Time-of-use electricity tariffs: when changing the clock can help

Identify the demand you can move

Some demand is flexible: a car charging overnight, a battery charging for later or an appliance cycle that can run at a suitable time. Other demand is harder to shift without changing comfort or normal household routines.

List the flexible loads and their actual energy needs. Avoid counting the same energy twice, such as adding the EV’s whole annual demand to a battery model when the car would already charge directly at the cheap rate.

Calculate an energy-weighted rate

For an example tariff with 10p cheap energy and 30p higher-rate energy, a household buying 40% in the cheap period pays a weighted 22p/kWh. The calculation is 0.40 × 10p + 0.60 × 30p.

If only 10% of energy is cheap, the weighted rate is 28p. The cheap headline has not changed, but the household outcome has. Add the daily standing charge before comparing a full-year bill.

Match a battery to its window

A large battery cannot necessarily fill during a short cheap period. Charging power, system limits and other simultaneous loads matter. Ask whether the proposed charge schedule is achievable within the allowed window.

Include losses. Delivering 8 kWh at 90% round-trip efficiency needs about 8.89 kWh of charging energy. If the useful tariff gap becomes too small, a cycle may provide little benefit or even increase energy cost. The battery scenario tool can show that effect.

Treat heating and exports separately

A heat pump’s cheapest permitted operating period is not automatically the period in which the home needs heat. Comfort, the building and the hot-water system affect how much demand can shift. Use a realistic seasonal efficiency and weighted electricity rate.

For solar exports, use the rate earned when energy actually leaves the property. A valuable evening rate does not apply to midday exports. Storage can change timing, but it also changes losses and the opportunity to use the energy at home.

Before changing tariff

• Check eligibility, metering and equipment compatibility.
• Read the complete import and export terms, including restrictions.
• Model the expensive periods and standing charge.
• Test a less favourable schedule or tariff spread.
• Understand the automation settings and what happens if connectivity fails.
• Check exit conditions and save a copy of the dated offer.

The tariff tool compares two offers using your weighted rates. It keeps imported and exported volumes fixed, so it is a first check rather than an optimisation engine. A supplier-specific analysis should use your interval data and the actual control strategy.