Assuming the heater is an "electrical resistance" type (vs. a heat pump or electronic ignition for some other fuel), a 3kW heater will put out about 10,240 BTUs whether its a residential model, commercial or industrial model. While the industrial unit may have a larger fan that dissipates the heat more, to actual heat should be the same. Why the difference? Let's consider a 1500 watt blow driers, for drying one's hair.
Here in Alaska you can buy a cheapie for about $10. That unit will be very noisy and likely give off a wee bit of burning smell as it is used. It has cheap, poorly made bushings and a cheaply made fan for cooling the motor and blowing the air. It's a disposable unit and if it lasts you two years using it once a day (100 times), you replace it.
The barber / hair stylist cuts perhaps 15 heads of hair a day. At that rate the cheap dryer would last about a week, so they buy a commercial model that sells for perhaps $100 or so. This fan likely has well made, sealed bushings and a reasonably-made fan. It also likely carries enough air that you don't get the burning smell. It lasts perhaps 1-2 years being used 15 times per day.
Then there's the industrial heater. It might still be 1,500 watts but is used 24 hours per day, 36 days a year. When it fails the assembly line its in must be shut down until it is replaced. This heater likely has sealed ball bearings and moves enough air to ensure no internal burning. It is likely thermally insulated so the outside of the heater doesn't get hot enough to burn a worker. It likely has a timer circuit that lets the fan run for a bit after the heater is turned off, to cool the insides. This unit is likely designed to be able to go 12 months or more without maintenance (or at least minimal maintenance) and may last 2 - 5 years, and cost $500 - $1,000.
In your home heater, what you should expect to be buying is quality of components and life expectancy. What you actually get will depend upon the vendor. What's right for you depends upon your needs, wants and of course, willingness to pay.
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