Installed photovoltaic capacity has been rapidly increasing as the price of solar modules has continued to fall. When photovoltaic (PV) generation approaches the mid-day electric load demand, actions must be taken to balance the supply and demand. The challenge of matching the generation anddemand is expected to vary with location. The goal of this project is to estimate the amount of PV that Regional Transmission Organizations (RTO) in the United States can afford to have on its grid before solar curtailment, storage, or load shifting will become important at mid-day to maintain balance of supply and demand. This paper uses historical renewable generation and load data for each RTO to assess imbalance between electricity supply and demand as solar generation increases. Imbalance was estimated as a function of solar penetration. Two scenarios were used to model the imbalance: 1) when the net load is calculated to be negative (PV generation exceeds the demand); and 2) when net load is less than a minimum generation limit value, e.g., determined by policy for baseload generation from conventional sources. The impact of increasing solar generation on real-time pricing is studied for the case of California, which presently has the highest solar penetration of the U.S. RTOs. This paper thus provides data-based projections for U.S. RTOs on the degree to which storage, load shifting, or trading on energy imbalance markets will need to be adopted to avoid severe curtailment of solar PV electricity generation.