We use a one-dimensional model to calculate the electrostatic energy of several horizontally extensive, electrified clouds. The model calculation is initiated with known charge distributions. By making an assumption of the way in which a hypothetical lightning flash changes the charge distribution, we use the model to determine if this flash is energetically allowed. The model shows that the charge structure in the cloud can constrain the types of lightning flashes that occur. A single-charge cloud-to-ground flash of one polarity can be absolutely prohibited, while the opposite polarity flash is allowed. Discharging a screening charge layer may require another charge region of the opposite polarity to be involved in the flash. Small reductions in the charge density of a region often can provide a relatively large amount of energy to drive a flash. Certain intracloud flashes are always allowed. Model calculations suggest that positive cloud-to-ground flashes that produce Q-bursts have energies of about 1 ¿ 1010 J and discharge an area of about 40 km ¿ 40 km. Model estimates of the total electrostatic energy stored in two stratiform clouds of mesoscale convective systems were 5 ¿ 1011 J and 2 ¿ 1012 J. These energies are sufficient to support hundreds or thousands of typical lightning flashes, but only 10--100 of the energetic positive cloud-to-ground flashes with associated Q-bursts.