Three ways to run
A system that makes its own power from sun and wind can run in one of three ways. It can be fully off grid, it can be a grid hybrid that leans on the utility when needed, or it can carry a fuel generator as backup. The right choice depends on the site, the load and how much an outage would cost.
Fully off grid
Panels, and a turbine where the site is windy, charge a battery that carries the load through the night and through dull spells. Nothing connects to a utility. This is the simplest arrangement to permit and install, with no trench, no meter and no energy bill.
The trade-off is that the generation and storage must be large enough for the worst week of the year on their own. It suits lighting, cameras, sensors and communications where no grid is nearby, where trenching would be cost prohibitive, or on temporary sites that move.
Grid hybrid
Sun and wind supply the load first, and the grid tops up the batteries or takes over when they run low. Because the grid covers the hardest weeks, the array and the battery can be smaller than in a fully off-grid design.
It needs a grid connection, and with it the utility's approvals and any costs to connect. It makes sense where a supply exists but is unreliable, or where a site wants clean energy and lower bills while keeping the grid as a safety net.
With generator backup
A fuel generator, running on diesel, propane or natural gas, starts automatically when the batteries run low and sun and wind cannot keep up. It runs only as long as it needs to recharge, then shuts down. Most of the energy still comes from sun and wind.
This suits critical loads that cannot be allowed to stop, and remote sites with no grid at all. The generator also lets the array and battery be sized for normal conditions rather than the rare extreme. In return, it brings fuel deliveries, regular maintenance, noise and exhaust.
How to choose
Start with two questions: is there a grid you can reach at a sensible cost, and what happens if the load stops? The answers usually point to one option, and the sizing then follows the same steps for all three.
- No grid nearby, or trenching is cost prohibitive: fully off grid
- Grid available but unreliable: grid hybrid
- Critical load on a remote site: generator backup
- Temporary or moving site: fully off grid, with a generator if the load is critical