Disadvantages of PHEVs include the additional cost, weight and size of a larger battery pack. According to a 2010 study by the National Research Council, the cost of a lithium-ion battery pack is about /kW·h of usable energy, and considering that a PHEV-10 requires about 2.0 kW·h and a PHEV-40 about 8 kW·h, the estimated manufacturer cost of the battery pack for a PHEV-10 is around and it goes up to for a PHEV-40. According to the same study, even though costs are expected to decline by 35% by 2020, market penetration is expected to be slow and therefore PHEVs are not expected to significantly impact oil consumption or carbon emissions before 2030, unless a fundamental breakthrough in battery technologies occurs.