Using my two Garmin devices, I lay down for 30 minutes watching a film.
Both recorded an average heartrate of 60 bpm. The 810 gave 35 kcal, the 910xt 45 kcal
So 70 - 90 kcal per hour : 1680 - 2160 kcal per day. It looks like the BMR is included in the figures, so that I have to subtract 100 kcal an hour or so to estimate my additional expenditure.
Garmin Connect shows the same figure as the device. Syncing the data over to Strava, it turns out that Strava doesn't estimate Calories without a GPS track or power data. So it looks like I'll have to try the next experiment in the garden.
Upate - I've finally gotten round to sitting still in the garden for 10 minutes with the 810 and a heart-rate strap. The result - Garmin Connect shows 16 kcal - my BMR, whereas Strava showed 0 kcal. So it looks like Strava only shows work based on the track data.
Update - I freewheeled down a 4 minute descent with my Vector pedals measuring no power input. The Calories consumed as displayed on the 810 did not go up at all. So it looks like the Garmin is inconsistently not including BMR when it has a source of power data.