In the world of electricity, there is a thing called "demand response." This is not a pushy bill-collection strategy. It's a critical component of the electric grid of the future, and it's an area where we should expect steady growth over the coming years. Long-term investors should get in now before everyone else does. We'll take a look at four stocks that could interest you.

I've been running a series of articles related to the investment opportunities in smart grid development. If you're not fully up to speed on the basics, start with our smart grid primer. Then mosey on back here.

Demand response defined
Demand response is a mechanism in electric grids that allows customers to reduce their consumption at critical times or in response to market prices. In practical terms, it allows you to, say, set your dishwasher to run only when power is cheaper, or dim lights and adjust your thermostat if brownouts are threatened.

Demand response helps to reduce peak load pressures on the grid. The electric grid is designed to accommodate a certain base demand for electricity, but it can be challenged to respond to significant demand spikes. Predictable spikes occur every afternoon and evening. Less predictable spikes happen when the temperature soars and everyone cranks the air conditioning, fires up the blender, and camps out in front of the Wii with a pina colada until it's all over.

On the flip side, demand response can use periods of low electricity demand to, for instance, drop the temperature in your freezer by a few degrees, allowing the temperature to rise back up to freezing again during peak load while using less energy to sustain that temperature. Studies find that demand response produces a "conservation effect" among customers that lowers overall electricity consumption anywhere from 3% to 7%.

Demand response enables greater renewable energy adoption. Many renewables are variable, meaning that electricity is not generated at a constant rate. Think about solar at night, or wind on a still day. Demand response's feature of preserving peak load power for later use is the perfect complement to renewable energy, in that it can smooth the peaks and valleys of generation loads.

Show me the players
EnerNOC (Nasdaq: ENOC) is a key player in the demand response arena. The company managed more than 7.1 gigawatts of electricity at the end of 2011, more than twice what it managed at the end of 2009. To put this in context, one gigawatt can power anywhere from 750,000 to 1 million homes. EnerNOC's share price has suffered mightily in the last year; as of this writing it was trading at $6.18, down from a 52-week high of $18.75. Considering its price-to-book of 0.8, and my belief that EnerNOC's products will continue to meet a long-term need, I think this one's a steal.

CPower -- a part of Constellation Energy, which is in turn a part of Exelon (NYSE: EXC) -- provides demand response programs to its primarily commercial clients. For end users, such programs can be an important element of their overall energy management strategy. CPower has steadily added to its gigawatts under management over recent years. Exelon's stock is also near its 52-week low, but while this company may be down, it's not out. My colleague Rex Moore thinks there are reasons to consider buying. Of course, given that CPower is just a small part of Exelon's business, you'll want to research the rest of the company before you settle on your opinion.

Honeywell (NYSE: HON) has thrown its considerable weight behind the Open Automated Demand Response solution, or OpenADR, which aims to automate the demand response process. Rather than sending a message to a building manager to turn up the thermostat, this approach uses a set of standards to make automatic adjustments. Honeywell's stock is not nearly as cheap as the others mentioned here, but if you're looking for a blue chip dividend payer in this space, it may warrant your consideration.

Maxwell Technologies (Nasdaq: MXWL) develops and produces advanced energy storage and power delivery products, which contribute to various links in the smart-grid value chain. Regarding demand response, it provides components of smart reader systems that help to moderate energy demand. Basically, Maxwell makes parts for the sensors that would go in your house to monitor your electricity use and provide feedback. Volatile little Maxwell's stock price has been all over the place in the last year, and it is currently trading at near-record lows. I think it's down but not out. Dive in, do the research, and see if you agree.

Demand response is not going away. Indeed, as we continue to try to wring every last watt out of an increasingly overworked power grid, this technology will be an important tool in electricity management for years to come. Investors interested in playing this space should get in now while no one is paying attention, then sit back and let that investment play out over the long term.

Speaking of the long term, you'd probably rather retire rich than poor. You really ought to check out our special free report "3 Stocks That Will Help You Retire Rich."