05Energy
What the house uses and what the roof gives back
A heat pump and solar panels on the roof. Change the assumptions and watch the electricity bill follow.
Panels on the roof
36 panels of 430 Wp, all on the south plane. Tap a roof plane to switch it off or on.
Room for 36 panels on the selected planes, 76 panels on the whole roof.
Money
Electricity bill a year
- Imports
- CZK 5,741
- Standing charges
- CZK 7,200
- Export income
- CZK 9,143
- Net a year
- CZK 3,798
By month
Electricity use by purpose and solar production. Tap a month to see its average day.
JulyUse 694 kWhSolar production 2,038 kWh
| Month | Use | Production |
|---|---|---|
| January | 693 kWh | 567 kWh |
| February | 538 kWh | 843 kWh |
| March | 456 kWh | 1,466 kWh |
| April | 376 kWh | 1,795 kWh |
| May | 706 kWh | 1,956 kWh |
| June | 675 kWh | 2,010 kWh |
| July | 694 kWh | 2,038 kWh |
| August | 695 kWh | 1,873 kWh |
| September | 680 kWh | 1,554 kWh |
| October | 394 kWh | 1,095 kWh |
| November | 514 kWh | 566 kWh |
| December | 673 kWh | 441 kWh |
Average day, July
Where the house gets its power, hour by hour: the average day, or only clear, partly cloudy, overcast or very overcast days.
Where the heat goes
| Element | Aream² | UW/(m²·K) | LossW/K | Share |
|---|---|---|---|---|
| Ground-floor slab | 205.5 | 0.147 | 30.1 | 22% |
| 28.3 | 0.950 | 26.9 | 20% | |
| south | 21.4 | 0.950 | 20.3 | 15% |
| west | 7.0 | 0.950 | 6.6 | 5% |
| Ceiling below the cold loft | 205.5 | 0.112 | 20.8 | 15% |
| 109.0 | 0.155 | 16.9 | 13% | |
| north | 41.9 | 0.155 | 6.5 | 5% |
| east | 30.1 | 0.155 | 4.7 | 3% |
| south | 27.4 | 0.155 | 4.2 | 3% |
| west | 9.5 | 0.155 | 1.5 | 1% |
| Ventilation (137 m³/h, heat recovery 88%) | 16.5 | 12% | ||
| 12.0 | 0.800 | 9.6 | 7% | |
| east | 5.9 | 0.800 | 4.7 | 4% |
| north | 5.0 | 0.800 | 4.0 | 3% |
| south | 1.1 | 0.800 | 0.9 | 1% |
| Thermal bridges | 357.7 | 0.020 | 7.2 | 5% |
| Walls and doors to unheated spaces | 17.6 | 0.432 | 3.8 | 3% |
| External doors | 2.9 | 0.900 | 2.6 | 2% |
| Total | 134.3 |
The U-value of the floor includes the soil beneath the house (EN ISO 13370), so it differs from the floor build-up alone. An allowance of ΔU = 0.02 is added for thermal bridges.
Where the electricity goes
- Appliances and lighting3,000 kWh
- Ventilation fans421 kWh
- Heating (4,011 kWh of heat)989 kWh
- Hot water (3,124 kWh of heat)1,041 kWh
- Pool (filtration and heating)1,643 kWh
The pool has its own line: filtration and its heat pump from May–September. It is not part of the house's heating demand.
Design heat load
At −12 °C the heat pump delivers 6 kW (8 kW rated) and covers 153% of the design heat load.
Summer sun through the glass
In summer (June–August) 2.9 MWh of solar heat comes in through the glazing. The external blinds, which lower themselves in direct sun, keep 2.2 MWh of it out.
An indicator, not a cooling calculation.
How it is calculated
The heat loss comes from the areas and build-ups of the model, measured to the wall axes. The ground floor follows EN ISO 13370, losses to unheated spaces EN ISO 13789. The heat demand is the monthly balance of EN ISO 13790: losses minus the usable gains from people, appliances and sun through the windows, with the shade of the roof overhang; the blinds stay up in the heating season. The design heat load to EN 12831 is for an outdoor temperature of −12 °C.
A further 6% is added to space heating for distribution and storage losses, and 15% to hot water for tank and pipe losses. The heat pump's efficiency varies with the outdoor temperature; your SCOP sets the annual average. A standing charge of CZK 7,200 a year applies with or without panels.
Solar production comes from PVGIS 5.3 (EU JRC, database PVGIS-SARAH3) for the house's location, a plane tilted at 22° and system losses of 14%. Each panel is calculated for the plane it sits on; the layout, the margins at the roof edges and the light pipes come from the model. Self-consumption comes from an hourly balance of four typical days in each month, including charging and discharging the battery at an efficiency of 90%.
Not modelled: cooling and summer overheating, night setback, shading by neighbours and trees (see the Sun page) and price rises. The payback allows for the lifetime and ageing of the panels and the battery, not for interest.
Main assumptions
- Thermal bridge allowance
- 0.02 W/(m²·K)
- Internal heat capacity
- 260 kJ/(m²·K)
- Fresh air per person
- 25 m³/h
- Minimum air change
- 0.3 h⁻¹
- Soil conductivity
- 2.0 W/(m·K)
- Design outdoor temperature
- −12 °C
- Heat recovery efficiency
- 88%
- Usable battery capacity
- 90%
- Status of the assumptions
- checked against the standards
Indicative figures, not an energy performance certificate. Climate data: PVGIS, EU JRC. Prices: indicative, 2026.