It's actually very simple. Think of it like buying petrol when the price is low.
The number one reason people hesitate to join Powershop is they think buying power is "complicated". It's not. Here is the 3-step guide.
Instead of waiting for a bill at the end of the month, you can log in to the app and buy power in chunks (called Powerpacks). You are essentially pre-paying for your electricity at a discounted rate.
Don't worry. If you forget to buy a pack, the lights won't go out! Powershop simply charges you for what you used at the standard rate.
The best strategy is to log in once a month, buy the "Future Pack" for the upcoming month, and then relax. It takes 30 seconds and saves you about 5-10% off the standard rate.