Accounts receivable, net

12 Months Ended

Dec. 31, 2021

Accounts Receivable, after Allowance for Credit Loss [Abstract]

Note 4. Accounts receivable, net

Accounts receivable consisted of the following:


(in thousands)	December 31, 2021			December 31, 2020
Trade receivables	$	38,597		$	23,691
Revenue recognized in excess of billings		72,676			1,224
Other receivables		147			47
Total		111,420			24,962
Allowance for doubtful accounts		(3,872	)		(1,228	)
Accounts receivable, net	$	107,548		$	23,734

Included in total receivables above are amounts billed under retainage provisions totaling $11.6 million and $4.5 million as of December 31, 2021, and 2020, respectively, which are due within the upcoming year.

Activity in the allowance for doubtful accounts for each period was as follows:


	Year ended December 31,
(in thousands)	2021			2020		2019
Balance at beginning of period	$	1,228		$	441	$	—
Additions charged to earnings		4,045			787		441
Write-offs of uncollectible accounts		(1,401	)		—		—
Balance at end of period	$	3,872		$	1,228	$	441

At December 31, 2021, four customers accounted for 29%, 23%, 19% and 18%, respectively, of total accounts receivable. At December 31, 2020, three customers accounted for 32%, 25% and 14%, respectively, of total accounts receivable.

During the year ended December 31, 2021, three customers accounted for 37%, 20% and 15%, respectively of total revenue. During the year ended December 31, 2020, four customers accounted for 21%, 19%, 10% and 10%, respectively, of total revenue. During the year ended December 31, 2019, three customers accounted for 59%, 21% and 13%, respectively, of total revenue.