Storage heater wiring

It means you don't get to use any more cheap electricity in the fourteen hours than you would in seven.
Ignoring interim losses, you can't anyway. A heater which stores XkWh will store that much whether you fill it up over 14 hours or 7.
Isn't that what I wrote?
XkWh in T hours = XkWh in 2T hours.

I did mean it as a benefit to the supplier; not the consumer.

Do your interim losses include 'letting it all out' ?
Why do they normally only give you seven hours, then?

Apart from that, I thought they only let you have a cheaper rate during the night because demand was low.
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Usually with tariffs that give more than 7 hours at a cheaper rate, the additional hours come in the late afternoon or early evening as a top up. This was to get around one of the disadvantages of storage heaters used with economy 7, in that they would start to cool just as everyone was getting home. The additional top up period gives them a boost.
This thread on Money Saving Expert details the Option 14 hours, and the third post mentions that you only get the value out of Option 14 if you have "half rated" storage rads. this ties in with the OP's original query.

The Scottish power tariff document has the following description

Option 14

This tariff has a Daily Service Charge, which applies regardless of usage and 2 different kWh rates. This tariff provides 14 hrs of off peak electricity

(defined as low rate) and 10 hrs of peak electricity (defined as peak rate), in each 24 hr period.

The off peak (low) period is split into two segments, one overnight and the other during the afternoon. At ScottishPower’s discretion the times are

defined as:

3hrs between 1300hrs and 1600hrs (GMT)

11hrs between 2100hrs and 0800hrs (GMT)

Option 14 is controlled using meter equipment with an electronic timeswitch and should be consistent across all sites.

Peak is defined as being at all other times.

The tariff was designed specifically for use in conjunction with specially designed storage heaters equipped with half rated elements. The principle

being that the heaters would charge for twice the normal period (compared with Economy7) but with the half rated element would only consume the

same number of units, thus making for more comfortable heating without a significant increase in running cost.

Under normal circumstances this tariff would only be made available to customers with the appropriate heating system designed for a 14 hour

charge. Customers with heaters containing elements that are suited to a 7 hr charge would have to install a timing device to prevent overheating of

the full rated elements.

In addition there are a small number of domestic customers with particularly high-energy requirements who may benefit from this tariff. Typically

customers who have high underlying consumption (such as swimming pools) can benefit by maximising the use of such installations during the 14

hours of the low rate.

When compared to Economy 7, the Option 14 Peak kWh rate is higher than the Economy 7 Day kWh rate and the Low kWh is higher than the Economy

7 Night kWh rate.
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