Well first, one light bulb will do a LOT and if only run at night has a very minimum impact on an electric bill. Keeping in mind this is for a quite small green house.
The methods you mention should be effective to a degree. Here are a few things I have learned this year thus far.
1. My experiment to grow pole beans failed, even in the green house the first 29 degree night killed the beans. Everything else made it.
2. I just bought a 32 gallon black rubbermaid trash can at target (12 bucks). I filled it with water and I have one of my reptile thermo probes in the water. Yesterday it was really warm (high 70s) but at night when the air temp was down to 57 the water was still at 67. I am thinking that will make a difference tonight when it goes down to 28! I have pretty much decided to buy a second one based on my temp readings thus far. I never though about antifreeze and it is probably a good idea just use the stuff that cats, dogs and children won't drink and kill themselves with. I lost a cat two years ago that drank antifreeze and I decided after seeing how he died that if I ever found someone that did it on purpose to an animal that they just might have be "vanished"
Seriously it is one of the most horrible of all deaths that can occur. Spend the extra 2 cents a gallon.
3. The only reason I don't just use a small heater or some bulbs myself is that the only place in my yard really suitable this time of year for my house is at the very farthest part of my yard from the house (of course
). Keep in mind a few bulbs, even a small heater (set on the lowest setting) would only need to be run on nights below say 32-34 degrees. You know it will work and you don't have to run them during the day. In fact if you set up a timer you could run them say from 2 hours after dark for a few hours, then off, then back on for a few hours, then off, then on right near morning and off when the sun comes up. Specifically with a small heater in a modest green house that would be enough.
4. A bed of rotting compost will add some heat. The key is it must be already starting to rot, but not finished yet.
5. The milk jugs full of hot water from the sink, work great but only in relatively small environments.
Just realize if you go with some solar heat sinks and stick to cold weather crops (many that are hard to grow in the heat by the way) you can get by with nothing. If you want peppers, tomatoes, etc. it is going to be really tough to get by without some form of "help". So either stick to stuff like lettuce, broccoli and other winter crops or bring in just a bit of heat.
By the way if anyone thinks they know a better way, please speak up my experience here is a bit limited and you won't hurt my feelings.
