Interested in your calculations for 2kl, do you have a small, highly efficient house? For my house, IIRC I needed something like 3000L (glycol, so a little less capacity than pure water) at 30C to maintain 16C in the house for 12h. That's calculating losses at average winter night temps of -8C and a relatively efficient adobe house of 150m2, and including estimated losses for a buried tank surrounded by foam installation.

Roughly: 3kw/hr worst case home losses, times 12h is 36kw, 36/0.00114 kWh/L is 31.5k liter-degrees, 31.5k/14 degree temp drop is 2.2kL, so 3kL inclusive losses. Experimentally verified heat loss calcs after installation of the 9kw resistive boiler which used around 30kwh for the coldest 12h winter nights which ends up being about a 50% duty cycle at the medium heat setting of 5kw. Yes my electricity bill was $500/month for two months a year when pulling it all from the grid.

If I was building the house i'd spec a 1m mixed layer slab and run two layers of hydronics through it. The bottom layer is the heat storage side and the top layer is the home comfort side. The waste heat from the storage dumps into the house and you've got a ready made heat battery right where you need it. Run your resistive boiler while the sun is shining to get your heat battery toasty and at night use your pumps to move the heat up when home envelope losses are more than the heat battery leaks up through the floor.

Heat pump didn't make sense in my climate because there is no need for cooling, when heat is needed it's usually way too cold for heat pumps to be efficient, and we have basically unlimited sun and therefore energy. High desert New Mexico.

I found the book "heating with renewable energy" helpful when designing my system