Hey guys! I know I am a little late to the conversation but hopefully this helps someone out there.
If you require a generator that can run a 13,500 BTU AC unit you will need to look in the 3000W range. An inverter generator such as the
Briggs and Stratton 30545 has 3000 surge/starting watts and 2600 rated/running watts and would be able to run a 13,500 BTU AC unit. It also has a display screen which tells you what percentage of the overall output is being consumed.
In addition to power output you should also consider noise emissions. No one wants their campsite to sound like a construction zone. If you are looking for quiet portable power you are in the market for an INVERTER generator (as opposed to a conventional generator). Inverters also produce a "cleaner" power source that is suitable for sensitive electronics such as laptops and smartphones. The inverting process produces a smoother sine wave that will not damage the microprocessors within these electronics.
Many people swear by Honda's and Yamaha's and they are the industry leaders when it comes to inverter generators BUT there are many alternative companies (ie Champion, WEN, Westinghouse) putting forth very competitive products at a lesser price that merit consideration when shopping around for a generator.
I found this website to be extremely helpful when deciding which generator best suits your needs;
quietpowergenerators.comThe "inverters vs generators" page is very insightful and arms you with the knowledge necessary to make an educated purchase. They also have detailed reviews of the latest and best selling inverter generators on the market.
The units that would be capable of powering a 13,500 BTU AC unit are the Briggs and Stratton 30545, the WEN 56310i and the Champion 75537i - all of which are reviewed in detail on the site.
I hope this helps! Happy RVing/camping and all the best,
Dave