Occupants of dwellings with grid-connected photovoltaic (PV) systems can often benefit financially from exporting electricity to the grid. When export prices are lower than import prices, however, occupants are incentivised to time-shift demand in order to avoid exports and reduce imports. To maximise this potential financial benefit, the addition of batteries to the PV system has been proposed to take advantage of the specific commercial opportunity presented to the occupant of trading exported power during the day for imported power during the evening. This paper therefore assesses the economic and environmental impact of the use of lead-acid batteries in grid-connected PV systems under current feed-in tariff arrangements in the UK. The development of a lead-acid battery model is described, which is used to simulate hypothetical power flows using measured data on domestic PV systems in the UK. The simulation results indicate that the net benefit of the battery is negative, even when considering an idealised lossless battery. When realistic energy losses and lifetimes are accounted for, the financial loss for the systems considered here can approach £1000/year. The environmental impact of the use and production of the lead-acid battery is also described, and also found to be negative, further strengthening the argument against the use of lead-acid batteries in domestic grid-connected PV systems.