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Loss on sale of assets - Narrative (Details) - USD ($)
 12 Months Ended 24 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2017
Disclosure on Sale of Assets [Line Items]      
Loss on sale of assets $ 4,049,000 $ 1,365,000  
Proceeds receivable 2,000,000 0  
Cash proceeds received on sale 1,345,000 981,000  
Power Manager assets      
Disclosure on Sale of Assets [Line Items]      
Cash proceeds received 2,000,000    
Note receivable 2,132,000    
Maximum possible earn-out amount $ 11,250,000    
Earn-out period 12 months    
Loss on sale of assets $ 3,957,000 0  
Proceeds receivable 2,000,000    
Cash proceeds received on sale 2,000,000 0  
SOFC assets      
Disclosure on Sale of Assets [Line Items]      
Loss on sale of assets 94,000 508,000  
Cash proceeds received on sale 50,000 0  
Telecom backup power assets      
Disclosure on Sale of Assets [Line Items]      
Loss on sale of assets 0 866,000  
Cash proceeds received on sale $ 0 $ 972,000  
Estimated fair value of earn-out payments     $ 0