Two months ago the energy company Vector announced that the Auckland Harbour Bridge would have permanent coloured lighting powered by solar energy.

It called the $10 million project a "world first" and signalled that it marked the next step in a 10-year partnership with Auckland Council on sustainable generation and energy efficient technology.

The exciting news was accompanied with an eyecatching video of how the bridge could be illuminated by its solar cloak, and a breathless claim that the innovative use of clean energy would put Auckland on the map as a creative and innovative city.

The source of power for the solar project was to come from more than 630 solar panels at Wynyard Quarter, about 2km from the bridge, which would feed a large battery to store the energy.

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But questions have arisen about how power from the battery will light up the bridge. Will it be connected to the proposed network of 90,000 LED lights and 200 spotlights on the bridge, or will it feed the grid? If it is to be connected to the bridge, will this affect the budget for the project?

These issues have surfaced after a source in the electricity industry suggested the grid solution looked more likely given the investment required to hook up the bridge. For a start, cables would need to be laid round Westhaven from the power source at Wynyard Quarter. This would duplicate the existing cables which feed the bridge's lights.

The power company has suggested that the bridge would always be connected to the national network to ensure security of supply, though this caveat was not mentioned in the solar plan statements. The installation is technically challenging. The bridge environment is demanding. Measures to protect supply make sense.

Vector says "connection options" are still under consideration, with both "direct and virtual" arrangements in the mix.

New Zealand gets about 80 per cent of its electricity production from renewable sources, mostly hydropower and geothermal power. Wind energy is part of the picture, and solar is coming into the frame.

But on any given day, and particularly in winter, thermal power helps keep the lights on. This is produced in gas plants, especially in Taranaki. It is not a renewable source of energy and it will be part of the generation mix for some time.

It could be that the energy delivered to the grid from the Wynyard solar field would offset power drawn from the network to feed the light show.

Tapping into the grid though presents a different picture to the one portrayed by Vector and the council when the scheme was unveiled.

The shift to sustainable energy is an economic challenge.

Projects which showcase this transformation, such as the bridge plan, need to be credible and transparent if the embrace of renewable sources is to gain traction.

Vector and the council should be open about the project and candid with their claims. If it is to be fed from the grid, so be it. It will still be an appealing idea and an asset to the city.