I have a laptop which I use 24/7 for crunching with 100 % CPU utilisation. Electricity consumption is not really a concern with around 1KW per day and I don't feel the heat impact in the summer either with a 40W system.
Current hardware is meant to run 24/7 so it shouldn't be an issue to crunch non-stop. What is critical is heat management (especially for laptops). To achieve maximum output but also maximum lifespan, I'm removing dust from my system every three to six months and let it run non-stop again.
Another option to consider is upgrading your CPU/GPU/Casing coolers, which will help as well to improve the lifespan of you system.
Just my 2 cents...
Laptops are not very good at CPU crunching, as the frequency gets down with the temperature increase. My experience with a i5 3.2GHz CPU is disappointing, as the CPU does not go higher than 1.4 GHz
I agree that heat management in laptops is difficult where dust is reducing the performance very quickly, this is why I remove dust every 3 to 6 months.
Another element is the thermal compound paste. My experience with some brands is that they use a low-quality paste. I once changed the CPU paste from a Lenovo T520 where the CPU (i5-2520m) was running at 80 degrees with the fan at max speed. After applying the new paste, the laptop was running at 60 degrees with the CPU at 100% load and no throttling.