Exactly.
Or if you don't like the whole case idea you can just get a rechargeable portable charger, such as one of these:
http://www.amazon.co.uk/Ultra-Compact-Lipstick-Sized-Flashlight-Lightning-Blackberry/dp/B00EAOGYNY/ref=pd_sim_sbs_ce_2
I've not used it myself but it's very cheap and has...
You can get yourself a battery case
http://www.amazon.co.uk/s/?ie=UTF8&keywords=xperia+z+battery+case&tag=googhydr-21&index=aps&hvadid=24515734254&hvpos=1t1&hvexid=&hvnetw=g&hvrand=153804572397383706&hvpone=&hvptwo=&hvqmt=e&hvdev=c&ref=pd_sl_8mchod9zua_e