Cogeneration CHP Sizing Calculator

Calculate

Enter the facility electrical demand — the electric load the CHP system is being sized to serve or partially serve.

Enter the usable thermal demand in MMBtu/h. Automatically converted to kWth for calculation (1 MMBtu/h = 293.071 kWth).

Enter the CHP heat-to-power ratio — usable thermal output per unit electric output. Typical screening values: reciprocating engines 0.8–1.2, gas turbines 1.5–4.0. Use manufacturer or project-specific data for final sizing.

Overview

The Cogeneration CHP Sizing Calculator estimates CHP system size using a fixed screening model based on facility electrical demand, usable thermal demand, and the selected CHP heat-to-power relationship.

The result is intended for preliminary cogeneration review. The calculator classifies the output as SMALL, STANDARD, LARGE, or VERY LARGE based only on the calculated CHP size in kW.

The result should be treated as a preliminary CHP capacity estimate. Real project sizing must still consider operating hours, part-load behavior, standby strategy, thermal-use continuity, prime mover type, emissions, interconnection, and project economics. A CHP system must be justified not only technically, but also by operating profile and economic performance such as annual running hours and practical payback.

How to Use This Calculator

  1. Enter Facility Electrical Demand — the electric load the CHP system is being sized to serve, in kW.

  2. Enter Usable Thermal Demand — the recoverable and usable heat from the CHP system, in kWth (Metric) or MMBtu/h (Imperial).

  3. Enter Heat-to-Power Ratio — usable thermal output per unit electric output (dimensionless). Typical screening values: reciprocating engines 0.8–1.2, gas turbines 1.5–4.0.

  4. Click "Calculate" — get the Thermal-Limited Electric Capacity and CHP Size in kW.

  5. Review the CHP Size — the result is classified as SMALL (< 100 kW), STANDARD (100–999 kW), LARGE (1,000–4,999 kW), or VERY LARGE (≥ 5,000 kW).

  6. Compare with facility load profile — verify that the entered electrical and thermal load assumptions match the intended operating case.

CHP sizing is a thermal-electric matching exercise. Both electrical demand and usable thermal demand must be greater than 0. The result is displayed in kW in both Metric and Imperial modes.

Inputs & Outputs

Inputs

•Facility Electrical Demand (kW)
•Usable Thermal Demand (MMBtu/h / kWth)
•Heat-to-Power Ratio (kWth/kWe)

Outputs

•Thermal-Limited Electric Capacity (kW)
•CHP Size (kW)

Formula

Calculator Formula

This calculator uses a fixed CHP sizing screening model.

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

Where:

  • CHP Size — selected CHP electric capacity in kW
  • Electrical Demand — facility electrical demand in kW
  • Thermal Demand — usable thermal demand in kWth (converted from MMBtu/h in Imperial mode: 1 MMBtu/h = 293.071 kWth)
  • HPR — heat-to-power ratio in kWth/kWe

Step-by-Step Calculation

Step 1: Determine the thermal-limited electric capacity

Thermal-Limited Electric Capacity = Thermal Load / HPR

Step 2: Select the smaller of the two

CHP Size = min(Electric Load, Thermal-Limited Electric Capacity)

Variable Reference

Variable Meaning Units
electricLoad Facility electrical demand kW
thermalLoad Usable thermal demand kWth (metric) / MMBtu/h (imperial)
heatToPowerRatio Heat-to-power ratio kWth/kWe
thermalLimitedCapacity Thermal-limited electric capacity kW
chpSize CHP Size (output) kW

In Imperial mode, Usable Thermal Demand entered in MMBtu/h is automatically converted to kWth before calculation (multiplied by 293.071).

Size Classification

The calculated CHP size is classified into the following screening categories:

Classification CHP Size Range
SMALL < 100 kW
STANDARD 100 – 999 kW
LARGE 1,000 – 4,999 kW
VERY LARGE ≥ 5,000 kW

These are qualitative screening categories based on typical CHP project scales. They do not represent code-mandated thresholds or performance guarantees.

What is Cogeneration CHP Sizing

Cogeneration CHP sizing is the process of determining how large an onsite combined heat and power system should be so that it matches both the facility's electric demand and its usable thermal demand. In practical engineering terms, a CHP unit that is too large may create unused heat or inefficient part-load operation, while a CHP unit that is too small may leave recoverable thermal and electric value untapped.

The fundamental constraint in CHP sizing is thermal-use continuity. Unlike standby generation, which is sized for maximum electrical load, CHP must be sized to a level where the recovered heat can actually be used. This is why CHP sizing is fundamentally a thermal-electric matching problem, not just a generator sizing problem.

This calculator uses a fixed screening model based on facility electrical demand, usable thermal demand, and the heat-to-power ratio. The result is the smaller of the two possible sizing limits — electrical demand or thermal-limited electric capacity — and is classified as SMALL (< 100 kW), STANDARD (100–999 kW), LARGE (1,000–4,999 kW), or VERY LARGE (≥ 5,000 kW).

CHP sizing screening is different from a full CHP feasibility study. Screening tools provide first-pass indicators during concept and pre-design phases. They are not a replacement for detailed energy modeling, equipment selection, or economic analysis — but they help engineers identify whether a proposed CHP size is in the right range before committing to detailed engineering.

Key Facts

  • CHP sizing is constrained by both electrical load and usable thermal load.
  • CHP sizing is a thermal-electric matching exercise, not just generator sizing.
  • A CHP project with weak thermal utilization usually performs worse than one with strong year-round heat recovery.
  • A higher heat-to-power ratio reduces the electric capacity supported by a given usable thermal load.
  • Final CHP sizing should consider operating profile, economic justification, and useful heat continuity.
  • Classification is based only on the final CHP size in kW — not on the HPR or the thermal-limited capacity individually.
  • Reciprocating engines typically have heat-to-power ratios of 0.8–1.2; gas turbines typically 1.5–4.0.
  • Thermal-use continuity is often the key constraint in CHP sizing — intermittent heat demand weakens the CHP case.
  • CHP sizing is different from boiler sizing and different from standby-generator sizing.

Applications

  • Preliminary CHP feasibility screening
  • Hospital and campus cogeneration review
  • Industrial process heat and power screening
  • District-energy pre-sizing
  • Checking whether a CHP requirement is small, standard, large, or very large
  • Early comparison of thermal-driven and electric-driven CHP sizing cases

Example Calculation

Example Calculation

Given:

  • Electrical Demand = 900 kW
  • Usable Thermal Demand = 2.40 MMBtu/h (Imperial) = 703.37 kWth
  • Heat-to-Power Ratio = 0.90 kWth/kWe

Step 1: Convert thermal demand to kWth (Imperial mode)

Thermal Load = 2.40 × 293.071 = 703.37 kWth

Step 2: Calculate thermal-limited electric capacity

Thermal-Limited Electric Capacity = 703.37 / 0.90 = 781.52 kW

Step 3: Select the smaller value

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

Result: 781.52 kW — STANDARD

This falls in the STANDARD range and indicates a moderate CHP size where usable thermal load, not electrical demand, governs the preliminary sizing result.

Standards & References

  • ASHRAE CHP Technical Committee — combined heat and power systems context
  • EPA CHP Guidance — combined heat and power performance and application context
  • ISO 50001 — energy management systems context
  • IEC 60034-1 — rotating electrical machines context where CHP includes engine- or turbine-driven generator equipment
  • IEC 60034-3 — large turbine-generator application context

Limitations

  • This is a preliminary CHP sizing calculator, not a full CHP feasibility study.
  • It uses a fixed calculator-specific thermal-electric matching model.
  • It does not calculate annual dispatch optimization, fuel cost or tariff savings, emissions credit value, interconnection study outcomes, or resilience strategy.
  • The model assumes steady representative loads, not full annual load-shape behavior.
  • The model assumes usable thermal demand is truly recoverable and consistently valuable.
  • Real CHP projects may require thermal storage, dump-heat strategy, or absorption chilling to align heat recovery with actual load profiles.
  • It does not replace a detailed CHP feasibility study, energy model, or manufacturer application review.

Common Mistakes to Avoid

  • Sizing CHP from electrical demand alone.
  • Ignoring whether recovered heat is actually usable.
  • Using total thermal demand instead of usable thermal demand.
  • Assuming all operating hours have the same thermal-electric load ratio.
  • Using an unrealistic heat-to-power ratio.
  • Treating CHP as the same as standby generation.
  • Ignoring part-load operation and redundancy strategy.
  • Assuming this calculator alone finalizes CHP selection.

Frequently Asked Questions

What does this calculator estimate?
It estimates the electric size of a CHP system in kW based on facility electrical demand, usable thermal demand, and the selected heat-to-power ratio.
Why does usable thermal demand matter?
Because CHP only performs well when the recovered heat can actually be used. If usable thermal demand is weak, the thermal side may limit the justified electric size of the CHP system.
Why does a higher heat-to-power ratio reduce electric CHP size?
Because a higher heat-to-power ratio means more thermal output is produced per unit of electric output, so the usable thermal demand is reached with a smaller electric CHP size.
What does a SMALL result mean?
It means the preliminary CHP requirement is relatively modest and may correspond to a small facility, limited thermal opportunity, or a conservative load-matching case.
What does a VERY LARGE result mean?
It means the preliminary CHP requirement is substantial and may correspond to a large industrial, campus, district-energy, or continuous-process application that needs deeper project review.
Does this calculator include economic savings or payback?
No. It estimates CHP size only. Fuel cost, tariff savings, emissions economics, and project payback require separate analysis.
What is CHP thermal efficiency and why does it matter?
Useful heat recovery determines how much of the thermal output can actually be captured and used. Better effective heat use improves the technical and economic case for CHP, while poor heat use can limit the justified project size even if electrical demand is large.
Is this enough to finalize a CHP project?
No. Final design should also consider operating profile, thermal-use continuity, prime mover choice, redundancy, interconnection, emissions, maintenance, and project-specific economics.

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

Microgrid Stability Estimator

Three Phase Power Calculator

Ups Sizing Calculator

Related Calculators

Explore similar calculators that might be useful for your project:

Microgrid Battery Storage Sizing

Wind Turbine Power Output

Kva To Kw Calculator

Waste Heat Recovery Efficiency

Boiler Efficiency Calculator

Battery Bank Sizing For Off Grid

7 Mistakes Electrical Engineers Make — Free NEC Reference

Avoid the classic errors: ampacity vs. voltage drop, one-way CT lead length, running vs. starting current. 8 formulas, 12 checks, 10 pages.

  • Instantly check voltage drop, ampacity & motor current
  • Catch the 7 wiring errors that fail code inspections
  • 12 design checks to run before submitting drawings

No spam. Unsubscribe any time.