The statistical war about Scotland's future has broken out again, as it does every year around midsummer. Well, it beats looking out at the rain.

For 17 years, GERS has meant the reckoning by St Andrew's House number-crunchers of Scotland's profit and loss accounts.

That is, Government Expenditure and Revenue in Scotland seeks to calculate (and sometimes guess, though in an educated way, of course) how much of the UK's revenue is raised in Scotland, and how much is spent in Scotland.

The big picture has become particularly important this year, as these figures now feed directly into the debate about Scotland's constitutional future, looking towards the independence referendum near certain within the next five years.

It's harder to draw conclusions for then, however, as the most significant factor of the 2009-10 figures in this year's GERS report is the dramatic lurch into the red of the UK's finances, Scotland's included.

Unsustainable

By this reckoning, Scotland would have had a £14bn deficit, or just under 10.6% of gross domestic product (GDP). With expenditure of £62bn, and revenue of £48bn, that's a big gap. Unsustainably so.

But then consider the UK's actual deficit of £157bn, or 11.1%. That's right — the UK's fiscal gap is bigger.

The Scottish government is citing a smaller deficit calculation of £9bn, or 6.8% of GDP.

Instead of the 'net fiscal balance', that smaller 'current budget balance' doesn't include capital spending.

That figure can be used on the basis that capital investment has payback over subsequent years.

However, it's still money exiting the government's bank account, and it tells a similar story of being a smaller share of GDP than the UK figure.

Those figures assume that Scotland would have a share of the UK's oil revenues based on the 81% of oil and gas that is reckoned to come from under its waters (97% of the oil, and 58% of the gas).

GERS also offers calculations of Scotland's finances without oil revenue, which look horrendous (the 2009-10 figures show an 18% deficit, which is heading towards Irish proportions).

You can see a population share of oil and gas tax too, and that still looks grim, but neither of these figures tells us anything about the fiscal position at the heart of the political debate about Scottish independence.

Indeed, it's worth repeating that there's a risk of using the 2009-10 figures as a guide to anything at all other than the precipice over which the UK's finances hurtled that year.

Volatile oil

But it tells us something of the difference between the UK and Scottish fiscal positions, depending how you want to use the figures.

Those arguing for retaining Scotland's place within the UK say GERS shows Scotland can't go it alone, unless it relies on oil revenues. And the most recent figures show how volatile that can be.

In the year after 2008-09, when oil hit $147 per barrel, total UK North Sea revenue halved from £13bn.

But since 2009-10 when the price was dropping to $50, oil is riding high again, and UK revenue is heading back towards £13bn again.

The next two years of GERS figures will look good for the Scottish balance sheet for that reason.

Taxing offshore hydrocarbons in most years determines whether Scotland's government is in profit or loss. Without it, Scotland's running a fairly consistent 6% deficit.

If oil and gas tax has to fill the gap between other tax and expenditure, it requires some large-scale contingency planning in case it doesn't deliver on forecast.

It also means a Scottish government working with these figures would struggle to put much North Sea revenue aside for a Norway-style trust fund.

In passing, it's worth remembering how big the revenues were in the 1980s and what they could be worth if invested. In 1983-84, but at 2010 prices, the North Sea delivered £35bn to the Exchequer.

Future Generations

However, the SNP government is talking rather less about a Norwegian-style oil fund these days.

The current campaign is to wrest powers over the seabed from the Crown Estate Commission, the unaccountable body that supplies its surplus direct to the Treasury and which, as custodian of Her Britannic Majesty's seabed, holds the keys to the hoped-for boom in offshore renewable energy.

The income from that could be placed in a Fund for Future Generations.

In the most recent accounts, the Crown Estate had revenue of £6.8m from its marine assets in Scotland, much of that fish farm rents, and £6.3m from onshore.

Some £9m of revenue became surplus for the Treasury.

But how much more could it be when offshore wind, wave and tidal turbines are in place and generating power?

The answer is not in the case for the Crown Estate being devolved to Holyrood published by the Scottish government at the same time it was publishing GERS.

But the answer I've been given by St Andrew's House reckons on the Crown Estate rental charge per megawatt/hour generated.

One gigawatt of power was reckoned to be worth around £3m in seabed rent in 2009.

So far, there are lease agreements for offshore wind power up to 2020 with a potential capacity of 10 gigawatts.

Leases are also agreed for wave and tidal energy over this decade with a capacity of 1.6 gigawatts.

If it's all built and reaches it's potential, that could add up to revenue of £35m in 2020-21. A nice nest-egg, but hardly on a Norwegian scale.

Unleashed dynamism

The next use of GERS deployed by nationalists is to highlight the finding that Scotland's deficit, relative to the size of its GDP, is rather smaller than the UK's — and it's been that way for five years.

The deficit ends four years when it could claim to be in surplus, on a modest scale, assuming a large share of oil revenue and not including capital spending.

The other nationalist argument is that these figures paint a picture of a Scotland operating within UK constraints. It is argued that independence, or at least more fiscal powers, could unleash dynamism and a higher growth rate.

Scotland couldn't know if that's true unless and until it gave it a try. An academic paper last year arguing that other countries show fiscal devolution tends to deliver faster growth rates was hotly disputed and ran into a ferocious political spat at Holyrood.

Scots share

But there's another political argument that focuses on those GERS figures — resentment at Scotland's relatively generous allocation of spending power.

GERS helps illuminate the claim that Scotland is subsidised by the rest of the UK, a claim that sets aside questions of revenue and the North Sea.

The report indicates that Scotland has 9.3% of UK expenditure, while it has 8.3% of the UK population. That's clearly a higher level of spending per capita.

Total expenditure per capita in Scotland in 2000-10 was estimated to have been £11,370 in 2009-10. That's £1,050 or 10.2% higher than the UK average.

Social protection, meaning welfare and pensions, came in at £3,880 per head — 8% above the UK average, or £298.

Transport spending was a long way ahead too, with £182 more spending per Scot, or 50% on top of the UK average.

The average Scot had £2,058 spent on health. That's £141 more than the UK average, or a 7% edge.

GERS concludes the only areas where Scottish spending per head came in below the UK average were in employment and in public order, including policing. That was 9% below the UK average, or £51 per head.

Eastenders

There are disagreements over the way shared expenditure is shared out, however. Scotland is allocated its population share of defence, for instance, when it can be argued much more of the economic impact of defence spend is felt south of the Border.

Most contentious, for now, are the benefits to be felt from the London Olympics. The Treasury makes a bold assumption that everyone in Britain benefits equally from the capital spend, and that £92m can therefore be attributed to Scotland.

The Scottish government begs to differ. Its statisticians put the value of the construction works in London's East End during 2009-2010 somewhere closer to a meagre £100,000.