Do I need a dehumidifier?

It's much easier and cheaper to check the humidity levels than it is to get a decent dehumidifier. I live in a house that was built in 1877 and is now plasterboarded and insulated, and the windows (which were installed before I moved in - not my choice of frames) do not have trickle vents. Therefore, mucho condensation. If your house is warm and the humidity is below about 65% you should be fine. Lower humidity is better but 65% as a maximum isn't really bad if it's warm. If your humidity is over 70% and/or you have visible condensation on your windows in the mornings, then you'll probably want to do something about that. I have a 16-litre dehumidifier which I run when necessary. But honestly opening the windows for a bit usually does the trick. I have little digital thermometer/hygrometers in every room, I use the extractor fan when cooking, I open the bathroom window after a shower or bath. One thing you can do when it's dry outside is to open a window at the front of the house and another at the back of the house, then set up a fan in front of one window, pointing into the house. It'll pull dry air in from outside, which then funnels through and pushes the damp air out of the opposite window. To directly answer your question - I don't use my dehumidifier very often overall. Just ventilating the house seems to work 99% of the time. I have no idea how much it costs in electricity, I don't have a smart meter and I don't track my usage that closely. It's honestly most useful in summer, when my house seems to naturally hit 80% or more when it's hot outside, compared with winter when just 15 minutes of open windows keeps it around 65% most days.