The meter is a good idea, they come usually with a master card and instructions, and can be topped up fairly easily or set to free for the owner, the rate can be set by master card. 100 cards usually come with the meter and more can be purchased.
Meter can be fixed so guest pays for all electric or Just air con
Manageing the cards and keeping the meter topped up and having cards available may be a problem if you are not on site?
Cant remember the cost now Camposol Heating & Maintenance fitted ours