Essentially, large companies make monthly declarations because they have a large sales volume. When they buy a carbon emission allowance, they will indicate a percentage of the price of the allowance in their declaration, depending on their province. In Ontario, for example, it is 13%. So that is how they will self-assess the amount in their declarations. If all their activities are commercial in nature, as is generally the case for most companies that buy carbon emission allowances, they will at the same time request an input tax credit. In the example I have just given, nothing will be payable.
The GST program anticipates other cases where a company would self-assess. For example, if a company buys a carbon emission allowance overseas for use in Canada, and if the seller of the allowance is not registered, the company must self assess, unless it has an automatic right to input tax credits.