No, because it will back feed into what ever else you have connected and is dangerous. It can be done with interlocking switches, but its NOT a diy job. Usually cylinders with economy 7 have two immersion heaters, one at the bottom for economy 7 and one at the top for "boost"
Economy 7 is a tariff not a system, what you can do depends on how it is wired. In the early days you had a white meter and a completely separate fuse box for the items using economy 7, however later there was a signal to tell the heaters when to switch on, and all fed from same fuse box/consumer unit, this had the advantage that washing machines could also use off peak power, disadvantage if the clock was wrong it could cost a lot of money.
There is a range of switches designed to work the immersion heaters with economy 7 cost between £50 and £80 and these often have a boost option.
It will depend on how your house is wired, as to what can be done, the old way was two immersion heaters in the same tank, economy 7 at bottom of tank set to a higher temperature to the second one at top of tank.
In some cases specially in Ulster the Wilis system was used, where the immersion heater was outside of the tank, and heater tank from top down instead of bottom up, so one immersion can give hot water to taps very fast, and has two thermostats one at top and one near the bottom, but not seen much in main land UK, the Irish plumber it seems are more cleaver than main land plumbers and on mainland UK plumbers did not seem to be able to get the system to work.
Since plumbers often failed, I think DIY is not really the way forward, I am an electrician not a plumber, and where I worked we had gas, so economy 7 was rare, so although I know there are special controllers, I am not sure which is which.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!