7
Overall Energy Consumption
Our condo provides more than 3 times the living space targeted for your 300 square foot tiny house. Our energy use for two people living in a 1000 square foot Kingston condo in November 2018 was:
| November | Annual | |||
| Electricity | \$ 69.22 | \$ 710 | ||
| On Peak | 209 kWh | \$ 0.132 / kWh | ||
| Mid Peak | 88 kWh | \$ 0.094 / kWh | ||
| Off Peak | 513 kWh | \$ 0.065 / kWh | ||
| Daily Average | 27 kWh / day | 21 kWh /day | ||
| Delivery | \$ 39.80 | \$ 454 | ||
| Regulatory | \$ 3.53 | \$ 38 | ||
| HST | \$ 14.62 | \$ 156 | ||
| Provincial Rebate | -\$ 9.00 | -\$ 96 | ||
| Total | \$ 118.17 | \$ 1262 |
The condo is all electric, with baseboard resistance heat, electric hot water, electric washer/dryer, so this represents a total energy consumption averaging 27 kWh/day in November or about 21 kWh/day over the year. Your off-grid tiny home will need to get by on much less than this!
Despite complaints about the high cost of power in Ontario, it would be difficult to replace this supply economically from another source. It would also be hard to be less carbon intensive, as the vast majority of electricity in Ontario is supplied by nuclear or hydro generation systems.
Condo Model Characteristics
Later on we will use this condo as an example and will need some basic information at our fingertips. This model of our condo is approximate and requires considerable guesswork about the construction details. 1000 square feet floor area with 8 foot ceilings yields a total volume of 8000 cubic feet, or 227 cubic metres.
The floor plan is rectangular, approximately 27 feet by 37 feet on the third floor above grade. The long outside wall faces due south in full sun, while the shorter outside wall faces west and is shaded. The interior walls, floor and ceiling are all shared with other heated spaces. Thus the gross exterior wall area is about (27+37)x8 = 512 square feet or 48 square metres, some of which is occupied by windows. The wall construction dates from the late 1980’s and a reasonable guess might be a thermal resistance of $R = 2.5\;\rm m^2K/W$. (about $14\;\rm ft^2Fh/Btu$ in US units commonly used in North American construction.)
Heat Losses through Walls and Windows
Resistance, or R value, is important because it describes how well a wall will prevent heat losses. The higher the resistance, the lower the heat loss $q = \frac{A}{R}(T_i-T_o)$. The larger the wall area, $A$, or the temperature difference between indoors and outdoors, $(T_i-T_o)$, the larger the heat loss will be.
R has units of $\rm ft^2Fh/Btu$ or $\rm m^2K/W$ depending on unit system. Insulating materials are often described as R12, R20, R5 per inch, and similar terms, all in US units. The metric values, often denoted as RSI, are about 5 times smaller, and usually obvious from context.
In the south wall there is balcony door system of about 7 square metres. There are two windows in the south wall and one in the west wall, each of about 1.5 square metres. All are about 80% open area double glazing and about 10 years old. A reasonable guess for an effective resistance might be $ R = 0.6\;\rm m^2K/W$.
Thus, the total exposed area would be windows of $11.5\;\rm m^2$ at $ R = 0.6\;\rm m^2K/W$ and remaining wall area of $36.5\;\rm m^2$ at $ R = 2.5\;\rm m^2K/W$.
The construction relies on natural ventilation through air leakage (infiltration) that probably accounts for about 0.5 air changes per hour, or about $0.032\;\rm m^3/s$. That leakage is important because we need energy to warm up all the cold air that leaks in to replace the warm air that is leaking out.
