Whether you use an mppt-type controller or not, when you tie to an array directly you have to match the array's "equivalent resistance" (working voltage / working amps) to the heating element's resistance. E.g., a common 4500 watt 240v element has 12.8 ohms resistance, so for that element you'd set up your array to provide [working volts/amps] = as close to 12.8 ohms as possible. This is what David Poz eventually discovered. (And series or parallel is not really relevant, except that generally you do want to get voltage as high as possible while balancing the equation, so to speak.) But, even when you do this balance, without mppt the performance will become terrible when the light decreases. This is because the [volts/amps=equivalent resistance] value of the array changes. Anyway, my controller gives me 800-950 watts in good sunlight, from a 960 watt array, and 150-300w in overcast conditions.