🧱 Wall Loss
100 m² × 2.10 × 15 degrees = 3,150 Watts (3.15 kW).
Uninsulated vs. Insulated House: Calculating kWh and Real Cost
We all know insulation saves money, but how much exactly? To stop talking in vague percentages like "up to 50%", it is time to get the calculators out.
In this article, we will take the standard virtual "10x10 House" (100 m²) and put it through the test of a typical Greek winter. We will calculate the energy leak (in kWh) and convert it into hard euros.
1. The Basic Calculation Rule
To find how much heat a wall loses, physics uses a very simple multiplication: Energy Loss = Area (m²) × U-Value × Temperature Difference (ΔT).
Our Scenario: It is January. Outside it is a steady 5°C. We want the thermostat in the living room fixed at 20°C. So the "Temperature Difference" (ΔT) our wall must fight is 15 degrees.
For simplicity, we will focus on the two largest surfaces of the house: the external walls (100 m² net) and the roof (100 m²).
2. Scenario A: The Uninsulated "Sieve"
Our house was built in 1980. The walls are double brick without insulation (U-Value: 2.10 W/m²K) and the roof is bare concrete (U-Value: 2.50 W/m²K).
Let us see what we lose in just one hour:
🔝 Roof Loss
100 m² × 2.50 × 15 degrees = 3,750 Watts (3.75 kW).
📊 Total Loss
The house steadily loses 6.9 kW every hour.
🔢 To maintain 20°C for 24 hours, the boiler must replenish this loss by producing 165 kWh of thermal energy (6.9 kW × 24 hours).
3. Scenario B: The Energy Shield
We decide to renovate. We install External Insulation (EPS 8cm) on the walls and insulate the roof with extruded polystyrene (XPS 8cm). The U-Value plummets to an impressive 0.35 W/m²K for both.
Let us redo the maths for the same hour, with the same cold:
🧱 New Wall Loss
100 m² × 0.35 × 15 degrees = 525 Watts (0.52 kW).
🔝 New Roof Loss
100 m² × 0.35 × 15 degrees = 525 Watts (0.52 kW).
📊 Total New Loss
The house now loses only 1.04 kW per hour!
🔢 Over 24 hours, the boiler needs to produce just 25 kWh (instead of 165 kWh). We just "eliminated" 85% of the losses!
4. How Many Euros Are Those kWh?
Let us translate this huge gap into real money. We will assume we burn heating oil and that, due to old boiler inefficiencies, each thermal kWh costs about €0.12.
Let us see the bill for a "heavy" 100-day winter (where the average outdoor temperature is 5°C):
❌ The Uninsulated House
Burns 165 kWh/day. For 100 days, we need 16,500 kWh.
Winter Cost: 16,500 × €0.12 = €1,980.
✅ The Insulated House
Burns 25 kWh/day. For 100 days, we need 2,500 kWh.
Winter Cost: 2,500 × €0.12 = €300.
💰 Your Net Gain: €1,680 every winter! Over 10 years, you save almost €17,000, a sum that more than covers the cost of the entire external insulation and roof insulation.
💡 Final Conclusion: The maths are relentless and never lie. An uninsulated house is not merely uncomfortable, it is a "black hole" that swallows your salaries. The U-Value is not a theoretical concept for engineers; it is the ultimate multiplier of your bank account!
Related Articles
The 10x10 House: The Standard Model for Calculating Heat Losses U-Value (Thermal Transmittance): What It Is & How It Is Calculated Insulation Thickness: Are 10cm Worth It Over 5cm? (Calculation) Insulation investment Payback Period
Thermal Physics & Calculations: From Theory to kWh and Euros
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