your comparison (unless I totally missed your point) there is no need to factor in the cost or efficiency of the DC-AC converter because it would be needed in both systems. It only helps you understand your total costs, which would only help you determine if the whole thing is worth it or not. If you want to calculate this then first subtract the percentage of the total load that will run off pure DC because there would be no conversion losses for these items.

One thing about the solar panel setup: you are basically locking in the cost for the next 20 to 30 years. Historically, utility rates have gone up 3.78% per year. So, in other words, even if the solar panels are not the cheapest option this year they might be cheaper after year 5 or year 8 or whatever.

Here's a good site to start your solar panel cost per kWh:
http://greenecon.net/understanding-the-cost-of-solar-energy/energy_economics.html

Here's a solar map for PV (answers question #6):
http://www.nrel.gov/gis/solar.html