How to Estimate Power Output Capacity of a Microwave Oven Transformer (MOT) Core?

Hi, The microwave oven transformer (MOT ) is often reused by hackers and DIYers in a number or roles. Google is awash with videos, but I think some of the posters have a poor understanding of transformer theory. The advantage of using the transformer core is that it easily splits open and allows you to fit your own secondary. It's also free, whereas transformers that size are often quite pricey for a hobbyist. But it isn't that simple IMO. Something doesn't add up. A core that size would be rated around 200VA ( give or take), but the oven manufacturers claim an output of 800W. I do have several questions about this device, but first I would like to know if there is a 'rule of thumb' which tells how much power can be drawn from a core of any particular size?

There's a site here that gives wattage rating vs core size, but it only goes up to 300W,  Can you measure your core (the middle part, not the outside) and advise the dimensions?
http://www.smoke.com.au/~ic/ham/transformers/used_transformers.html

Most microwave transformers I have seen are fairly beefy nut I've never compared them to a table like this.

Hello David and thanks for that. I rather think the guy has made a serious mistake, though: he takes the flux  'area' as the width of the middle section ( correct ) multiplied by the length ( wrong ). It's the cross-sectional area of that central 'tongue' you want, IMO. If you put the numbers of a nominal 800W microwave transformer into that formula you get 250W which is roughly what I reckoned it should be.I think I now understand what is going on: the 'power output' is defined as Volts X Amps, but theres no definition of the VA rating. I think what happens is that the core heats up because of losses. The more you try to pull, the hotter it gets. Eventually the thing self-destructs. The quoted VA rating is just a guide-line to keep well within safe limits.Now, a manufacturer of ovens makes his core as small as possible to save on metal costs.  The core could comfortably handle 250W, but they try to pull three times that amount. The shunts help prevent the thing from saturating. They gamble that the oven only operates on full power for short periods ( say 5-10 mins ).  And they use a fan to force-cool it. I'd bet that if you tried to pull 800W from a microwave oven continuously, without the fan, it would melt the insulation pretty quick. The downside is that, because the manufacturer is being a cheap-skate with his cores, the end-user has to pay for the electricity to cool it. Bloody cheek. Does that sound correct?