If mains gas is available in the street, oddly enough, use that :-)

Taking 15,000kW total heating cost for heating.

E7 cost...
- Energy = 5p / unit
- Maintenance = zero
- Depreciation = £100/yr or 10000/15000kW = 0.7p / unit
- Effective Cost = 5.7p / unit

Gas cost...
- Energy = 5p / unit
- Maintenance = £100/yr or 10000p/15000kW = 0.7p / unit
- Depreciation = £300/yr or 30000p/15000kW = 2.1p / unit
- Effective Cost = 7.8p / unit

Depreciation is the 10yr replacement of boiler, powerflush, odd
radiator, TRV, improved programmer and such odds n sods.

Notes :-)
1 - If the heating is 30,000kW per year figures change.
2 - If you need peak elec boost, cost is 2.5x HIGHER.



For electric boilers...
#1 Insulation must be very high
a) recent house b) Cavity & Loft insulation and chimney's trickle
vented.

#2 Thermal store is required
The bigger the better to best use cheap rate electricity, with
75-100mm rockwool insulation "sock" rather than the miserable 25-50mm
expanding-foam-in-place stuff.

#3 Electricity rate must be E7 or E10
E7 requires a gargantuan thermal store, E10 can reduce it a little.

#4 You hit the limits of the supply cable.
7hrs at 9kW = 63kWhr... that is not actually a lot.
10hrs at 12kW = 120kWhr... that is pretty reasonable.

Realise the volume of thermal store you need after the house is
initially heated overnight, it's really enormous - 1000L or 1500L. The
figures do not work well, you end up needing peak or E10 because you
just don't have a liquid sodium thermal store handy at B&Q.

Hence they are favoured for well insulated flats, anything else gets
tough - you need 3-phase E7.

The solution would be a 9kW heat-pump (£1k), plus thermal store, plus
backup E7 "brick" storage heaters for the coldest days. That will
become quite a common solution in about 10yrs, CO2 heat pumps whilst
nutty price now (3.7k vs 1k) will eventually drop in price, it is just
a dual-stage DC compressor - hardly rocket science just lack of
economy of scale and screw the early adopters as usual.

As to actual install - make sure the terminals are tight, because
quite a few burn-out the PCBs if they are not.

Some idiot-installs have a) not used a thermal store b) not used a
suitable E7 E10 rate - in either case the result is horrific as you
can imagine.