Compare inputs and outputs to common rules of thumb

Rules of thumb are guidelines used to make estimates based on experience rather than precise calculations. They are helpful for providing a quick and easy check of input and output data, but they are only approximations and should be treated as such.

Considerations for using rules of thumb

When performing QA/QC of energy models, rules of thumb can be helpful for making sure that inputs and outputs are not wildly different than commonly used values, however there may be valid reasons for differences to exist, so rules of thumb should always be taken with a grain of salt and not be used as the basis for design. Newer versions of building codes may also have specific requirements that are not adequately captured by rules of thumb (based on past experience).

Common rules of thumb

This table includes some commonly used rules of thumb. Because designs are variable, the table includes a range of possible values. "Low" values are on the more efficient end of design, "high" values are greater than average, and "medium" values are in between.

(Source: IBPSA-USA BEM Workshop)

Rating metric	Units (I-P)	Low	Med	High	Notes
Building EUI	kBtu/ft2	25	60	95	1
Cooling design	ft2/ton	600	400	250
Cooling design	kW/ton	0.6	0.9	1.2
Cooling loop	gpm/ton	2.5	2.5	2.5
Heating design	Btu/ft2	15	20	30
Fans	W/cfm	0.8	1.0	1.2
Supply air	cfm/ft2	0.6	1.0	1.2
Ventilation air	cfm/ft2	0.1	0.2	0.3
Lighting	W/ft2	0.7	1.0	1.8
Plug loads	W/ft2	0.5	1.0	1.5

Notes:

1. EUI is highly variable depending on specific building types. This range covers many building types, however it is strongly advised to compare your project's EUI against a building-specific benchmark.