$4k less 30% for tax credit. so $2.8K for extra 25% production capacity. Not sure what your total cost is at this point--most likely above $16K, so cost/kwh should be averaged down. Trust me, you'll find a way to use up the extra capacity plus you'll love the fact that you really don't need to worry about turning off your lights
Additionally leaves you optionality for EV in the future.