If a laptop costs 1000 dollars to buy, it couldn't have used more than 1000 dollars worth of hydrocarbons to create, unless firms are operating at a loss, right? Yes, the laptop required mining lithium, mining steel, turning hydrocarbons into plastics, growing silicon crystals, photolithography for the chips, running the conveyor belts for the assembly lines, etc and all those things required electricity and the electricity was mostly provided by fossil fuels, but the total amount of fossil fuels used (when considering price) couldn't have exceeded the cost of the laptop, because that would mean that some firms are spending more on fuel than they're receiving in profit, and such firms in the general case don't survive long. So if you take the cost of the laptop and then convert that to the mix of hydrocarbons used for energy at the time of the product's manufacture, that gives you an absolute upper bound of how much embodied carbon that thing must represent. It also gives you a lower bound of how efficient something has to be before you've paid for the old thing being thrown out and the new thing being manufactured.
So consider this: you have a desktop from 2010, it cost 1000 dollars, and operated at 150 watts. You consider getting a laptop today for 500 dollars, and it would have twice the nominal performance and operates at 50 watts. The total amount of embodied carbon for both of those devices has to be less than $1,500 worth of carbon dioxide produced by hydrocarbons. It can't be higher than that. Then you consider the running cost of 150 watts per hour vs 50 watts per hour. Well, back in 2010, 1000 (2010) dollars could buy you about 6000 to 9000 kilowatt hours worth of electricity when adjusting for conversion rates and electricity cost in China. Today, 500 dollars can buy about 3500 to 6500 kilowatt hours depending on whether you're buying in the US or China. So in order for the embodied carbon to be paid off for the laptop vs the desktop, let's take 7500 kilowatts for the desktop (a fair midpoint) plus 5000 kilowatts for the laptop, and then divide that by the running difference in power of the two systems: 100 watts. So if you plan on operating the laptop continuously for 13 years, the carbon savings from the efficiency gain of the new device would offset any possible carbon generated from the old device. But the laptop is twice as powerful, and what I gave was an absolute upper bound, and cannot be taken as a good ballpark estimate for how much carbon was actually produced. In the example that I gave, there was a 15-year age difference between the old system and the new system. Depending on how the devices were used, it's reasonable to assume that the right time to have replaced the desktop was back in 2023. Depending on how you use your device, it may never end up paying itself off before using less carbon than the older device. Waste is possible. But if the new device is on longer than 125,000 hours, it will have. It's just a sanity check, but it's good to have an upper bound.