Well..the more watts, the more money. The more watts the more heat. If you want a higher temp at night, you will need to turn up the heater, or increase the outside temp. Remember, if the outside temp goes below 30 say into the 20s, your 1500 might not be able to keep it at 60 on the inside. If you have anytype of wind, you also change the heat effect of the heater. There are a lot of variables to this. You kinda of have to play with it and see what works best for you. Also, the lower the temp at night, the harder it is to keep the inside warm. Its amazing what 10 degrees can do. You ask about cheaper, there is no cheaper in this case. It all cost money, the more you use, the more you pay. Good luck.

Well, is it better to have shorter or longer heating cycles??

An electric heater that is on for 10 hours per night set at 750 watts uses exactly the same amount of electricity as one set for 1500 watts and is on for 5 hours. The heat output will also be exactly the same. The only difference may be more wear and tear on the mechanical parts, which would go through more on/off cycles on the high setting. This assumes your night temperatures do not drop too low for the low setting to be able to maintain the desired temp. inside the greenhouse.

If its the type with a fan, the bearings in the motor are the weak point, and will likly fail with a year or two of heavy duty use, no matter what setting you use.
As for saving power, the only way I know is to use a higher voltage heater. Generally the higher the voltage the less amps it will pull.

My heater does have a fan. it actually have a four year warranty but for 30 bucks, I don't really care if it wears out in 3 years anyways.

Right now I have orchids, hoyas and African violets. I may move the violets in for the winter. Then the greenhouse can be a little lower in temp without me worrying.

It doesnt matter about the heating cycles as long as you have the correct temp you want. The lower the temp the longer your cycle might be. I bet you money, if you have a temp of 10°F outside your heater might stay on all day and night.

I assume you are correct.
Right now it is 28 degrees and the heater is on the low setting and still cycling on and off.
I am going to take out the cold sensitive plants tonight and then put up some bubble wrap I bought.

If it gets colder you will need to put it on high but if you have the heater on a long extension cord the high setting could cause it to heat. Heats causes resistance and that will take more power to run it so if it is on a long cord it probably will be more efficient on low.
The heater should be on a 12GA. wire for 1500 watts

No, I don't have an extension cord. I hate them! I did check the cord after a long cycle and haven't found that it heats up at all yet. I made sure my new heater had a nice cord. I have a milk house heater but the cord isn't as thick. I have that one for backup.

If it is cycling quickly on low that means you need more hysteresis. You probably can't adjust the setting on the heater, but you could increase the thermal mass of the greenhouse.

Not sure what that means! I have the greenhouse pretty full of buckets. I don't think I can add more.
What exactly would you call a quick cycle??
I am not sure off the differential on my cheapy heater. That could be a problem also.

well adding more buckets probably won't help, because there is too much resistance to getting the heat into the water. You need to couple the water more directly to the heater (how is left as an exercise).

Does your heater have a fan? Is the thermostat separate?

yes, it has a fan and a thermostat, which is a dial. The dial doesn't have temps on it though like "real" greenhouse heaters.

Odd. How often is it cycling at 750W? Once every few minutes seems reasonable for a cheap heater. You can use your heater to work out the insulation value of your greenhouse - if it is on 30% of the time when it is 70F inside and 30F outside, and the heater is delivering 750W, then you are using 225W to maintain a 40F difference, that's about 20 btu / hour.degF for the whole greenhouse. Going to 20F outside will require on average an extra 112W. More wind can mean more power depending on how well sealed your GH is.

does your electric co. have demand rates? if so, I'd keep it at 750 instead of 1500. but, I'd keep it at the lower setting for other reasons anyway and point it directly at something with some thermal mass. my gas furnace is pointed at my horse trough "pond"

Hi!
I don't think my electric company has demand rates. i did turn up the heater to high last night. It was 15 degrees out and pretty windy so the low setting had a difficult time keeping the temp steady. with Hi, it cycled on and off, not sure how long it was on and off though, but it kept the temperature much more steady than at the low power. It was 58 to 60 degrees through the entire night.
As first I was kinda worried about power consumption but then I thought that hey, I don't spend money anywhere else so why can't I spend an extra 100 dollars a month for this? I doubt it would be that much extra really. My stupid electric company doesn't read our meter so we were about a month behind in calling it in. I sent in the new numbers for last month so it won't be untill the end of this month that I can figure out how much extra electricity it will be.

Also we did have alot of snow during the night. I did notice that some there was some ice on the greenhouse, meaning some heat must be escaping however there was still snow on it so I can't imagine too much heat is escaping. I have double walled polycarb adn then I just put up two layers of plastic sheeting on the inside. I can't tell if the sheeting made a difference really.

this is obviously my first year with the greenhouse. I saved a long time for it and really want to use it all year long. I may ponder buying a vented empire heater for next year. I don't know if they can run gas to the greenhouse or not. We have one of the empire heaters in our dining room and I really love it. I don't want to have an unvented heater because of my orchids and it is a smaller greenhouse and I don't want to worry about carbon monoxide.

You seemed unsure of the low wattage setting. I use an electric heater for my 8.5 x 8.5 ft Rion and the low setting is 1300 watts, the high is 1500W. That's also the case for all the similar heaters I have seen. Double check your unit, the difference may not be as much as you think.

You might be able to read the electric meter yourself to get an idea of consumption.