Vauxhall eventually beat nine other manufacturers to secure one of the biggest fleet deals in its history, for 10,000 vehicles worth £150 million.
The bidding process was handled by Selectusonline, a buying consortium representing a number of power companies, to give them economies of scale when awarding supply contracts.
It said the final decision in Vauxhall's favour was based on wholelife costs, breadth of model range, fleet experience and fleet support infrastructure.
The new fleet order will see the supply of 5,500 cars and vans to Scottish Power, and a further 1,500 cars on a three-year replacement cycle and 3,000 vans on five-year cycles to United Utilities, replacing a Ford solus-badge deal.
Scottish Power runs about 1,700 cars, currently Toyotas, mainly for engineers and field staff, and will also take 800 perk cars. The two day online auction was organised on behalf of Scottish Power and United Utilities by the Dutch-based Eutilia.
All manufacturers who were bidding met with Selectusonline individually before moving to the online bidding process for 'lots' of vehicles, defined by vehicle type.
Alan New, programme manager at United Utilities, said: 'This is the first time Scottish Power and United Utilities have worked together to purchase vehicles. 'We did it to maximise our resources and optimise our discounts.
'We found that Vauxhall had the best range of cars and vans, and the most fleet experience in meeting our requirements.'