Until recently, you’ve had no choice but to be a passive participant in the electrical grid, paying exorbitantly for the juice that flows into your home. But you’re about to play a much more active role — if you’re not already — and get paid to send power from your home battery or electric vehicle to the energy system. Even if you’re a renter, you may be able to buy solar panels that plug into a wall outlet, turning your balcony into your own tiny power plant .

Earlier this month, a punishing California heat wave showed just how this more flexible grid can help absorb spikes in demand.…