Roger, I don't have any experience with the Nomad 300. I do have 12 years experience living with solar panels and batteries, though. I looked at the Nomad 300 ad. Yes, you can put an auxiliary battery onto the system. But, more than one is overkill. You only get 21 watts, maybe 1.5 amps/hours. If you use your system on a regular daily basis, you'll probably have just enough panel power to recharge the battery that's included. If you use your system every few days or once a week, yes, a second battery might be useful.(posted 9635 days ago)If you do have a second battery, be aware that your batteries will try and equalize with each other. In other words, if you've got an older battery, both batteries will only be as good as the weaker one.
If you're looking for a small home system, barebones, or even a semi portable system, think about this. Buy a regular size solar panel, or whatever size you can live with. Some panels are self regulating, in other words, you don't have to buy a charge controller (keeps your batteries from boiling away when overcharged). Buy either a 12v deep cycle battery, or better yet, go to Sams and buy two 6 volt golf cart batteries. For four hundred bucks (300 for panels, 100 for batteries) you can get a system with spunk! Looking at the size of the Nomad 300, you've got limited amperage. I've got a small battery the same size, and I get about 8 hours playtime on my boombox. Not much. If I hooked my notebook up, I'd get two hours. I think you'll be much happier with a larger system. Nomad 300 would be great for intermittent use or while travelling. Good luck.