5000 BTU or 5000 KW...?

It is listed as 5200 btu in the amazon website https://www.amazon.com/dp/B01M0AGJIQ/ref=cm_sw_r_other_apa_i_XsE-Eb60BXKDM

A 1500 W electric space heater will provide about 5000 BTU. That is probably enough to keep the a 500 square foot space in the 45-50 degree range to keep pipes from freezing. It is hard to tell without knowing the insulation values and the lowest outdoor temperature.

Are you worried the main heater will run out of propane? If there is a power outage the electric heater will also fail.

Thanks for the information and I am mainly worried about the main heater going out as it is a Siegler heater and looks like it was built in the 1960s. It is run fine but who knows how long it'll last. I was just thinking that if it went out during a holiday or difficult time to find somebody come out for a replacement it would be nice to have a backup system.

Where do the pipes run? If they are inside the house, that should be fine. If they are in an unheated crawl space or attic, then who knows?

In the dead of winter, when you aren't at the home, do you have a smart remote device that will sense the temperature and turn on the heater?

Another option is to install electric heat cable along the water pipes. Frost King makes various lengths with a built-in thermostat. You can buy it Home Depot.

Thanks for the additional replies and I will check out the Frost King product. The pipes do run inside and normally in the winter I drain out the water but going forward him I'm going to be spending more time here in the winter.

Use a modern self-regulating heating cable such as Raychem Winterguard. The old Frost King style heating cables with a thermostat were notorious for burning out if not properly wrapped with continuous, intimate pipe contact. The modern semiconductor based cables self-regulate along their entire length and cannot overheat, they are worth the higher cost.

https://www.outdoorradiant.com/index.php?route=product/category&path=72