If you assume that your 100V home generator produces a sine wave with an RMS value of 100V, then its output waveform is described by the function

v(t) = 100*sqrt(2)*sin(2*pi*60*t)

This waveform has peaks of 100*sqrt(2), about 141 volts. If a 100 V battery is connected in series, the voltage function becomes

v1(t) = 100 + 100*sqrt(2)*sin(2*pi*60*t)

This hypothetical waveform will have peaks of

100 + 141 = 241 volts

v(t) = 100*sqrt(2)*sin(2*pi*60*t)

This waveform has peaks of 100*sqrt(2), about 141 volts. If a 100 V battery is connected in series, the voltage function becomes

v1(t) = 100 + 100*sqrt(2)*sin(2*pi*60*t)

This hypothetical waveform will have peaks of

100 + 141 = 241 volts