Most businesses that own assets or hold inventory routinely value those assets and inventory, the change in value between two points in time is the profit or loss
Some assets, like computers, simply depreciate, and a simple depreciation percentage is used each year
Other assets, like buildings, vary in value with the market conditions, and are generally valued using a mark to market principle (that is we simply look and see what the building is worth at the end of the accounting period)
It's important to realize that each trade is an asset (or a liability) - it's a firm contract and must be valued like any other asset
The current value of a single trade is the difference between the price paid, and the value of the deliveredcommodity when it is delivered (which may be a cargo, a day of gas delivery, or a half hour of powerdelivery)
We can find the value of the commodity once it has delivered by looking up the spot trading price on the day of delivery
But to value the deal before delivery we must mark the value of the deliveredcommodity to the market; that is we set the value of the commodity to the price that is currently being paid for the delivery period
For example:
We have bought 10,000 therms of gas for delivery in March next year at 25p per therm
Each day we can look at the average traded price for that month, and mark the value of the 10,000 therms to that price
After two days we note that March is trading at 27p a therm, so we mark the physical value to the market price of 27p, and subtract the price we will pay, 25p. The unrealizedP&L is therefore £200 ((27p - 25p) * 10,000). Each day we will need to repeat this calculation until the delivery is complete