There is a point in this category where a machine stops being an air fryer with extra functions and starts being an oven that happens to air-fry. The DT251 is well past it, and the review has to be written accordingly: judged as an air fryer it is enormous and overkill, and judged as a countertop oven it is excellent.
Ninja publishes the numbers that matter. It is an 1,800-watt machine with ten functions — air fry, air roast, bake, whole roast, broil, toast, bagel, dehydrate, reheat and pizza — built around what Ninja calls True Surround Convection, which it describes as delivering up to ten times the convection power of a traditional convection oven. The cavity takes two levels with no rotating, and Ninja gives three concrete capacity figures rather than a vague claim: a five-pound chicken plus a sheet pan of vegetables, two 12-inch pizzas, or a 12-pound turkey.
It also carries the integrated Foodi Smart Thermometer, which is the feature we consistently rate above every other “smart” addition in this category, because it changes the food rather than the experience.
What “replaces” actually means here
The only way a machine this size earns its counter is by removing another one. The DT251 does that convincingly: toast and bagel functions make a toaster redundant, bake and broil across two racks make a toaster oven redundant, and air roast plus the thermometer take most weeknight protein off your full-size oven entirely. If it is replacing two appliances, the footprint is not a cost — it is a net saving.
If it is not replacing anything, it is the most expensive way to make chips on this site. That is the whole decision, and it is the same test we apply to the Breville Smart Oven Air Fryer and the Cuisinart TOA-70. The category is ranked in best toaster ovens.
1,800 watts: the cost, and the circuit
Published wattage makes this reproducible. At 1,800 watts a 20-minute cook is 1.8 × 0.333 = 0.6 kWh, about ten cents at the US average residential rate — a little more than a basket machine because there is a larger cavity to bring up to temperature, and still a fraction of what a full-size oven draws for the same job. Your own rate goes into what an air fryer costs to run.
The electrical note is the more important one. At 120 volts, 1,800 watts is 15 amps — the entire nominal capacity of a standard 15-amp circuit. Run this alongside a kettle or a microwave on the same circuit and you will find the breaker; run it through an extension cord and you have a worse problem than a tripped breaker. The full explanation, with the codes and the manufacturer guidance, is in can you plug an air fryer into an extension cord.
Two-level cooking without rotating
This is the specification that separates a good countertop oven from a mediocre one. Most countertop ovens have a hot spot and a cool spot, so anything on a second rack needs turning halfway through. Ninja’s True Surround Convection is its answer to that, and the practical claim — two levels, no rotating — is the one worth buying on. It is what makes a sheet pan of vegetables plus a chicken a single operation rather than a choreography problem.
Where the format still loses to a basket is raw crisping intensity. Food on a rack in a large cavity sits in gentler airflow than food in a drawer directly above a fan, so chips brown less aggressively at the same setting. That trade is the whole of basket vs oven-style air fryers, and it is why we would not recommend this machine to someone whose main use is frozen food.
The thermometer, again
We keep coming back to this feature because it is the only smart addition in the category that improves the food. Cooking a chicken to 165°F measured is categorically better than cooking it for 22 minutes and hoping — and on a machine this size, where you are roasting whole birds and large cuts, the difference is bigger than it is on a 4-quart basket. Our own cooking-times guide is an approximation of what this probe measures directly.
Who should skip it
Anyone with a toaster oven they are keeping. Anyone whose counter is genuinely tight — the clearance requirements alone are worth reading before you buy, in air fryer clearance and placement. And anyone whose real need is a fast weeknight chip machine, which is a 5-quart basket from the best air fryers roundup at a third of the price and a third of the space.
