1Hz will be fine. You'll lose no more in the off period with 1Hz than you will do a 1kHz.
PWM systems at low freq are only less efficient with highly inductive loads. There is a trade-off then between inductive losses (at low freq) and switching losses (at high freq).
But for a mostly resistive load (there will be a small amount of inductance) like yours higher freq's are actually slighlty more in-efficient due to switching losses.
1Hz will be fine!
