First off, the story you want is A Christmas Carol. A Christmas Story is the one where Ralphie wants a BB gun for Christmas, but everyone tells him "You'll shoot your eye out!"

That being said, I'm not sure how much historical information can be used to answer this question. Spending Power, by which I assume you mean Purchasing Power, is an economic measure of how much goods and services you can buy. You can calculate it, which means the question you've asked is a math question, not a history one.

The answer to your question that is the easiest to understand is to convert 15 shillings in 1843 money (the story takes place in 1843, not 1840) to 2015 money (the most recent year for which full financial data is available). A simple way to do this is to use the either the Consumer Price Index (CPI) or Retail Price Index (RPI), both of which are tools used to measure inflation, specifically measuring the change in costs of specific goods and services from one year to the next. There are plenty of sources for these. If you live in the UK, here is the Office for National Statistics' page for it. If you live in the US, here it is for the Bureau of Labor Statistics.

As for calculating the amount, you can find a calculator for CPI or RPI online very easily, and since they're all going to work off the same data (historical data you can find on the above sites), they should all give you the same answers. The problem is that a lot of them don't go back far enough to convert 1843 money. I found one that did, and your answer is here.

15 Shillings per week in 1843, according this calculator (which uses the RPI), will be £66.40 in 2015. And if you live in the US, the IRS says the average exchange rate from US Dollars to British Pounds in 2015 was .681, which turns £66.40 into $97.50. That site gives alternative methods to calculate purchasing power, but this is going to be the generally accepted answer.

So, imagine being paid £66.40 or $97.50 weekly, or $2.44/hour last year. That's Bob Cratchit's Purchasing Power.

This will be off by about 40 years, but "Life on a Guinea a Week," which appeared in The Nineteenth Century in March of 1888, offers some insight into what a clerk, earning 50 guineas, (i.e. £52/10/0) - might have in the way of expenditures.

The weekly expenditures listed are as such:

Now, to be fair, the article in question seems to be concerned with proving that it is possible to subsist on one guinea "... and yet life not be a burden and perpetual misery to an individual." so, remember that. It's also supposed to be about a single man, not a family. Feel free to read the article linked above yourself. Since it was published in 1888 it's free to read on google scholar and it is quite short.

Roberts, W. "Life on a Guinea a Week" In The Nineteenth Century, 464-67. Vol. 23. Henry S. King & Company, 1888.

Adding to this: was Bob Cratchit's income supposed to be realistic, or was it exaggerated for literary effect?