I have 240w lm301B and now bought 301h. 120x120 tent. Temperature is 28 around

2 lights dimmed will give you much better overall canopy coverage vs one light on full power in the middle. One light full power will create a hot spot directly underneath it with not so good side coverage. In a 120 x 120, bare minimum of 480w of light via quantum boards to obtain decent bud development ideally 640w. However, since your landlord is on to the increase in electricity usage, you can and I have flowered out autos on a 14/10 light schedule and still achieve great results. This will cut your electricity usage down 2.5 kw per day. If you adjust amount of time on, just stay consistent and slowly cut back the time daily. If you decide to dim the lights, I would go with 2 light sets and move them as close to the plants without burning them. If your dimming to 75%, try hanging at 25 to 30 cm