Feel free to download and use the following spreadsheets. The calculations are accurate but the assumptions you plug in are your own and can dramatically affect the results. The numbers already entered are mine and should be replaced with your own.
Both spreadsheets are based on the work of Richard Graylin. The original and complete spreadsheet can be found on CamperTrailers.org website. I encourage you to visit their webpage as there is a lot of very interesting information. I have simplified the spreadsheet here for a more basic and approachable look but the results are not affected.
Please note that all the technical information is provided in good faith and as an educational resource to help the reader understand how such a system works, but it is no replacement for professional advice if you intend to build such a system. The accuracy of the spreadsheets results directly depends on the accuracy of the data you input in the Assumptions.