No. That return on equity number is a target that the regulators consider when approving price hikes. If PG&E tried to get a 20% RoE, the regulator would deny the request. Utilities are basically compelled to accept price regulation in return for a monopoly on utility business in a geographic area.
There are obviously no guarantees that a utility will make money, but these good utilities are good stable investments that generally speaking will not make you rich, but appreciate nicely over time. Due to deregulation, however, they are a more complex investment than they once were.
Basically, the utility builds and maintains a bunch of physical infrastructure, buys fuel and turns it into electricity. So they have fixed costs, regulated pricing, market-driven costs for fuel, and market-driven demand for electricity.
Also consider that the marginal cost of adding capacity to the electric grid is incredibly high, so uneven demand growth or economic disruption in the utility service area can hurt the firms return on equity (and thus the stock price). Compare the stock performance of HE (the Hawaiian electric utlity) to ED (Consolidated Edison, the NYC utility) to SO (Southern Companies, the utility for much of the South). You can see that the severe impact of the recession on HE really damaged the stock -- location matters.
Buying strategy is key as well -- during bad market conditions, money flows into these stocks (which are considered to be low-risk "defensive" investments) and inflates the price. You don't want to buy utilities at a peak... you need to dollar-cost average a position over a period of years and hold it.
Focus on the high quality utilities or quality local utilities if you understand your local market. Look at Southern Co, Progress Energy, Duke Energy or American Electric Power as high-quality benchmarks to compare with other utilities.