How to Size a Cogeneration CHP System: Screening Electric Capacity from Thermal and Electrical Loads
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HVAC Design April 26, 2026 12 min read

How to Size a Cogeneration CHP System: Screening Electric Capacity from Thermal and Electrical Loads

Problem Framing

Sizing a combined heat and power (CHP) system from electrical demand alone is the most common and costly mistake in preliminary cogeneration design. A 900 kW electrical load does not justify a 900 kW CHP unit unless the facility can productively use roughly 800-1000 kWth of recovered heat (assuming a typical reciprocating engine heat-to-power ratio of 0.9-1.1 per EPA CHP Catalog of Technologies 2017 Table 2-1). When the usable thermal demand is lower, the CHP unit operates at partial load, wastes heat, and erodes the economic case. CHP installations sized at 1.2 MW based on peak electric load with only seasonal thermal demand commonly run at 50-65% capacity factor during mild weather periods, extending simple payback periods from a target 5-7 years to 10+ years per EPA CHP Partnership feasibility analysis methodology.

A proper screening calculation must compare the electric demand against the thermal-limited electric capacity, which is the electric output that can be sustained while fully utilizing the available thermal load. This is the core of the CHP sizing screening model. For thermal load profiling methodology used in determining usable thermal demand, see How to Calculate Cooling Load for HVAC Sizing (cooling load profiling provides a parallel approach for absorption-cooling-fed CHP applications) and How to Calculate Boiler Efficiency for heating-side load validation against existing boiler plant capacity.

Exact Formula / Method

The screening model uses a single equation to select the smaller of two constraints:

Thermal-Limited Electric Capacity (kW) = Usable Thermal Demand (kWth) / Heat-to-Power Ratio (kWth/kWe)
CHP Size (kW) = min(Electrical Demand (kW), Thermal-Limited Electric Capacity (kW))

Where:
- Electrical Demand (kW): Facility electric load. Typical range: 100–50,000 kW for commercial/industrial. This is the maximum electric output the CHP unit could supply if thermal were unlimited.
- Usable Thermal Demand (kWth or MMBtu/h): The portion of the facility's thermal load that can be recovered and used year-round. In metric: kWth. In imperial: MMBtu/h (converted to kWth by multiplying by 293.071). Typical range: 50–100,000 kWth. This is the constraint that most often limits CHP size.
- Heat-to-Power Ratio (kWth/kWe): The ratio of recoverable thermal output to electric output at full load. Reciprocating engines: 0.8–1.2. Gas turbines: 1.5–4.0. This value depends on prime mover type, not on facility loads.
- Thermal-Limited Electric Capacity (kW): The electric output that corresponds to fully using the usable thermal demand. If this is smaller than the electric demand, the thermal side governs.
- CHP Size (kW): The preliminary electric capacity of the CHP system. This is the output used for initial screening.

The formula represents a fundamental physical constraint: for every kW of electric output, the CHP unit produces a fixed amount of thermal output (determined by the heat-to-power ratio). If the facility cannot use that heat, the unit must be downsized or the heat rejected, which negates the efficiency advantage. ASHRAE Applications Handbook 2023 Chapter 7 (Combined Heat and Power Systems) and the EPA Combined Heat and Power Partnership Catalog of Technologies (2017 edition, Section 2.3 'CHP Sizing Considerations') both establish this thermal-electric matching principle as the primary CHP sizing constraint. The model is a fixed-ratio screening tool; it assumes a constant heat-to-power ratio at full load, which is reasonable for preliminary sizing but breaks down at part load (see 'When This Method Is Not Enough').

Inputs Explained

The two most critical inputs are the facility electrical demand and the usable thermal demand. Electrical demand should be the average hourly load during the CHP operating period, not the peak instantaneous load. Using peak demand oversizes the unit and leaves thermal capacity stranded. For a 24/7 industrial process, use the average load over the year. For a building with diurnal variation, use the typical daytime load during the heating season. A common mistake is using the utility bill peak demand, which is typically 20-30% higher than the average load per EPA CHP Catalog 2017 Section 4 load profiling methodology.

Usable thermal demand is even more frequently misestimated. Engineers often enter the total boiler capacity or the design heating load. That is wrong. Usable thermal demand is the heat that can be recovered and put to productive use, not the peak heat loss of the building. For example, a hospital may have a design heating load of 2,000 kWth, but its actual usable thermal demand (from hot water reheat, domestic hot water, and space heating at typical conditions) might be only 1,200 kWth. The difference can halve the justified CHP size. Obtain this value from hourly thermal load profiles, not from nameplate ratings. If only annual fuel consumption is available, divide by annual operating hours to get an average usable thermal load, then apply a diversity factor of 0.6-0.8 per ASHRAE Applications Handbook 2023 Chapter 7 Section 7.3 thermal load characterization guidance.

The heat-to-power ratio is a characteristic of the prime mover, not the facility. For screening, use 1.0 for reciprocating engines and 2.5 for gas turbines per EPA CHP Catalog 2017 Table 2-1 (Reciprocating Engines) and Table 3-1 (Gas Turbines), unless specific manufacturer data is available. The catalog also provides combustion turbine HPR ranges 1.7-2.5 (small turbines <5 MW), 1.4-2.0 (medium 5-25 MW), and 0.9-1.4 (large >25 MW with combined cycle). Overestimating the heat-to-power ratio (e.g., using 2.0 for a reciprocating engine) reduces the thermal-limited capacity, potentially undersizing the CHP. Underestimating it (e.g., using 0.5) inflates the thermal-limited capacity and may lead to an oversized unit that cannot use its heat.

Worked Example

Scenario: A hospital campus has an average electrical demand of 900 kW and a usable thermal demand of 2.40 MMBtu/h (imperial) during the heating season. The proposed prime mover is a reciprocating engine with a heat-to-power ratio of 0.90 kWth/kWe.

Calculation:

Step 1: Convert usable thermal demand to kWth.

Imperial input 2.40 MMBtu/h × 293.071 kWth/(MMBtu/h) = 703.37 kWth

(CHP electric and thermal capacities are universally expressed in SI units of kW and kWth; imperial thermal input MMBtu/h is converted before further calculation.)

Step 2: Calculate thermal-limited electric capacity.

Thermal-Limited Capacity = 703.37 kWth / 0.90 kWth/kWe = 781.52 kW

Step 3: Select CHP size as the smaller of electric demand and thermal-limited capacity.

CHP Size = min(900 kW, 781.52 kW) = 781.52 kW

Interpretation: The thermal side governs. The CHP should be sized at 782 kW (rounded), not 900 kW. If the engineer had sized from electrical demand alone (900 kW), the unit would be 118 kW larger, producing 118 × 0.90 = 106 kWth of excess heat that would need to be rejected. Heat rejection at this rate represents approximately 15% of the unit's recovered thermal output wasted (106/782 × 0.90 = 15.2%), reducing overall CHP efficiency from typical 75-85% (electric + thermal recovery) to approximately 65-72% per EPA CHP Catalog 2017 Section 2.4 efficiency definitions.

Decision matrix:

(1) Select 782 kW reciprocating engine package matching thermal-limited capacity. Use grid for residual electric demand (900 - 782 = 118 kW deficit).

(2) Investigate thermal load enhancement: add absorption chiller for summer cooling demand to extend usable thermal load year-round, potentially raising thermal-limited capacity to 900+ kW.

(3) Reduce target CHP utilization to 70-80% of thermal-limited capacity (≈547-625 kW) to ensure full thermal utilization at part-load operation per EPA CHP Partnership feasibility methodology, accepting smaller capital investment with cleaner economics.

What the Result Means

The output number (CHP size in kW) tells you which side of the project is more constraining. If the CHP size equals the electrical demand, the project is electric-limited: thermal load is abundant, and the CHP can be sized to match the electric load. If the CHP size equals the thermal-limited capacity, the project is thermal-limited: the usable thermal demand constrains the electric size. A thermal-limited result is the more common scenario in commercial and light industrial applications.

A decision rule: if the thermal-limited capacity is less than 70% of the electrical demand, the thermal side is strongly constraining. The engineer should either reduce the CHP size further (if thermal utilization is intermittent) or investigate thermal load enhancement (e.g., adding thermal storage, absorption chillers, or connecting to a district heating network). If the thermal-limited capacity exceeds the electrical demand, the project is electric-limited, and the CHP can be sized to the electric load, but only if the thermal load is truly usable year-round. Otherwise, the thermal load may be overestimated. For more on evaluating thermal loads, see How to Calculate Cooling Load for HVAC Sizing, which covers methods for developing accurate load profiles.

Common Mistakes

Sizing from electrical demand alone. This is the most frequent error. Engineers treat CHP like standby generation, which is sized for peak electric load. But CHP is a thermal-electric system; the thermal side often governs. In the hospital example, ignoring thermal constraints would oversize the unit by 15% and waste 106 kWth of heat whenever the unit runs. The consequence: lower CHP efficiency (down from 75-85% to 65-72%), longer simple payback (extending from 5-7 years to 10+ years per EPA CHP Partnership benchmarks), and potential heat rejection regulatory issues under local thermal discharge permits.

Using total thermal demand instead of usable thermal demand. Entering the design heating load of a building (e.g., 2,000 kWth) instead of the actual usable thermal load (e.g., 1,200 kWth) inflates the thermal-limited capacity. This leads to selecting a CHP that is too large to run at full thermal utilization during most operating hours. The fix: use hourly or monthly thermal load data, not design conditions. A 30% overestimate in usable thermal demand can result in a 30% oversized CHP.

Assuming a constant heat-to-power ratio across all operating conditions. The screening model uses a fixed heat-to-power ratio, but real CHP units have part-load heat-to-power ratios that differ from full-load values. For reciprocating engines, the heat-to-power ratio increases by 10-20% at 50% load per EPA CHP Catalog 2017 Section 2.5 (Part-Load Performance), because electric efficiency drops 5-10% (relative) faster than thermal recovery. Ignoring this part-load behavior can reduce actual thermal-limited capacity below the screening calculation by 10-20% during typical operating periods. The screening result should be treated as a starting point, not a final selection.

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When This Method Is Not Enough

The fixed-ratio screening model assumes that the CHP operates at full load with a constant heat-to-power ratio and that the thermal load is continuous. Both assumptions are often violated in real projects. In buildings with variable thermal loads (e.g., offices with nighttime setbacks or hospitals with seasonal shifts), the usable thermal demand may exist for only 4,000-6,000 hours per year per EPA CHP Catalog 2017 Section 2.3 typical operating profiles by building type. The screening model does not account for operating hours; it assumes the thermal load is always available when the CHP runs. If the thermal load is intermittent, the actual CHP size that can be economically justified may be smaller than the screening result. A CHP that runs at 50% load for 2,000 hours/year because thermal load is absent will have poor economics.

Additionally, the model does not consider part-load electric efficiency or thermal recovery degradation. At 60% load, a reciprocating engine has 5-10% lower electric efficiency (relative) and 10-15% higher heat-to-power ratio per EPA CHP Catalog 2017 Table 2-3 part-load performance characteristics, meaning the thermal-limited capacity decreases. The screening result should be refined using manufacturer part-load data and hourly load profiles. The method also ignores prime mover type-specific constraints: gas turbines have minimum load limits (typically 30-50% of full load) per ASHRAE Applications Handbook 2023 Chapter 7 Section 7.4 prime mover constraints, and reciprocating engines have emission compliance issues at low load. These factors can make a screening-sized unit infeasible in practice.

FAQ

What is the difference between CHP sizing and standby generator sizing?

CHP sizing matches both electric and thermal loads, while standby generator sizing only matches peak electric load. A standby generator can be sized to 100% of peak demand because it runs only during outages and has no thermal recovery requirement. A CHP unit must be sized to the lower of electric demand and thermal-limited capacity because it runs continuously and must fully utilize its heat output.

Why does usable thermal demand matter more than total thermal demand?

Usable thermal demand is the heat that can actually be recovered and put to productive use: for space heating, domestic hot water, process heating, or absorption cooling. Total thermal demand includes heat that may be rejected or lost. If usable thermal demand is overestimated, the CHP will be oversized and will either reject heat (wasting fuel) or run at part load (reducing efficiency).

Can I use this screening result for final equipment selection?

No. The screening result is a preliminary capacity estimate, not a final selection. Per EPA CHP Partnership feasibility methodology, detailed selection requires three additional analysis layers beyond screening: (1) Operational analysis: hourly load profile matching, part-load efficiency curves, thermal load continuity, and prime mover minimum-load constraints; (2) Code/regulatory analysis: emissions compliance (typically 40 CFR Part 63 NESHAP, EPA New Source Performance Standards, state air quality permits), utility interconnection requirements (IEEE 1547 for distributed energy resources), and safety codes (NFPA 850 for stationary engines); (3) Economic analysis: capital cost vs operational savings, fuel pricing escalation, maintenance contracts, and financing structure. Use the screening result as the starting capacity for these three layers of detailed feasibility analysis.

What heat-to-power ratio should I use for a gas turbine?

For a gas turbine, use a heat-to-power ratio of 1.5 to 4.0, depending on the turbine model and whether supplementary firing is used. A typical value for a small gas turbine (1–5 MW) without supplementary firing is 2.0–2.5. Check manufacturer data for the specific unit.

How do I convert MMBtu/h to kWth?

Multiply the value in MMBtu/h by 293.071 to obtain kWth. For example, 2.40 MMBtu/h × 293.071 = 703.37 kWth.

What is the difference between Power-to-Heat Ratio (PHR) and Heat-to-Power Ratio (HPR)?

Both ratios describe the same physical quantity (relative magnitudes of electric and thermal output) using inverse expressions. Different documents use different conventions: Power-to-Heat Ratio (PHR) = P_electric / P_thermal: used by US Department of Energy, EPA CHP Partnership Catalog 2017, and most American CHP literature. Values typically less than 1 (thermal output exceeds electric output for most prime movers). Heat-to-Power Ratio (HPR) = P_thermal / P_electric: used by ASHRAE Applications Handbook 2023 Chapter 7 and most European CHP literature. Values typically greater than 1 for the same prime movers. This calculator uses HPR convention. For a reciprocating engine: PHR = 0.83-1.25 corresponds to HPR = 0.8-1.2. For a gas turbine: PHR = 0.25-0.67 corresponds to HPR = 1.5-4.0. When citing manufacturer specifications, verify which convention is used to avoid errors in capacity calculation. The relationship: HPR = 1/PHR.

How do CHP system efficiency metrics work, and what target values are realistic?

CHP systems have three efficiency metrics per EPA CHP Catalog 2017 Section 2.4: Electric Efficiency (η_e) = Electric Output / Fuel Input. Typical values: 25-35% for reciprocating engines (1-5 MW), 25-40% for gas turbines (1-25 MW), 28-45% for combined cycle (>25 MW). Thermal Efficiency (η_th) = Recovered Thermal Output / Fuel Input. Typical values: 35-50% for reciprocating engines, 30-50% for gas turbines, depending on heat recovery method (jacket water, exhaust, both). Total CHP Efficiency (η_total) = (Electric + Thermal Output) / Fuel Input = η_e + η_th. Typical values: 65-85% for properly sized and operated CHP systems. EPA CHP Partnership defines a CHP system as efficient if η_total is at least 60% with 5% minimum η_th. ASHRAE Applications Handbook 2023 Chapter 7 Section 7.5 provides similar benchmarks. Target η_total at least 75% for cost-effective installations; below 65%, evaluate whether CHP is justified vs separate purchases of grid electricity and boiler-generated heat.

Related Calculation to Check Next

After determining the preliminary CHP size, the next step is to evaluate the economic feasibility using annual operating hours and utility rates. The screening result assumes the CHP runs at full load whenever thermal load is available, but real operating hours are limited by thermal load continuity and maintenance. Calculate the annual runtime based on hourly thermal load profiles. For a hospital with 8,000 hours/year of thermal load, the CHP may run 7,000 hours/year; for an office with 4,000 hours of heating, runtime may be only 3,000 hours. Use that runtime to estimate fuel savings and payback.

Additionally, verify that the electrical interconnection is feasible. The CHP size from screening must be within the capacity of the facility's electrical switchgear and the utility's interconnection requirements. For large systems (>1 MW), a utility interconnection study is typically required. For a deeper look at electrical system impacts, see How to Calculate Cable Ampacity: Applying Correction and Adjustment Factors for Conductor Screening, which helps ensure the feeder conductors are sized for the CHP output. Also review How to Calculate Capacitor Bank Size for Power Factor Correction: A Practical Guide for Electrical Engineers to assess whether the CHP's power factor requires correction.

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