have enjoyed subsidies or premium prices (feed-in-tariff). The price reduction could be a result of these subsidies. On the other hand, one could also argue that if carbon prices are introduced, the price reduction effects of RE sources hold even if they do not receive any subsidies. It should also be noted that the studies cited in Würzburg et al. (2013) are relatively old (about a decade at least); costs of electricity from RE sources were much higher a decade ago than now. The average prices of electricity are higher now than a decade ago.

**Table 4. Selected empirical studies showing impacts of renewable energy on electricity supply costs**

||Methods|Findings|
|---|---|---|
|Schulte & Fletcher<br>(2021)|Useful clean energy<br>analysis in the USA.|Due to intermittency, RE leads to diminishing returns for<br>useful clean energy. Every utility has a hypothetical<br>“inflection point” where increasing high levels of<br>renewables begin to exceed hourly load and must be<br>turned down or otherwise curtailed. Utilities would “stall<br>out” in useful energy when RE reaches levels at about 60<br>%–80 % of their annual load.|
|Mai et al. (2021)|Optimization-based<br>capacity expansion model<br>and Regional Energy<br>Deployment System are<br>applied in the USA from<br>2020 to 2050.|The expansion of non-fossil electricity generation to<br>account for 45% of the total generation would increase<br>electricity system costs by 0.4% to 0.8% depending upon<br>the discount rate. If the share of non-fossil fuels in the<br>total generation reaches 78%, the system costs would<br>increase by 2.7% to 3.6%.|
|Vellini et al. (2020)|Cost-effective estimation<br>of mitigation costs of the<br>power sector in Italy.|The mitigation costs CO2emissions reduction of the<br>Italian electricity sector in 2030 vary widely ranging from<br>null up to €140 per ton. The larger the use of gas and<br>electricity generation from the solar source, the higher are<br>the mitigation costs. The complete phase-out of coal<br>exhibits the highest migration costs that are not as<br>beneficial from an economic perspective.|
|Liang et al. (2019)|A simulation method<br>using the LEAP model is<br>applied in China from<br>2015 to 2050.|Increasing the share of renewables in total electricity<br>generation capacity from 27.1% to 50.7% in 2050 would<br>increase the electricity system costs for the 2015-2050<br>period by 7%. It would reduce 41% of the power sector's<br>CO2emissions in 2050.|
|Adelman & Spence<br>(2018)|Low-cost electricity<br>generation optimization is<br>applied in the USA from<br>2016 to 2031.|The low natural gas prices—below US$3.50/MMBtu<br>through 2023 and below US$4.50/MMBtu through<br>2031—inhibit the construction of new renewable capacity.<br>The significant impact of inexpensive gas is likely to be<br>limiting the entry of renewables. This is mainly due to two<br>phenomena: 1) renewables pose a much greater threat to<br>the viability of base-load generation in the longer-term<br>than a natural gas-fired generation; and (2) when gas<br>prices set market prices, they also determine the<br>economics of renewables, and thereby the volume of new<br>renewable capacity that enters the market.|
|Kumar (2016)|A simulation method<br>using the LEAP model is<br>appliedin Indonesia and|In implementing renewables at a large scale in both these<br>countries the cost of production increases substantially.|

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