When should I use 兆 instead of 万亿? In the following sentence:


  据悉，日本政府此次提出了在东京奥运会召开的2020年，使访日游客增至4000万人次，消费额达到8万亿日元的目标。


In this sentence, 万亿 is used instead of 兆, to express 10^12 digit.

In Chinese, now 兆(zhào) might be rarely used to express "trillion" and instead 万亿 is used.

So when should I use 万亿 and when 兆 to express such a large number?

And also what about the case to express more than 万亿, such as 10^16? 

Japanese use 万, 亿, 兆, 京... but how about Chinese? Should I write it as 91万万亿2513万亿522亿6915万159 to express 912513052269150159? or 91京2513兆522亿6915万159?
兆 was used in ancient China to represent trillion, it is no longer used in mainland China, but still used in Taiwan, it uniquely represents the mega prefix in the International System of Units currently. But in Taiwan, 一兆 means one trillion, while 兆赫兹 means MHz (megahertz). The largest number that can be expressed in a single Chinese character is 亿 in mainland China, to express larger numbers, use the phrases like 万亿 or 亿亿. Of course, there are also Chinese characters that represent numbers larger than 亿 (100 million), but such a large number is not used generally, and nobody use them, except the ancient literature. You may learn them here.