Electric Tillers, Petrol Micro Tillers, Petrol Mini Tillers and most Petrol Front Tine Cultivators/Tillers are not suitable for breaking up hard ground and should only be used for cultivating and turning over ready-made beds and pre-broken soil.*
Attempting to break up hard, impacted ground with the wrong machine can damage your gearbox and will be inconvenient as well as costly.
Generally speaking, according to the experts at the Royal Horticultural Society, Light soils (sandy soils) can be turned over by a 2-5 HP machine, but heavy (clay) or impacted soils soil need more power.
Rotavating wet soil is extremely damaging so do wait for drier conditions to work this soil.
*NB: It is VERY important that you only attempt to break up virgin ground with a heavy-duty petrol rear tine tiller.