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If you need 20kW and install a system that can output 20kW under the worst situation (resistive heating), you are probably averaging about 7kW of electric usage when the heating is on to generate 20kW (about 3:1 ratio on average)

Setting aside capital costs that's going to cost you 7kWh per hour of heating. An oil boiler will cost 20kWh per hour of heating.

If your oil costs 40c per litre/$1.50 per gallon and each litre delivers 10kWh, that's about 80c/hour to heat

If your electricity costs 10c per kWh, that's 70c/hour to heat, that's a win

If your electricity fosts 15c per kWh, that's $1/hour to heat, that's a loss