This is a weird and partly incomplete question
1) it's allmost impossible to mine when you're offline... Theoretically, you could mine a block using pen and paper, but in reality, it's next to impossible to do so
2) you could theoretically mine using a raspberry pi using a couple of watts, or use your cellphone... Only on average it'll take several thousands of years to find a block.... If you want to have a decent chance, you'll need to buy an ASIC, and those things do use a lot of enegery
Here are two lists of devices you could mine with, with an indication of their power usage;
https://en.bitcoin.it/wiki/Mining_hardware_comparison
https://en.bitcoin.it/wiki/Non-specialized_hardware_comparison
This being said, unless you have really, really, really cheap electricity, you could only make a little profit using the very latest gear.
EDIT: just to prove it's a bad idear to mine with your phone or a rPi, i calculated how long it would take with a cortex-A9 (common cellphone CPU)
http://www.coinwarz.com/calculators/bitcoin-mining-calculator/?h=0.00057&p=0.5&pc=0.01&pf=1&d=286765766820.55000000&r=12.50000000&er=770.69000000&hc=0.00
At current diff, on average, it would take you 68.518.161 years (YES, that's 68,5 MILLION years) to find a 12.5BTC block (only, by the time you'll find a block, there probably will only be fees, the block reward will be 0).
Next to this, even when using allmost free power (1cent/Kwu), you'll still burn 4 cents/year in power * 68 million = 2.7 million USD. And you'll probably have to replace your phone several million times
This theoretical example was made with current diff, block reward and btc price... These things will change over time, so my calculation will not be correct (it'll probably be way to optimistic)
That's some detailed calculation on an inefficient device for mining. Yes, the calculation did not include halving and the increasing prices of power. But good illustration regards using a phone.
To continue OP's question. If I would buy an ASIC S9 right now and mine 24/7 how much electricity am I look at? I live in a tropical country so I think I would also need cooling, but if it's only one miner will I need cooling?