Scottish and Southern Electricity Networks has said 65 postcodes in the city have been affected, including the city centre.
They include properties in the PO1, PO3 and PO5 postcode areas.
The fault was first reported just before 12.30pm, and engineers are hoping to have fixed the problem by about 3.30pm.
Woman, 44, whose large Dobermann dog mauled girl, 3, on Leigh Park street released back into neighbourhood by police
Unidentified woman’s body found in water as Hampshire police investigate ‘unexplained’ death
Plans for redevelopment of Knight and Lee in Southsea get the green light - and work will begin soon, vows developer That Group
Seven - including five teenagers - arrested after Lower Drayton Lane burglary in Portsmouth
Portsmouth businessman turns 'detective' to catch brazen men who took air conditioning units feels ‘unsafe’ after men return to warehouse
On its website it said: '˜We apologise for the loss of supply. We currently have a fault affecting the areas listed. Our engineers are on site working to get the power back on as quickly as they can.'
If anyone would like more information they can call 0800 072 7282 and quote reference '˜EV0030'.
You can also look on the SSEN website.