Many with access to electricity in sub-Saharan Africa (SSA) are subject to frequent outages due to insufficient generation capacities and/or poor transmission and distribution infrastructure. These outages result in increased use of backup diesel generators. We use a Monte Carlo Analysis framework to estimate changes in net air emissions, consumer costs and fossil energy consumption that result from the use of backup diesel generators in SSA. We show that reliance on backup diesel generators can lead to increased air emissions in all countries. Use of backup diesel generators also increases fossil fuel energy consumption by a factor of 1.5–1,000 compared with current grid levels throughout SSA. Finally, we estimate that the costs of generating diesel backup power are millions of dollars higher than the costs of grid electricity in all countries. These results suggest that increasing power system reliability for those with existing electricity access is a key component of meeting sustainable electricity access goals. Provision of electricity in sub-Saharan Africa is often affected by outages leading to increased use of backup diesel generators. In this study, the authors estimate the air emissions, consumer costs and fossil energy consumption resulting from the use of such generators.