Well, there are many ways to calculate; but currently, if the BEV-specific incentives were removed tomorrow, and the Model S were instead taxed like a plug-in hybrid, the only applicable taxes would be VAT.
Aside from VAT, there are four factors that are taxed:
1. CO2 emissions (This is zero for BEVs, and leads to a 101k NOK deduction which applies to the other factors.)
2. NOX emissions (This is zero for BEVs.)
3. ICE hp (This is zero for BEVs. All electric hp are tax exempt, for hybrids, plug-in hybrids, BEVs and FCVs.)
4. Weight (This is progressively taxed per kg. Plug-in hybrids get a 26% deduction to the weight and traditional hybrids get a 10% deduction.)
What happens if a Model S is taxed like a plug-in hybrid is that the weight will get a 26% deduction, so the 2100 kg Model S will be taxed like it weighs 1554 kg. That amounts to a tax of 94k NOK. From this sum, the 101k CO2 deduction is applied, and the resulting tax bill ends up at -7k NOK (rounded up to zero). Plus VAT.
What happens if a Model S is taxed like a traditional hybrid is that the weight will get a 10% deduction, so the 2100 kg Model S will be taxed like it weighs 1890 kg. That amounts to a tax of 161k NOK. From this sum, the 101k CO2 deduction is applied, and the resulting tax bill ends up at 60k NOK. Plus VAT.
What happens if a Model S is taxed like a regular fossil car is that the weight will be taxed by 202k NOK. From this sum, the 101k CO2 deduction is applied, and the resulting tax bill ends up at 101k NOK. Plus VAT.
You can of course argue that the full power of the car should be taxed, not just the ICE hp. This requires a reshaping of the car tax system, affecting hybrids, plug-in hybrids, BEVs and FCVs, and the net result will be that the power of a P85D, with 66 kW (maximum constant power, according to some EU standard) still wouldn't be powerful enough to result in a tax.
Okay, so maybe you also change the car tax system to use the peak power output - in this case the Model S would definitely result in a major tax, though still significantly less than it's ICE equivalents, due to factors 1 and 2.
Basically, Norways entire car tax system is built up around rewarding the environmentally friendly cars and punishing the dirtier cars. You have to throw all that out the window for a Model S to be taxed as heavily as it's ICE equivalents, and that's just not going to happen. It would require a major political shift, especially considering that the current BEV incentives have a 75% approval rating in the population.