Backup Generators (Diesel & Natural Gas)

While a generator motor can respond quickly, loads often change faster than the ability of a generator to follow. This is why we ask for load interval data when sizing your backup generator – We need to get a handle on how quickly your loads change. Slow-to-moderate changes can be managed by almost all generators. Fast changing loads might require a rich burn natural gas or diesel unit. Very fast changing loads can require the addition of batteries to compensate for the generator lag. 

Fuel choice is a function of:

• Fuel availability;
• How you use a generator; and
• Your budget.

If natural gas is available at your site, or inexpensive to bring to your site, this is typically the best fuel to use. The equipment is a bit more expensive, but you save money on maintenance and fuel costs, and pipeline gas eliminates concerns about refueling during an extended outage. Natural gas generators certified for non-emergency use can participate in grid services, such as demand response, allowing you to earn substantial profits. 

If natural gas is not available, or you have significant budget constraints, your best choice would be a Tier-2 or Tier-3 diesel generator. These units can be used only during emergency situations, and they are not allowed to participate in grid services. Tier-4 diesel generators can be used in non-emergency applications and grid services; however their additional emission controls make the cost of these units comparable to natural gas units. Considering the cost of diesel fuel, participating in grid services is typically not economically justified.

One of the most confusing aspects of generators is their power rating. The 4 ratings are:

1. Standby
2. Demand Response
3. Prime
4. Continuous Duty

These designations tell you how much power they can generate, and for how long.

Standby units typically provide max output for only a few hours at a time, and they typically can operate less than 200 hours per year. 

Demand Response units provide slightly less output than the standby rating (15%) to compensate for greater run hours per year. A vitally important item is the wording on the engine nameplate: “Non-Emergency Rated” must be printed on the nameplate to be permissible for grid services. If you plan on participating in grid services or peak demand management, you must specify Non-Emergency Certified or Demand Response Certified, and it must be listed on the nameplate. In general, manufacturers will nameplate the unit as standby, even if it can meet the non-emergency requirements. This is an administrative cost saving measure. Many customers who later decide to participate in demand response are unable to because the nameplate is labeled as standby only. 

Prime Power units also typically provide 15% to 20% lower output than standby rated units, which indicates that the unit can run more hours during the year than a standby-only system. The difference between prime and demand response is often subtle and not clear: In general, a prime unit will be warranted to run longer than a demand response unit.  It is vitally important to note that prime power does not mean it can operate continuously for very long periods of time – a prime unit is not capable of operating 24x7x365.

Continuous Duty units are heavy duty systems designed to operate uninterrupted 24x7x365.