There's a question that comes up almost as often as "how much will this save me?"—and it's "how big a battery do I actually need?" Get it right and you're using far more of your own solar power, cutting your evening grid costs and getting genuine backup when the power goes out. Get it wrong and you're either paying for storage capacity that sits half-empty most nights, or watching your battery run dry by 9pm and buying electricity back at peak rates anyway. There's no single