The naquadah of which the stargate is made is a superconductor, and it will absorb energy directly in many forms, including heat, nuclear, solar, and static. Internal functions will convert and store that energy in capacitor-like reservoirs, and the inside wheel will unlock and spin freely once there is enough reserve power. It is possible to dial the stargate manually by using an external power source and spinning the inside wheel by hand. The archival footage of the stargate research from 1945 showed that the team experimenting with the gate had first learned to turn the gate manually while charging it with electrical surges, and they had successfully made a connection to P3X-972.
SG-1 has dialed the gate manually in emergency situations when no DHD was available, relying at various times on the power from a bolt of lightning, cold fusion, two truck batteries, reserve power stored in the disconnected gate from the SGC, and the naquadah core from a Goa'uld bomb. SG teams will often carry a portable naquadah generator on off-world missions to planets for which MALP telemetry gives no clear indication of the presence of a DHD, should a power source and manual dialing procedure be required.
Cross Reference: Cold Fusion, DHD, Naquadah Generator, Sekhmet's Ark, Stargate
Episode Reference: The Torment of Tantalus, Prisoners, 1969, Nemesis, Bad Guys