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the big picture
By Jeremy Harrison
Photo book


Many people interested in micro CHP are unsure whether it is economically worthwhile to install such a system.  This simple calculator is intended as a tool for you to calculate your potential energy savings, based on your current fuel bills.  From this you can work out how long it will take to pay back the investment in micro CHP.

Please note that this spreadsheet is provided only for you to undertake an estimate and that each home and household is different.  You should satisfy yourself that the result is reasonable.  You will not be asked for your personal details.  If you want more information on the product you have selected, please contact the manufacturer (or other supplier) via the links on the products page.

Download the MS Excel spreadsheet and then enter your details to work out how much you can save.

Clicking on the "calculator" button will take you to a Microsoft Excel spreadsheet.  You need to have MS Excel installed to run this calculation.


This calculation assumes that you are considering replacing your boiler (which has reached the end of its useful life) with either a micro CHP unit or a new 88% efficient condensing gas boiler.  It is based on replacing the boiler with a WhisperGen micro CHP unit; other products may be evaluated if you know the respective heat and power outputs.  You will need to know your gas and electricity costs for a complete year, which you can obtain from your annual energy bills.

Simplified Example

An example of a simplified calculation is shown below for illustration only.  It assumes that the system is installed in parallel with the existing power supply and that surplus generation is exported to the grid and any shortfall in requirements is imported from the grid.  It is based on the WhisperGen (1.0kWe/7kWt) micro CHP unit in a family home with annual heat demand of 28,000 kWh, all of which is met from the engine (not the auxiliary burner).

It is based on a residential installation in the UK eligible for FiT (Feed in Tariff) payments.  Other financial incentives including tax relief, grants and subsidies are available in other regions.

Annual heat demand 28,000 kWh
Running hours 4,000 hours
Electricity generated 4,000 kWh
Own use of generation 67 %
Unit cost of avoided import 15 p/kWh
Value of avoided import 400
Unit value of export 5.4 p/kWh
Value of export 72  
Value of FIT (UK only) 500
Total value of generation 972
Additional gas consumed (compared with new condensing boiler) 4,000 kWh
Cost of additional gas consumed @4.5p/kWh 180
Net saving = 972-180 792
Marginal cost of unit 5000
Simple payback 6.3 years
Page updated 9th March 2015      


the big picture
By Jeremy Harrison
Photo book



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This site was last updated on 12th August 2017 Jeremy Harrison