Update....
I spent some time mapping the circuit...
It is a 15 amp with outlets in Dining Room and Rear Bedroom and single Back Porch Outlet and two patio lights.
The Extension Cord is 14ga and 15' long...
At the buss I get 122 volts, at the breaker 121 volts and at patio GFCI 119 volts.
House wired in the 1960's and all two prong except kitchen is grounded and patio GFCI.
On a side note the house is full of outlet strips with lots of electronics and rice cooker, micro, toaster oven, hair dryer, etc
Tenant today said he is willing to pay for a 20 amp breaker and I said it doesn't work that way unless the wire is also sized for 20 amp.
To add a dedicated patio circuit which would mean a new split half size breaker as all panel slots used.