Not all reduction of consumption decreases output. For example, leaks could be plugged, insulation can be improved, more efficient machines could be used.

Also, not all output is equal, not does all output have to be done at a fixed time. Heating homes has to be done during winter, but some factories can postpone production a few months to wait for more favourable circumstances.

Furthermore, it's only a reduction in a few energy sources that are required, with gas being the most important. Other sources could be used to compensate to an extent. We're seeing some gas power plants moving back to coal for example. Not great for carbon emissions, but it has more and more flexible sources.