  Heat engines should ideally have large power output, operate close to Carnot
efficiency and show constancy, i.e., exhibit only small fluctuations in this
output. For steady-state heat engines, driven by a constant temperature
difference between the two heat baths, we prove that out of these three
requirements only two are compatible. Constancy enters quantitatively the
conventional trade-off between power and efficiency. Thus, we rationalize and
unify recent suggestions for overcoming this simple trade-off. Our universal
bound is illustrated for a paradigmatic model of a quantum dot solar cell and
for a Brownian gyrator delivering mechanical work against an external force.
