Leveen also claimed Bloom Boxes were economic only with subsidies, and schemes supporting their installation had cost California taxpayers hundreds of millions of dollars.
Matt Ross, chief marketing officer for Bloom, said the NBC story was misleading. "Their report is a hodge podge from different time periods. They're comparing apples to oranges to rocks," Ross said.
Data for the Delaware site show CO2 emissions had risen to an average 373kg/MW, from an initial reported performance of 350kg/MW.
Ross said the Delaware site had been operational for more than two years and was half-way through a maintenance cycle requiring replacement of fuel stacks, leading to anticipated loss of performance.
The reported Delaware emissions were within contractual limits, and compared favourably to other types of electricity generation, he said.
The US Energy Information Administration said the comparable kg/MW figures for nuclear power were 514.8 and for coal generation 1020.
Ross said subsidies were an important part of introducing new technology, and he expected within three years the company's product would be refined and a "very attractive value proposition to the customer without the requirement for ongoing incentive support".
Bloom Boxes rely on fuel, typically hydrocarbons and mostly natural gas, but generate electricity through an electrochemical reaction rather than traditional generators' combustion.
The NZSF invested US$50 million in Bloom in May 2013, and another US$50 million in March. A spokeswoman for the fund said the NBC report had crossed their desk.
"We are aware of the NBC story, have investigated all the claims and are not concerned about any aspects of it."
The stake in Bloom comprises less than half a per cent of NZSF's $26.7 billion investment pool, and was made after the fund opened a small slice of funds for relatively high-risk expansion capital. Most of the NZSF portfolio remains invested in relatively blue-chip listed companies.
Prior coverage by the American financial press suggested Bloom has been angling for an IPO, but Ross wouldn't say if listing was a short or medium-term goal.
Given NZSF said their US$100 million investment netted a less than 5 per cent stake, it seems Bloom has been valued by shareholders in excess of US$2 billion.