A low unit rate can look attractive on a comparison screen, but it does not automatically make it the best deal for your home. When weighing up fixed vs flexible tariffs, the right choice depends on how you use electricity now and how that use may change after adding solar panels, battery storage, an EV charger or a heat pump.
For many households, certainty is the priority. For others, the chance to shift energy use to cheaper periods could bring greater savings. The key is choosing a tariff that works with your property, your routine and your planned energy upgrades – not simply the deal with the boldest headline price.
What is a fixed energy tariff?
A fixed tariff sets the price you pay per unit of gas and electricity, and usually the standing charge, for an agreed period. This is commonly 12 or 24 months, although terms vary by supplier.
Your bill can still rise or fall because you use more or less energy. What is fixed is the rate you pay, rather than the total amount leaving your bank account each month. This distinction matters, particularly if you are installing a heat pump and expect your electricity consumption to increase while your gas use reduces.
The main benefit is predictability. You know the unit prices for the length of the deal, which makes household budgeting simpler and protects you from price rises during that term. The trade-off is that you may pay more if market prices fall, and leaving early can involve an exit fee.
A fixed tariff often suits households that prefer a straightforward arrangement, use most electricity at similar times each day, or do not want to actively monitor half-hourly pricing.
Fixed does not always mean the lowest cost
A fixed deal should be judged on the full picture: electricity unit rate, gas unit rate where relevant, standing charges, contract length and exit fees. A low unit rate can be offset by a high standing charge, especially in a smaller or highly efficient home.
It is also worth checking whether the tariff is suitable for smart-meter pricing, export payments from solar panels, or overnight EV charging. Not every fixed deal is designed around a modern, electric home.
What is a flexible tariff?
A flexible tariff, sometimes called a variable tariff, allows the supplier to change its rates. Standard variable tariffs can rise or fall, subject to the energy price cap where it applies. They generally have no fixed end date and no exit fee.
This flexibility can be useful if you do not want to commit to a long contract, are waiting for a better deal, or expect your home energy set-up to change soon. For example, you may be planning solar panels or a heat pump and want to review your tariff once the system is installed and your energy use is clearer.
The drawback is less certainty. If prices rise, your unit rates may rise too. A flexible tariff can therefore be harder to budget for, even if it provides freedom to switch without a penalty.
Time-of-use tariffs are a different kind of flexibility
Some flexible tariffs use time-of-use pricing. Electricity costs different amounts at different times of day, with cheaper periods often available overnight or when demand is lower. These tariffs usually require a working smart meter that can send half-hourly readings.
They can be a strong fit for homes with battery storage, an electric vehicle or a heat pump that can be programmed to run more at lower-cost times. However, the savings are not automatic. You need enough control over when you use electricity, and you need to be comfortable with prices that may be higher during peak periods.
Fixed vs flexible tariffs for a home with solar panels
Solar changes the question because you will not buy all your electricity from the grid. During brighter hours, your panels can supply appliances directly, charge a battery or export surplus power.
A fixed import tariff can still work well for solar households, particularly where most electricity is used in the evening and you want stable rates for the energy you buy after sunset. It can provide reassurance through winter, when solar generation is lower.
A flexible or time-of-use import tariff may be more useful if you have a battery. You may be able to charge the battery at a lower overnight price, use that stored energy during higher-priced periods, and reserve solar generation for daytime use. Whether this saves money depends on the difference between off-peak and peak prices, battery capacity, efficiency losses and your household routine.
Do not overlook your export tariff. This is the rate paid for surplus electricity sent to the grid. An attractive export rate may sound ideal, but it is not always best to export every spare unit. Using solar electricity in your home can avoid buying electricity at the import rate, which may be worth more than the export payment. A tailored solar and battery design helps you balance self-consumption, export income and practical daily use.
How heat pumps affect your tariff choice
An air source heat pump moves your heating demand from gas to electricity. It is highly efficient, but it is likely to make electricity a larger part of your household energy bill. That means the electricity tariff deserves careful attention before and after installation.
A fixed tariff can be a sensible option for homeowners who value stable costs and want uncomplicated heating controls. Heat pumps work best when they maintain a steady indoor temperature rather than being constantly turned on and off, so a predictable tariff may suit a consistent operating pattern.
A flexible time-of-use tariff can work well where the heat pump controls, hot water cylinder and property insulation allow some demand to be moved away from expensive periods. For instance, heating water during a cheaper window may reduce costs without reducing comfort. But the home must retain that heat effectively, and settings need to be designed around your household rather than copied from someone else’s.
Before replacing an older boiler with a heat pump, it is sensible to look at insulation, radiators, heat loss and likely electricity demand together. The Boiler Upgrade Scheme may help reduce the upfront cost of an eligible installation, while the right tariff can support lower running costs over the years ahead.
Questions to ask before choosing a tariff
There is no universal winner in fixed vs flexible tariffs. A practical decision starts with a few honest questions. Do you need stable monthly budgeting? Are you likely to move home or change supplier soon? Can you charge an EV, run appliances or store energy overnight? Will solar panels, a battery or a heat pump change how and when you use energy?
Also look beyond the tariff name. Check the standing charge, contract term, exit fee, off-peak windows, peak rates and any requirements around smart-meter data. If you are comparing an export deal, check whether it can be paired with your preferred import tariff and whether there are restrictions on who installed your solar system.
For a household without flexible demand, a competitive fixed rate can be the calmer and more practical choice. For a household with a battery, EV charging and the ability to shift usage, a flexible tariff may offer worthwhile savings. Many homes sit somewhere between those two positions.
Make the tariff part of your energy plan
Your energy tariff should support your home improvements, not be an afterthought once they are complete. A solar-only home, a solar-and-battery home and a property moving from a gas boiler to a heat pump can all benefit from different pricing structures.
Sunny Side Pro assesses the wider energy picture, including how your home uses power, its heating needs and the upgrades that will make the biggest difference to comfort and bills. With professional system design, clear pricing and finance options where suitable, it is easier to plan improvements with confidence.
The most useful tariff is the one you can live with comfortably while your home becomes more efficient. Start with your actual routine, choose technology that suits the property, and review your tariff once your new system has had time to show how it performs.