It doesn't really matter. Fine, use US$0.12/kWh, which is absurd for night rates. In Europe, that makes it $12k instead of $13k after 2000 days of 40km+ driving, and $20k+ over PHEV's lifetime. That easily pays for PHEV equipment and gives owners a net saving.
Here's what it looks like in Ontario (Canadian dollars):
http://www.ontario-hydro.com/index.php?page=current_rates
In the UK at least, you are missing:
a) A higher service charge
and
b) An increase in cost in the rest of the electricity used (for TVs, PCs, lighting, etc. etc.) unless you can hit somewhere in the region of 50% daily energy usage during off-peak times.
Some calculations using a purely electric vehicle:
The Nissan Leaf is £10,000 more expensive than a Golf Bluemotion 2.0L (£5,000 after a government rebate).
The bluemotion does 4.3L/100km. Diesel costs £1.35/L here at the moment, which works out as £5.81 per 100km. The Nissan Leaf travels 100km on 21KWh of power, which at 5p per KWh costs £1.05, so the Leaf ends up as £4.76/100km cheaper, or £0.0476/Km.
To break even on the initial £5,000 expense of choosing a Leaf over the Golf, you would need to travel (5,000 / 0.0476) Km. This works out as 105042 Km. At 40km/day, that is 2626 days. If you drive 40km *every* day for 7 years, you still won't have broken even on the initial outlay. If the government wasn't paying £5,000 for you to take the car, it would be almost 14.5 years before you were back where you started.
If you only commute to work and don't do any travelling at the weekend, it gets even worse, needing 10 years to break even with the government subsidized value, and 20 years without.
The Leaf's battery is guaranteed for 8 years or 100,000Km - Nissan won't even guarantee the battery long enough for you to break even on the initial overspend.
So overall you are left with a vehicle that costs more unless you own the car for at least 7 years, driving 40Km each and every day will cost you more money over the lifetime of the car. After 10 years, the car will only have 70-80% of its initial range due to battery degredation, and if you ever do make a long journey you have to stop for 30 minutes to recharge after an hour of highway driving, lengthening any journey over 100km or so by about 50%.
Now, how does that compare to the Volt?
The Chevy Volt is slightly worse in terms of KWh/100km (22.3 vs 21 for the Leaf) so it will take longer to recoup the cost. It is also more expensive, costing about £35,000 (before any rebates) in Europe, meaning that you'll need to recoup £10,000 - £15,000 (vs. £5,000 - £10,000 for the _more_ efficient Leaf). The fuel economy on the Chevy Volt is 4.7L/100km, which is worse than the Bluemotion Golf, and any time spent using this method of power is prolonging the time it takes to catch up with the ICE car. The Chevy Volt has the same warranty on the battery as the Leaf - given the decreased efficiency and increased cost of the volt over the leaf, you almost certainly won't make your money back before the battery is out of warrenty.
For me, it doesn't seem economical to run a pure-electric, or PEHV given current premiums paid for this sort of car over fuel-efficient ICE cars.