Rental fleet is capitalized at cost and depreciated over the estimated useful life of the unit using the straight-line method. Rental fleet is depreciated whether or not it is out on rent. Capitalized cost of rental fleet includes the price paid to acquire the unit and freight charges to the location when the unit is first placed in service, and when applicable, the cost of manufacturing or remanufacturing, which includes the cost of customizing units. Ordinary repair and maintenance costs are charged to operations as incurred.

We periodically review depreciable lives and residual values against various factors, including the results of our lenders’ independent appraisal of our rental fleet, practices of our competitors in comparable industries and profit margins achieved on sales of depreciated units. Appraisals on our rental fleet are required by our lenders on a regular basis. The appraisal typically reports no difference in the value of the unit due to the age or length of time it has been in our fleet. Based in part upon our lender’s third-party appraiser who evaluated our fleet as of August 31, 2019, management estimates that the net orderly liquidation appraisal value as of March 31, 2020 was approximately $1.0 billion. Our net book value for this fleet as of March 31, 2020 was also $1.0 billion.

Depreciation expense related to our rental fleet for the three months ended March 31, 2020 and 2019 was $7.8 million and $7.7 million, respectively. At March 31, 2020, all rental fleet units were pledged as collateral under our Second Amended and Restated ABL Credit Agreement dated as of March 22, 2019 (the “Credit Agreement”) with Deutsche Bank AG New York Branch (“Deutsche Bank”), as administrative agent, and the other lenders party thereto.

Rental fleet consisted of the following at March 31, 2020 and December 31, 2019:

	Residual Value as Percentage of Original Cost (1)		Estimated Useful Life in Years	March 31, 2020			December 31, 2019
				(In thousands)
Storage Solutions:
Steel storage containers		55%	30	$	624,731		$	627,230
Steel ground level offices	55		30		364,429			366,630
Other					6,122			6,565
Total					995,282			1,000,425
Accumulated depreciation					(168,564	)		(166,565	)
Total Storage Solutions fleet, net				$	826,718		$	833,860
Tank & Pump Solutions:
Steel tanks			25	$	82,264		$	82,015
Roll-off boxes			15 - 20		35,576			35,398
Stainless steel tank trailers			25		31,292			29,716
Vacuum boxes			20		17,242			16,775
Dewatering boxes			20		10,233			9,486
Pumps and filtration equipment			7		15,031			14,343
Other					9,896			9,776
Total					201,534			197,509
Accumulated depreciation					(68,075	)		(65,146	)
Total Tank & Pump Solutions fleet, net				$	133,459		$	132,363
Total rental fleet, net				$	960,177		$	966,223