This way, low initial rates can foster political acceptance and give stakeholders time to adjust, while increases over time guarantee stable revenue and maintain environmental effectiveness. If uncertainty about the predictability and longevity of a tax rate emerges, investors may regard the risk of low-carbon, energy-efficient or pollution-reducing investments to be too high. At the same time, incentivising private investment requires additional measures to minimize risk and create stable investment frameworks that guarantee - or at least increase the probability of - a safe return. Such measures can include low-cost loans for private investors, accelerated depreciation, preferential interest rates or, for renewable energy, long-term power purchase agreements (Cottrell, Fortier and Schlegelmilch, 2015).