How is qty on hand calculated?

I'm writing an application to speed up the product import procedure, but I'm stuck on the qty on hand part. It appears this is a calculated field (?) and there is no way to insert it directly into a table. Does anybody have any info/suggestions? I've got the import process to about 14 records per second, vs. the OpenERP import which takes almost a full second per record, so I'd rather not use the OpenERP import. TIA!