Gravitational potential energy storage with solid masses Alternatives include storing energy by moving large solid masses upward against gravity. This can be achieved inside old mine shafts or in specially constructed towers where heavy weights are winched up to store energy and allowed a controlled descent to release it. In rail energy storage, rail cars carrying large weights are moved up or down a section of inclined rail track, storing or releasing energy as a result; In disused oil-well potential energy storage, weights are raised or lowered in a deep, decommissioned oil well. Economics The levelized cost of storing electricity depends highly on storage type and purpose; as subsecond-scale frequency regulation, minute/hour-scale peaker plants, or day/week-scale season storage. Using battery storage is said to have a levelized cost of $120 to $170 per MWh. This compares with open cycle gas turbines which, as of 2020, have a cost of around $151–198 per MWh. Generally speaking, energy storage is economical when the marginal cost of electricity varies more than the costs of storing and retrieving the energy plus the price of energy lost in the process. For instance, assume a pumped-storage reservoir can pump to its upper reservoir a volume of water capable of producing 1,200 MW·h after all losses are factored in (evaporation and seeping in the reservoir, efficiency losses, etc.). If the marginal cost of electricity during off-peak times is $15 per MW·h, and the reservoir operates at 75% efficiency (i.e., 1,600 MW·h are consumed and 1,200 MW·h of energy are retrieved), then the total cost of filling the reservoir is $24,000. If all of the stored energy is sold the following day during peak hours for an average $40 per MW·h, then the reservoir will see revenues of $48,000 for the day, for a gross profit of $24,000. However, the marginal cost of electricity varies because of the varying operational and fuel costs of different classes of generators. At one extreme, base load power plants such as coal-fired power plants and nuclear power plants are low marginal cost generators, as they have high capital and maintenance costs but low fuel costs. At the other extreme, peaking power plants such as gas turbine natural gas plants burn expensive fuel but are cheaper to build, operate and maintain. To minimize the total operational cost of generating power, base load generators are dispatched most of the time, while peak power generators