5. REVENUE

The Group’s revenue streams are mainly composed of pipeline plus EPC services, BT, sales of solar modules, O&M services and income from the sale of electricity generated by IPP solar parks. The following table summarizes the categories of the Group’s revenue:


	Year ended December 31,
	2016	2017	2018
	Thousand USD	Thousand USD	Thousand USD
Related parties:
—EPC services solar energy system sales
Sales of permits	525	—	—
—Other sales
-O&M services	263	286	324
-Sales of solar modules	—	—	—
	263	286	324
Subtotal	788	286	324
Non-related parties
—Electricity sales income	53,658	53,614	61,438
—EPC services solar energy system sales
-Provision of Pipeline plus EPC services
Sales of permits	—	—	—
Provision of construction services	4,954	285	198
- BT	4,232	—	—
	9,186	285	198
—Other sales
-O&M services	2,290	2,548	2,708
-Sales of solar modules	3	—	1
	2,293	2,548	2,709
Subtotal	65,137	56,447	64,345
Total	65,925	56,733	64,669

Contract balances

The following table provides information about receivables, contract assets and contract liabilities from contracts with customers.


	December 31,	January 1,
	2018	2018
	Thousand USD	Thousand USD
Receivables, which are included in ‘trade and other receivables’	—	819
Receivables, which are included in ‘assets held for sale’	—	—
Contract assets	731	—
	731	819

The contract assets primarily relate to the Group’s rights to consideration for work completed but not billed at the reporting date on IPP sale of electricity. The contract assets are transferred to receivables when the rights become unconditional. This usually occurs when the Group issues an invoice to the customer.