Root cellar energy use

I just checked the wattmeter on my outside aboveground root cellar, which is heated with a 25 watt bulb controlled by a thermostat set for 40 degrees.

It has used 11.75 kilowatt hours over 725 hours (30 days), or 16.2 watts on average (55.3 Btu/hour).   During this period the average outside temperature was 21.4 F, so the average temperature difference inside-to-outside was about 20 F.  The heat loss coefficient "UA" of the box is therefore about 55.3/20 = 2.76 Btu/h-F.

If I calculate a U value based on the outside surface area of the box, U=UA/A = 2.76/150 ft2 = 18.4e-3.  The R value is 1/U = 54 in English units (h*ft2*F/Btu.)  This is pretty close to the nominal R value of the SIP panels I made it out of.