Electrical Efficiency vs Total Efficiency: Direct Answer
Direct Answer: Electrical efficiency is the share of fuel energy a gas generator set converts into electricity at the terminals. Total efficiency adds the recoverable heat from the exhaust and jacket water that the CCHP system can put to use on site. The difference is the energy boundary: electrical efficiency counts one output, total efficiency counts electricity plus usable heat.
For a CCHP plant, the two figures answer different questions. Electrical efficiency tells you how much power the natural gas generator set delivers per unit of fuel. Total efficiency tells you how much of the fuel's energy the site actually captures. Because heat recovery is added on top, total efficiency is normally well above electrical efficiency for the same unit — but only where the recovered heat has a year-round use.
Neither figure is comparable until both are quoted on the same fuel basis (LHV or HHV), load point and ambient conditions. A stated electrical efficiency advantage may simply reflect a different boundary, not a better machine.
The next section explains why the gap between the two figures is so large and which heat streams create it.
Why Total Efficiency Can Be Much Higher Than Electrical Efficiency
Direct Answer: Total efficiency exceeds electrical efficiency because a gas generator set rejects most of its fuel energy as heat, and in a CCHP system a large share of that heat is recovered and used on site. The main recoverable heat streams are exhaust gas, jacket water, lube oil cooler, and, on turbocharged units, the intercooler (charge-air cooler).
Where the heat comes from
A generator set converts fuel energy into shaft work and heat. Only part of the input leaves as electricity at the terminals; the rest must be dissipated. In open-cycle operation that heat is wasted through the radiator and exhaust. In CCHP it becomes a second product.
The recoverable streams differ in quantity, temperature, and how easily they can be used:
- Exhaust gas. The highest-temperature stream, typically leaving the engine at several hundred degrees Celsius. It suits steam generation, absorption chillers, and process drying. It may also carry the energy needed for exhaust after-treatment, so usable heat is not always the full exhaust enthalpy.
- Jacket water. Engine cooling circuits deliver heat at roughly 80–95 °C, well matched to space heating, domestic hot water, and low-temperature process loads.
- Lube oil cooler. A smaller stream at moderate temperature, often combined with jacket water in a common recovery loop.
- Intercooler (charge-air cooler). Present on turbocharged and aftercooled engines; the heat available depends on cooling architecture and ambient conditions.
What this means for procurement
Total efficiency is high because it counts electricity plus these recovered streams. It is not a measure of how well the engine converts fuel to power — a unit with modest electrical efficiency can post a high total figure if more heat is captured.
Two cautions follow. First, recovered heat only counts if the site can use it year-round; surplus heat in summer reduces the real benefit. Second, low-temperature streams are harder to use than exhaust heat, so the mix of temperatures matters as much as the total quantity. Ask suppliers to state each stream separately and at what temperature, not as a single aggregated percentage.
Next, we look at how to compare efficiency figures from different suppliers on a consistent basis.
How to Compare Efficiency Figures from Different Suppliers
Direct Answer: Never compare quoted efficiency numbers until all figures share the same fuel basis and reference conditions. Build a normalization checklist covering LHV/HHV basis, load point, ambient conditions, and parasitic loads — and require each supplier to confirm every item in writing.
Fuel basis: LHV or HHV? Ask whether the efficiency is calculated on fuel lower heating value (LHV) or higher heating value (HHV). HHV includes the latent heat of water vapour in the combustion products, so an efficiency quoted on HHV will always look lower than the same machine quoted on LHV. The gap is material for natural gas, so it is never safe to assume. Confirm also that electrical efficiency and total efficiency use the same basis — a common quoting error is an LHV electrical figure paired with an HHV total figure, which flatters the total efficiency. Require the fuel composition and calorific value assumed in the calculation.
Load point and ambient conditions. An efficiency figure without a stated load point is unusable. Ask for the guaranteed value at 100%, 75% and 50% load (and the part-load curve), plus the ambient temperature, altitude and humidity assumed. A supplier quoting a single efficiency point is not telling you how the machine behaves at your site's typical load.
Parasitic loads. Confirm whether pumps, fans, cooling tower and exhaust treatment are deducted from the net electrical output — and use net output as defined in section 2, without re-listing the auxiliaries here. HHV/LHV alone can swing a comparison by several points; parasitics can do the same.
Request in writing: fuel basis and calorific value; load point; ambient assumptions; net output definition; the test procedure behind each figure. Illustrative example: two bids may differ by a few points purely because one quotes LHV gross and the other HHV net — normalizing both usually closes the gap.
With the checklist applied, the next step is agreeing which parameters become contractual guarantees.
Key Parameters to Confirm Before Accepting an Efficiency Claim
Direct Answer: An efficiency guarantee is only enforceable when it is tied to defined operating conditions. Before accepting any figure, require the supplier to confirm turn-down capability, the part-load efficiency curve, fuel quality limits and explicit derating factors for ambient temperature, altitude and fuel variation — in writing, as part of the contract.
Part-load performance. Gas generator sets rarely run at 100% load continuously. Ask for the efficiency curve across the expected load range, not a single point. Turn-down capability defines how low the unit can run while remaining stable and compliant.
Derating factors. Request derating tables or curves for high ambient temperature, altitude and fuel quality variation. Illustrative example: a unit rated at a temperate site may need a lower output rating at high ambient temperature and altitude, so the quoted efficiency may not apply at your site. Confirm whether published figures already include these derating factors or are reference-condition values.
Fuel quality. Specify expected fuel composition ranges — methane content, inert gases, heavy hydrocarbons, contaminants — and require the supplier to confirm the acceptable envelope and any pre-treatment requirements.
Next, we examine how electrical and total efficiency trade off during CCHP sizing.
The Trade-off Between Electrical and Total Efficiency in CCHP Sizing
Direct Answer: The better choice between a higher-electrical-efficiency unit and a higher-total-efficiency CCHP configuration is decided by two site-specific numbers: the amount of recoverable heat you can actually use year-round, and the hours per year the plant will run at each load point. Neither efficiency figure settles the question on its own.
For CCHP sizing, start with the heat side. Build a 12-month heat demand profile (process heat, absorption cooling, district heating) at hourly or at least monthly resolution, then match it against the recoverable heat available at the expected operating profile. The overlap between the two curves gives annual useful heat, and that — not a nameplate total efficiency — is the quantity that drives the trade-off.
Illustrative example: Suppose a site needs electricity continuously but has process heat demand only in winter, with an absorption chiller running in summer. A configuration with modest electrical efficiency and generous heat recovery may deliver more useful energy annually than a high-electrical-efficiency unit whose heat is dumped for several months. The reverse can hold for a site with flat, year-round heat demand. These figures are illustrative only; actual results depend on the site's load profile and the supplier's guaranteed performance.
To compare total cost of ownership, request from each supplier an energy balance at the site's expected load profile, a part-load curve, and a written heat-output map at usable temperatures, so both configurations are priced on delivered energy rather than quoted efficiency. Confirm the guarantee wording and boundary conditions as set out in the earlier sections on comparing supplier figures and accepting efficiency claims. Tie any efficiency guarantee to defined reference conditions and a named test procedure so the number is enforceable. Only then can fuel consumption, operating hours, and maintenance exposure be compared on a like-for-like basis.
The verification route for those guaranteed numbers — factory acceptance testing and long-term performance checks — is covered next.
Verifying Efficiency Claims: Factory Acceptance Test (FAT) and Long-Term Performance
Direct Answer: Make the efficiency guarantee enforceable by tying it to a defined factory acceptance test (FAT) protocol — specify instrumentation accuracy, load points, stabilization time, fuel analysis and a heat balance check, with penalties for shortfall. For long-term performance, ask for the major overhaul interval under your duty cycle and for the full remote monitoring and predictive maintenance data-point list.
What to specify in the FAT
A FAT is only useful for efficiency verification if the protocol is agreed before the test, not after. Require a written procedure covering:
- Instrumentation and calibration: fuel flow meter, electrical power analyzer, temperature and flow sensors at each heat stream, with calibration certificates and stated accuracy.
- Load bank testing: load steps at 100%, 75% and 50% (plus your expected operating point), with stabilization time and readings averaged over a defined period.
- Fuel data: a fuel sample analysis taken at the test, so efficiency is calculated on the actual LHV and composition rather than a default figure.
- Heat balance verification: measured recoverable heat from exhaust, jacket water, oil cooler and intercooler, reconciled against the electrical output and fuel input within a stated tolerance.
- Acceptance criteria and remedies: guaranteed values, tolerance bands, and the consequence of a shortfall — re-test, correction, or price adjustment.
Maintenance intervals
Ask for the specified generator major overhaul interval in operating hours and years, and how it changes under continuous base-load versus cyclic or peak-shaving duty. Cyclic operation adds thermal cycling and start-stop wear; request the maintenance schedule for your specific annual hours and load profile, not a generic figure.
Remote monitoring and predictive maintenance
Request the sensor and data-point list: exhaust and jacket water temperatures, oil pressure and condition, vibration, fuel consumption, load and power quality. Confirm how data is transmitted, who owns it, and what alarms trigger service action. Monitoring supports predictive maintenance only if the alerts are mapped to defined interventions.
For natural-gas configurations, review the XE516-N and XE512-N specification pages as a starting point for the data you require.
Key Takeaways and Next Steps
The core of a fair CCHP comparison is normalization: same fuel basis (LHV or HHV), same load point, same ambient conditions, and net rather than gross output. Electrical efficiency alone does not decide the outcome; the useful energy actually delivered to the site does. Before accepting any figure, tie the guarantee to defined boundary conditions and a FAT procedure that includes load bank testing and heat balance verification.
Next steps for your evaluation:
- Build a normalized comparison sheet for every offer received.
- Request part-load curves, derating factors and heat recovery temperatures in writing.
- Confirm the FAT acceptance criteria and efficiency tolerance.
- Compare configurations on annual useful energy and total cost of ownership, not headline efficiency.
To move forward, send your site heat demand profile, expected operating hours, fuel specification and ambient conditions to the supplier and request a detailed efficiency comparison on a common basis. Models such as the XE516-N natural gas generator set and XE512-N natural gas generator set can serve as a starting point for that discussion.
Frequently Asked Questions (FAQ)
What should I request in a FAT to verify efficiency?
Ask for a witnessed load bank test at the guaranteed load points, with fuel flow, gross electrical output and heat recovery measured simultaneously. Specify the fuel analysis used (composition and heating value), the reference ambient conditions, and the correction method for site conditions. The heat balance should record exhaust and jacket water temperatures and flows so total efficiency can be recomputed, not just read from a datasheet. Acceptance tolerance and a re-test path should be written into the contract before the test. See our XE516-N natural gas generator set page for the configuration data typically requested at this stage.
How do part-load operation and fuel quality affect efficiency?
Both efficiencies fall as load moves away from the design point, so request the part-load curve rather than a single full-load figure. Ask how guarantees are adjusted when fuel deviates from the test gas.
What warranty terms are typical for efficiency guarantees?
Guarantees are usually tied to defined boundary conditions — fuel basis, load point, ambient conditions, and net or gross output. Confirm the measurement method, the party performing the test, exclusions for fuel-quality variation, and the remedy if results fall short. Frame these as items to negotiate; terms vary by supplier and contract.
Illustrative example: a guarantee stated at ISO conditions without a correction curve is difficult to enforce on a hot, high-altitude site.
Request our normalized comparison sheet and FAT procedure template when issuing an enquiry.
