Our pools are insulated to retain heat even during cold winters. To use your pool year-round, you simply need to set an ambient temperature on your thermostat, ensuring the pool will not drop below 0 degrees (if your water freezes it can damage the pool, pipes, and pumps), or higher to your preferred warmth.
If your pool has a cover, it retains heat extremely well, such that you can maintain the temperature consistently without too much cost—the average price for a natural gas heater is around $1200/year to operate; for propane heaters, the price is closer to $1400/year. Electric heaters will typically cost much more in the long term, depending on the size of your pool and the frequency of its use.
You also have the option to simply drain the Modpool and not use it over winter. If you choose to do that, you can follow these linked steps to protect your Modpool: Winterizing Steps