What heat demand means for your house
Heat demand is the amount of energy your home needs over a year to keep its rooms warm and its hot water running, measured in kilowatt-hours (kWh). It isn't the same figure as a boiler's rated output. A boiler or stove is sized to cope with the coldest day of the year, but the number that decides your fuel bill, and the number any fuel comparison has to start from, is the annual total.
Two houses of identical floor area can have very different heat demands. A solid-wall Victorian terrace with no cavity insulation and single glazing can need two or three times the energy of a well-insulated new build the same size. That gap changes everything downstream of it, from the size of a pellet hopper to the annual fuel spend to whether a heat pump will cope on the coldest nights.
Why the number comes before the equipment
Comparing oil, a pellet boiler and a heat pump without knowing your heat demand first is like comparing fuel economy figures without knowing how far you drive. The price per kWh of a fuel only tells you something once you know how many kWh you'll use across a year. A system sized against the wrong heat demand will either run short on the coldest days or sit oversized and inefficient for most of the winter.
This figure also decides how much pellet storage a house needs. A higher annual heat demand means more fuel burned over the year, and more storage space required, regardless of how the house looks from the outside. The pellet storage calculator guide sets out how storage volume follows from fuel use once you have a heat demand figure to work from.
How a heat demand figure is worked out
The calculation behind a heat demand estimate rests on a small number of inputs. The home heat demand estimator uses the same three.
Floor area and insulation standard
Floor area sets the starting point, multiplied by a factor for how well the building holds heat. Building Regulations Part L sets insulation standards for new work, but most existing homes were built to older, looser standards. An estimate has to ask what era the walls, roof and glazing belong to.
Temperature difference and location
Heat loss is driven by the gap between inside and outside temperature. A target indoor temperature is set against local winter averages. A house in the north of Scotland has a larger gap to heat against through the year than one on the south coast, even with identical construction.
Hot water use
Space heating is only part of the total. Hot water demand depends on the number of occupants and how it's used, and it's counted separately because it continues through the summer months when space heating demand drops close to zero.
Running these through the home heat demand estimator produces a range, because the inputs feeding it are estimates too. It also shows the working behind that range, which is useful for checking what a change, such as loft insulation, would do to the total without starting the calculation again from scratch.
Common mistakes when estimating heat demand
- Treating a boiler's rated output as the annual demand. A boiler is sized to cope with the coldest day of the year, while the house uses a fraction of that output across most of the year.
- Using a neighbour's fuel bill as a stand-in for your own demand. Occupancy patterns, thermostat settings and how long doors are left open shift the figure, sometimes by a large margin, even between near-identical houses.
- Leaving hot water out of the total because it looks small next to space heating. In a well-insulated house it can be a sizeable share of the annual figure.
- Treating an Energy Performance Certificate (EPC) rating as the same thing as heat demand. An EPC is built for comparing properties on a standard basis.
What the figure can tell you, and what it can't
A heat demand estimate is useful for comparing fuels on a like-for-like basis and for working out roughly what a system will cost to run. Multiply the annual kWh figure by the price per kWh of the fuel under consideration, adjust for the appliance's efficiency, and the result is a workable estimate of annual fuel spend. The running costs and maintenance page sets out that same arithmetic in more detail.
It can't account for how a household lives in the house. A home with the heating on from early morning will use more energy than the same house occupied by someone out at work all day. It also can't replace a heat loss survey carried out by a competent installer before work begins. A system specified by someone working to MCS (Microgeneration Certification Scheme) standards needs a room-by-room calculation. The home heat demand estimator is an arithmetic illustration built on the figures you put into it. It isn't a quotation, and it can give a wrong answer if your assumptions about walls, windows or occupancy don't match the house itself.
Where this figure fits into the bigger decision
Heat demand on its own doesn't settle whether biomass, oil or a heat pump suits a house. A high, draughty heat demand sits awkwardly with a heat pump's comfortable operating range, while a low, well-insulated demand suits one well. The suitability assessment and the biomass against a heat pump comparison both start from this same figure, so a reasonably confident estimate is worth having before working through either.