Question: I am planning on TDE encrypting a bunch of  large Sql Server databases. Due to potential IO contention issues during the work day , the plan is to encrypt these databases during the out of hrs period. If the Encryption is triggered , but there is a SQL Server service restart during the encryption process , will  the encryption process be impacted when the SQL Server service restarts?

Answer:   The TDE encryption is triggered by executing the following statement 

ALTER DATABASE myDB  
SET ENCRYPTION ON

You can track the progress of the encryption by executing this sql statement , use the percent_complete column to review the percentage. If the SQL Server restarts , the encryption process will just continue , and continue form where it left off . 

SELECT DB_NAME (e.database_id) AS DatabaseName,
e.database_id,
e.encryption_state,
CASE e.encryption_state
WHEN 0 THEN 'No database encryption key present, no encryption'
WHEN 1 THEN 'Unencrypted'
WHEN 2 THEN 'Encryption in progress'
WHEN 3 THEN 'Encrypted'
WHEN 4 THEN 'Key change in progress'
WHEN 5 THEN 'Decryption in progress'
END AS encryption_state_desc,
c.name,
e.percent_complete
FROM sys.dm_database_encryption_keys AS e
LEFT JOIN master.sys.asymmetric_keys AS c
ON e.encryptor_thumbprint = c.thumbprint

DatabaseName database_id encryption_state encryption_state_desc name percent_complete
tempdb 2 3 Encrypted NULL 0
test 5 3 Encrypted NULL 0
MYDB 9 2 Encryption in progress NULL 15.25823

From SQL Server 2019 there is also an option for a controlled pause of the encryption

ALTER DATABASE SET ENCRYPTION SUSPEND;

ALTER DATABASE SET ENCRYPTION RESUME

Read more on SQL Server TDE

Tested TDE with Instant File Initialization on SQL Server databases

How to measure Transparent Data Encryption (TDE) Performance