Ofgem had been administering the similar RHI scheme that was running in Great Britain before it took on the operation of the RHI scheme in Northern Ireland.

Image source, RHI Inquiry

It therefore had the necessary experience and IT systems in place to handle the Northern Ireland initiative - or so it seemed - and it was approached by the Stormont department that was setting it up.

Mr Nolan says it "seemed pretty logical" for the Department of Enterprise, Trade and Investment (DETI) to ask Ofgem to run its scheme.

Ofgem's primary function is as a regulator but a subdivision administers energy scheme, as it did with the RHI scheme.

Dermot Nolan has been its chief executive since 2014 - his previous work included time as a commissioner at the Republic of Ireland's independent energy regulator and he's got a PhD in economics from Yale University in the United States.

He's given a statement to the inquiry on behalf of Ofgem - it can be found in two parts here, external and here, external.

Image source, RHI Inquiry

The inquiry's junior counsel Joseph Aiken will be asking the questions.

He says Ofgem has submitted a colossal 196,423 pages of documents to the inquiry and a further 4,873 spreadsheets.

"There are some in the inquiry who love spreadsheets, so that's going to be keeping them busy," says the barrister with a smile.

BBC News NI

An email supplied to the inquiry by an ex-DUP adviser appears to show that someone in the party did not want cost controls introduced in the RHI scheme.

In the email, Tim Cairns told fellow DUP adviser Timothy Johnston that they needed to "catch up" about the initiative.

He also said that if they wanted to "deviate" from how a similar scheme in Great Britain had been set up with cost controls they would need to secure a formal command from a minister.

But Mr Johnston, now the DUP's chief executive, denied that he had given an order that cost controls would not be added to the scheme.

BBC News NI

An independent inquiry into the RHI scandal was established in January last year by the then finance minister Máirtín Ó Muilleoir.

He ordered it in the wake of the huge public concern and what was then a major political crisis surrounding the scheme.

The RHI Inquiry began in November last year and Sir Patrick Coghlin (below), a retired Court of Appeal judge, is its chair and has been given full control over how it will operate.

Image source, Pacemaker

It is looking at:

• the design and introduction of the RHI scheme
• the scheme's initial operation, administration, promotion and supervision
• the introduction of revised subsidies and a usage cap for new scheme claimants in autumn 2015
• the scheme's closure in February 2016

For more information on the RHI Inquiry, you can read our handy Q&A.

When the scale of the overspend emerged, public and political concern rocketed.

As the minister in charge of the Stormont department that set up the RHI scheme, the DUP leader Arlene Foster faced calls to resign from her role as Northern Ireland's first minister in December 2016.

Image source, PA

She resisted, and Sinn Féin's Martin McGuinness then quit as deputy first minister in protest at the DUP's handling of what had by then become a full-blown political crisis.

That move brought about the collapse of the Northern Ireland Executive. Now, well beyond a year-and-a-half on from that, Northern Ireland remains without a devolved administration.

You can find much more detail on the RHI scheme in our need-to-know guide.

The budget of the RHI scheme ran out of control because of critical flaws in the way the initiative was set up.

Claimants could effectively earn more money the more fuel they burned because the subsidies on offer for renewable fuels were far greater than the cost of the fuels themselves.

At one point the estimate for the overspend was set at £700m if permanent cost controls weren't introduced - temporary cuts have since pulled the budget back on track for now.

Whatever the scale of the bill, it will have to be picked up by the Northern Ireland taxpayer.

The Renewable Heat Incentive scheme - or RHI for short - came to the fore of the Northern Ireland public's knowledge in late 2016.

The fallout from the scandal attached to it is still being felt in the region's politics today.

Image source, Getty Images

The scheme was set up by the Northern Ireland Executive in 2012 as a way of encouraging businesses to switch from using fossil fuels to renewable sources for generating their heat.

Those who signed up were offered financial incentives to buy new heating systems and the fuel to run them.

Sound the trumpets and hand out the long-service medals - we've reached day 100 of the Renewable Heat Incentive Inquiry!

And even better, it's a Friday, which means everyone gets to clock off early.

It's been an incident-packed week so far with some stunning testimony emerging in the Senate chamber.

Today the inquiry will hear from Dermot Nolan, the chief executive of the RHI scheme's administrator Ofgem.

The proceedings begin shortly - watch our live stream at the top of this page and follow our extensive text commentary on all of the important bits.