Heating systems for Nepenthes greenhouses will be a needed piece of equipment unless you live in a very warm year round environment. Heaters will be needed primarily for winter nights, and to a less degree, very cloudy winter days. Any kind of natural light will heat a greenhouse, even on moderately cloudy day. Full sun, even on the coldest of days, will keep the interior of a greenhouse warm to hot. In fact, a well insulated greenhouse may have to vent hot air during sunny winter days, even if the temperatures are freezing outside.
The use of electric heaters in small structures can work, as long as the winters are relatively mild. Otherwise, your electric bill will become prohibitive, and small heaters will not be able to keep the temperatures up. In larger structures, natural gas or propane heaters are the only way to go. They use a small amount of electricity for the fan, but heat by combusting the gas. These heaters will need proper venting equipment to keep deadly carbon monoxide gases from building up inside the greenhouse.
Your greenhouse manufacturer will help you size the appropriate heater for your greenhouse. If you use propane, the heater size and amount of heating needed will also help you size the propane tank needed. (Most propane heaters can also be fitted for natural gas, if its available in your area.) In a very mild area, temperatures may only rarely approach freezing. In this case, small propane "bottles" might be more feasable than a propane tank, which would be needed where winters can be cold.
Heating is likely to be the largest dilemma growers in small greenhouses are likely to face. A small greenhouse in a cold environment is going to have to be heated. Whether or not you can get by with an inexpensive electric heater will depend on where you live, how insulated your greenhouse is, and how big of a structure it is. It may be more cost effective to install a gas heater capable of easily heating the structure, which will result in a lower utility bill, than to start off with an inexpensive electric heater(s). One bad month of weather, and the gas heater can pay for itself in utility savings alone. Don't think about heating when winter approaches. It needs to be in place well in advance. The price of propane is much less in summer than it is in winter, so that is the time you need to purchase it.
The heater must be operated on a thermostat, or better, computer controlled. You dont want to have to get up in the middle of the night to turn on the heater if the temperatures suddenly drop, and failing to keep the temps above freezing will kill nearly every Nepenthes. They certainly grow slower if the temps aren't kept in the optimum range.
For my large 22'x50' greenhouse, I use a Reznor 125000 BTU propane gas heater. A 75000 BTU heater would have worked as well. At night, I have the heater in the highland house turn on at 50 degrees. It runs for about 3 minutes before the temperatures hit 60. The computer turns the heater off at 60, but the heater runs for about another minute in cool down mode, and the temperature can rise to 64. Over 45 minutes to an hour and a half, the temps can slowly drop back down to 50. So the heating cycle is on for about 5 minutes, off for an hour. Even on the coldest of nights, the heater rarely operates for more than an hour of total time. The heater rarely operates during the day, and only on very dark days in the winter when the outside temps dont get out of the 30's. That in of itself is rare here in Memphis, Tn, as cloudy days in the winter usually keeps the temperatures warmer. More common is clear nights, which can cause a significant drop in temperatures. The heater is fed from a 200 gallon propane tank.
The fan located inside the heater is not large enough to circulate the hot air through a large greenhouse. The HAF circulation fans ensures that the air in the greenhouse is moving in a circular pattern, and this disburses the heat throughout the greenhouse.