Installing an amp and subwoofer in 04 HCH

Yes, there have been many Gen 1 Civic Hybrid owners that have upgraded the factory sound to something more powerful. So it is possible, and if done correctly should cause no ills at all and be quite cool.

Will it cause a loss of MPG's? Yes. Noticeable loss? Depends.

Please note that the charge for your 12V comes from your IMA battery. The more power is drained from the 12 volt system, the more energy is diverted from propulsion duties so that the 12V system may remain charged. That is reality.
In fact, people who seek to have the best MPG's will avoid draining their 12 V system. Not surprisingly, it is quite common for many folks to disable their Daytime Running Lights and other equipment so that more IMA state of Charge will remain available for better FE.

So what does this all mean?
Well, if your drives are short and mostly city based, then you "may" be more affected than other folks who's drives are long more highway based. This type of thing is very subjective depends heavily on the circumstances. Some folks will scoff at this issue and claim no loss of MPG's... To each their own. :)

Personally, I avoid unnecessary drain of the 12V system because the impact is measurable in my case.

MSantos

I'm having my 12'' sub and amp installed this Sat. 8/25. I'll report my experiences later.