With a 1:46 step up toroid, I am measuring the DC volts rectified across a 470 ohm secondary load resistor using a small 50uA meter in series with 240k ohms (120k twice) and a series silicon diode. This circuit is straight out of the "LF Today" book available from the RSGB bookshop.
DC current = 30uA
DC voltage across 470ohm secondary load = 0.6 + (240 x 10E3 x 30 x 10E-6) = 7.8V
DC voltage (rms) across 470 ohm = 7.8/1.414 = 5.52Vrms
I secondary = 5.52 * 10E3/470 = 11.7mA
I primary = 46 * Is = 46 * 11.7mA = 0.54A
Rrad = 160 * pi^^2 * Heff^^2/lambda^^2 = 160 * 3.14 * 3.14 * 2 *2/(630 * 630) =0.0159 ohms
P(erp) = I^^2 * R = 0.54* 0.54 * 0.0159 W = 4.6mW
My original guesstimate of 5mW ERP is not far out!
Clearly getting the antenna much higher would help enormously as ERP is proportional to Heff^^2. All one can really be sure of is the order of magnitude. I am running low milliwatts.