In my area of N. WI near Ladysmith, the off-peak electric rates are $.055/Kw. I would like to use an electric boiler to heat the slab. This will be a 28" x 48" SIP house. Is it possible to get enough heat into the slab in the 11 hours of off-peak time to carry me thru the rest of the day? I will also have a propane fireplace/stove for back-up & assist.

My radiant slab very rarely runs during the day. My house is a 24 x 50 SIP home.

If you insulate it well, I think you should have pretty good luck with it.

Most likely. We actually had a three story with 1600 sq ft per floor and I could carry the house using off peak electric only down to about 10F. The walkout basement was mid 70’s with the main floor being right about 70 and the upper floor being 65 when we set the floor temp up when it got colder to store more heat through the on peak time. I was going to say if you have natural gas in the upper mid west (where we are) natural gas and off peak electric will swap back and forth as to which is less expensive. We had a forced air natural gas furnace and I would heat which ever was less expensive at the time.

I suspect it will be a stretch in Ladysmith, although any heat you bank overnight at 5.5 cents is money in the bank, pun intended. The formula goes like this: Concrete weighs 145 lb/cubic foot. Specific heat (ability to store it) is .2 per lb. Thus a 1344 sf slab 4 inches is 448 cf, or 65000 lbs, and can store not quite 13,000 btu per degree F. The rest of it is your tolerance for cold and the performance of your radiant system. 85 degrees is considered the maximum comfortable temp for a radiant floor. Figure the air will be 10 degrees cooler than the slab. Cooling to 75 during the day (65 air temp) would get you storage of about 130,000 btu, with little discomfort except to roll out of bed in a warm house. (Radiant feels warmer.) Cooling to 65 (55 air temp) would get you 260,000 btus with modest discomfort but you may be pushing the envelope on your ability to meet heat loss overnight while adding degrees to the slab.

In my smallish house (1600 sf) I will have concrete walls in addition to concrete slab for 100,000 pounds of storage, or 200k btu to meet a design loss of 25k btu/hr. It should carry the house overnight in most weather conditions. My weather is milder at a design loss of 10 degrees, but your house is coasting during the day, while mine coasts at night under more extreme conditions.

A masonry fireplace could warm things up pretty quickly in early evening and add mass for overnight storage. Putting the fireplace in front of south facing windows would be even better.

Water is a more effective storage medium. Heating 200 gallons to 180 degrees would give you 144,000 btu, assuming your radiant system would work down to 90 degrees in the tank. (1 gallon = 8 pounds = 1600 btu/degree in a 200 gallon tank.) Builditsolar.com has lots of links of storage and masonry fireplaces.

Agreed, a water storage tank will store more energy per volume and will eliminate the need for room temperature swings. Off-peak plans vary widely in timing and prices - storage definitely makes sense in some places.

http://hearth.com/econtent/index.php/articles/fuel_cost_comparison_calculator/ is a good link for calculating costs for different fuels