Compare the whole system cost.
A fair DG-versus-BESS comparison serves the same load for the same backup duration. Include installed capital, charging or fuel, maintenance, replacements and financing before estimating savings.
What belongs in the comparison?
| Factor | Diesel generator | Battery storage |
|---|---|---|
| Power and duration | Output depends on generator rating, loading and fuel availability. | Output is limited by PCS rating; duration depends on usable battery energy and the load. |
| Energy expense | Measured litres × fuel price, divided by delivered kWh. | Charging cost divided by round-trip efficiency, plus auxiliaries. |
| Installed cost | Generator, ATS, fuel storage, enclosure, civil works and installation. | Battery, PCS, protection, switchgear, installation and any solar system. |
| Maintenance and replacements | Fuel system, engine servicing, consumables and overhaul. | Thermal management, controls, inspection and battery or power-electronics replacement allowances. |
| Backup transfer | Starting and transfer time depend on the generator and ATS. | Model-specific: under 10 ms for MTX Hybrid; external PCC switching for PTX and CTX. |
| Noise and emissions | Fuel combustion produces on-site exhaust; noise depends on enclosure and load. | No fuel combustion during battery discharge. Fans and cooling equipment can produce noise; lifecycle emissions depend on the charging source and equipment. |
| Site requirements | Check applicable emissions, noise, monitoring and consent conditions. | Check electrical, fire-safety and installation requirements, including any retained generator. |
How do you calculate total cost of ownership?
Total project cost includes the installed purchase price, operating costs, financing and replacements over the chosen period, less any residual value. Compare both options using the same time horizon and discount-rate assumptions.
Do not add an annualised capital charge to a calculation that already includes the full upfront purchase cost. Separate fuel or charging savings from net cash savings. Solar generation has an installed cost or an opportunity cost when exported energy could otherwise earn revenue.
Can you retain a generator as standby?
Yes. A design may use batteries for shorter outages and peak management, with a generator supporting longer interruptions. Define the dispatch strategy, reserve capacity, transfer arrangement and recharge plan together.
Neither lower runtime nor a smaller generator automatically removes site obligations. Check DG standards and monitoring requirements before including compliance costs as avoided.