How we estimate air conditioning running costs

Understanding our running cost estimates

At BOXT, we want to give you an accurate picture of what it costs to run your new air conditioning unit. Because every home is different, we use a standardised "Baseline Room" model to ensure our estimates are fair, consistent, and based on real-world UK data.

1. Room sizing

To calculate how much energy is needed to cool or heat your space, we look at four typical room sizes:

• Small (12m²): e.g., A home office or nursery.

• Medium (23m²): e.g., A standard double bedroom.

• Large (35m²): e.g., A spacious living room.

• Extra Large (50m²): e.g., An open-plan kitchen and dining area.

2. Our calculation methodology

We break your costs down into two distinct phases:

A. The startup phase (The initial push)

When you first turn your AC on, it works at its maximum power to reach your desired temperature quickly.

• Cooling: We calculate the energy needed to drop the room temperature by 7°C (e.g., from 29°C down to 22°C).

• Heating: We calculate the energy needed to raise the temperature by 8°C (e.g., from 16°C up to 24°C).

B. The maintenance phase (More efficient running)

Once your room reaches the right temperature, the smart "Inverter" technology in your unit slows down. It then uses only a small amount of power to maintain that comfort level, offsetting the natural heat loss of your home.

3. Where do the numbers come from?

Our estimates incorporate three key industry benchmarks:

• UK insulation standards: We assume your home meets "Average UK Standards" (Double glazing, wall insulation, and loft insulation) as defined by UK Building Regulations (Part L).

• Occupancy heat: Humans naturally give off heat. Our estimates account for adults resting in the room, using data from the CIBSE (Chartered Institution of Building Services Engineers).

• Real energy prices: We update our costs based on the latest Ofgem Price Cap.

The fine print

These figures are for illustrative purposes to help you compare your options. Your actual costs may vary depending on how much you pay for your electricity, how many windows you have, which direction the room faces (south-facing rooms get more sun), and your specific insulation levels.

Quick Reference Table

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Technical Substantiation: AC Operating Costs

1. Variables and Definitions

To ensure transparency, we define the variables used in the calculations as follows:

• Pmax (Maximum Power Input): Calculated as Highest Cooling Capacity/Lowest Efficiency. This represents the peak electrical draw during the "startup" phase.

• HL (Heat Load): The thermal energy (in Watts) required to maintain a specific temperature against external conditions.

• ST (Startup Time): The duration (in minutes) the unit runs at maximum power to reach the target setpoint.

• SEER / SCOP: Seasonal efficiency ratios for cooling and heating, respectively.

• R (Unit Rate): The cost of electricity, set by the Ofgem Price Cap.

2. Heat Load (HL) Derivation

The Heat Load figures are calculated based on room volume and a standard UK U-value (insulation performance).

Methodology: These figures assume a standard ceiling height of 2.4m and a "Modern Average" insulation profile (U =x 0.35 for walls, 1.6 for glazing). The cooling load is lower because the temperature differential (Delta T) is smaller (7 degrees celsius) compared to the heating differential (19 degrees celsius from 5 degrees celsius outside to 24 degrees celsius inside).

3. Step-by-Step Calculation Breakdown

A. Startup Costs (The "Sprint" Phase)

When the AC is first turned on, it operates at Pmax to bridge the temperature gap quickly.

Cost = Pmax x (ST/60) x R

• Logic: We convert the Startup Time (ST) into an hour fraction (ST/60), multiply by the power draw in kW, and then by the price per kWh.

• Assumptions: Small rooms reach temperature faster (20–25 mins) than Extra Large rooms (50–60 mins) due to lower air volume.

B. Maintenance Costs (The "Marathon" Phase)

Once the target temperature is reached, the unit modulates down to match the Heat Load (HL).

Cost-Maint = HL/(Efficiency x 1000) x R

• Logic: We divide the required Heat Load by the efficiency rating (SEER for cooling / SCOP for heating) to find the actual electrical input. We divide by 1000 to convert Watts to Kilowatts.

• Efficiency: Using SEER/SCOP accounts for seasonal variations in performance rather than a static EER/COP.

4. Data Sources and Standards

To maintain authority, the following sources are cited for the underlying data:

• Electricity Pricing: Ofgem Price Cap Data. National average for Direct Debit customers.

• Insulation Standards: Based on Part L of the UK Building Regulations, assuming "Reasonable" performance for existing dwellings with retrofitted double glazing and loft insulation.

• Occupancy Heat Gain: Based on Chapter 7 (Internal Heat Gains) CIBSE (Chartered Institution of Building Services Engineers) Guide A, which estimates a sensible heat gain of approx. 75W per resting adult.

• Equipment Performance: Efficiency ratings (SEER/SCOP) are sourced from standard ERP (Energy-related Products) Ecodesign labels for modern inverter-driven split systems.