Virtually any PC will be more than capable of doing it but with suitable kit
 
Off the shelf not available as yet may be in year or so.
 
you could have two and do some mental arithmetic?

Thanks Tony. Yeah - mental arithmetic is a possibility but the problem is that monitors like the OWL don't go backwards. eg if you are importing 200 watts it will show you are importing 200 watts. However if it's a really sunny day and you are exporting 200 watts the OWL will show that you are importing 200 watts. ie it doesn't differentiate on direction of flow.........

Yea but er but if you know how much you exported ...
 
More mental arithmetic

ALL serious PV traders will be able to provide you with such a meter. These meters are basic standard. On the EU continent there is a feed-in tarif, without these meters it wouldn't exist.
If one of them trys to sell you a 200 Wp installation for grid connection he/she is propably not a serious trader....
Toys don't pay. The costs (meter reading plus total capital costs) will be higher than the energy saved, in monetarian as well as environmental terms. Anything smaller than 1 kW is a figleaf, wilting on close examination and revealing naked ignorance.

The problem you will have is that you are consuming in AC but generating in DC. No-one that I know has bothered mixing the two yet because it is really only offgrid systems that want such accurate monitoring. I'' just going to write up a story on this subject for the summer edition of green building mag so if you get that you will be able to catch more on the subject then.
 
I currently use a trace meter which measures dc into and out of the batteries, that is just about as close as you will get but it does not measure any of the generator technologies independently and can easily fail to monitor (measure) very low inputs that are often common in these technologies.

Probably not a lot of help, but :
Assuming you have a generation meter with your PV inverter, and an import-export meter, you can measure deficit/surplus on a day-by-day basis. I have an import-export meter that only reads to the nearest kWh, so it's not fantastically accurate (but it's fine over weeks or months).
I'm assuming you really want an instantaneous figure - and as you say, the OWL / Current cost style monitors don't know whether you are importing or exporting. All I can suggest is that you use a bit of guesswork - if your owl meter says 200W, you switch on a 2.5 kw kettle and the OWL goes to 2300W then you were exporting; if it goes to 2700 then you were importing.
If you have a rough idea of what your panels generate under different weather conditions, and an idea of what's switched on in your house, you should be able to tell whether the OWL reading measures import or export.
The cumulative figures (last 24 hrs, last 7 days, etc) on the OWL meter are meaningless as far as I can tell.