I use these. Maybe it's cause I keep my house so cold, but I care barely tell mine are on.
You should also get a thermostat which will toggle the heating mat on and off and keep it at an ideal temperature. You set it to 72, put the probe at the top and it'll do the rest.
There's a lot of good information in this thread already so I'll just share why I'm using heat mats and lights and what I purchased 3 years ago and have served me well.
Total cost was under $100.00.
My backyard garden space is just over 1000 sq feet (about 93 sq meters). I wanted to reduce my expenses by growing seedlings for transplant instead of buying them at a local greenhouse/nursery. I also wanted to grow varieties that most nurseries don't carry around here. I don't have enough natural light in the room where I grow seedlings and needed to supplement with LEDs. The room is also significantly colder than the rest of my house and so heat mats were a necessity. I normally plant 70-80 peppers plants each year. I sow seed into cell packs indoors in early March and target early to mid-May for transplant (central VA, Zone 7a).
There may be better or less expensive products available but I've used these very successfully 3 years in a row and I would buy them again.
I highly highly highly reccomend Akiva Silver's book Trees of Power. It has incredible information on plant propagation but also just offers a refreshing new environment ethic for people. The trees he covers are based in the North East US, but can be applied to most temperate zones.
Sand is a great rooting medium because it drains well and retains moisture. The heat helps the cuts callous faster, and thus root. Cuttings are a race between rooting and rotting and we want roots to win. I'm a little behind this season but the goal is to have cool tops and warm bottoms for all hardwood cuttings. It will increase the success rate.
The cables are just heating cables for soil heating or gutter melting. I got mine here: https://www.amazon.com/gp/product/B00P218EFA/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1 (I know I hate amazon too but I live very rural).
And the thermometer is here: https://www.amazon.com/gp/product/B015F4VFGI/ref=ppx_yo_dt_b_asin_title_o00_s02?ie=UTF8&psc=1
When I bought this it was $17: the digital heat sensors https://www.amazon.com/VIVOSUN-Digital-Seedling-Thermostat-Controller/dp/B015F4VFGI/ref=pd_bxgy_86_2?_encoding=UTF8&pd_rd_i=B015F4VFGI&pd_rd_r=1HCKJXJJBBV0NW2W1AYK&pd_rd_w=fJNYN&pd_rd_wg=iKA5l&psc=1&refRID=1HCKJXJJBBV0NW2W1AYK
Here is the heat pad, it is always on full blast (you put it under the tray and the heat goes up, and put the heat sensor at the top so when the thing is fully heated, it turns off) : https://www.amazon.com/Plant-Babies-Heating-Waterproof-Seedling/dp/B013HFKRMK/ref=pd_sim_86_3?_encoding=UTF8&pd_rd_i=B013HFKRMK&pd_rd_r=1HCKJXJJBBV0NW2W1AYK&pd_rd_w=152tJ&pd_rd_wg=iKA5l&psc=1&refRID=1HCKJXJJBBV0NW2W1AYK
Or you can use this programmable controller the head: https://www.amazon.com/bayite-Fahrenheit-Digital-Temperature-Controller/dp/B011VGASLW/ref=sr_1_5?s=hi&ie=UTF8&qid=1481436370&sr=1-5&keywords=digital+heat+switch But you have to do some wiring (outlet cord, and wire an outlet) and you have to read up on how to program it
I use the programmable timer for Quail and the timer above in the first link for grow trays/starts. Either combination should work. Can you post pictures of your worm farm? Thanks