Help Center Pre-Design
How Consumption is Derived from Square Footage
Published on 10 Oct 2023
The following guide provides information on how the annual consumption is derived when a user enters the square footage of the project property in Solargraf when creating a new project.
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Linear Model of Consumption
Solargraf uses a linear function to derive consumption from the entered square footage.y = mx + b
where
y = total annual consumption (kWh per year)
m = predetermined fixed value (kWh per sqft)
x = entered property size (sqft)
b = predetermined fixed value (kWh)
The derivation of m and b is described in the following sections.
The annual consumption is then passed to a normalised consumption model (based on the region) to derive the hourly consumption.
This normalised consumption model comes from the following sources based on the project:
- Project located in the US, and Genability is available --> We use Genability hourly profile.
- Project located in the US, but Genability is not available --> We use Solargraf hourly profiles for each region.
- Projects located outside the US --> We use a generic hourly profile.
- Projects located in Germany --> Use an hourly profile obtained from the local Germany team.
Genability offers normalised hourly profiles (referred to as 'Base Typical Profiles') for a particular zipcode. They scale consumption based on these base profiles.
The value of m and b for Residential Projects
For Residential Projects, the following formula for annual consumption is used:
Annual Consumption (kWh) = [Base consumption + (x - Min. sq ft for given slab) * Add on Consumption/sqft] * 365
Where x is the property size in sqft and base consumption, minimum sqft for given slab, and add on consumption/sqft are as follows:
| Slab | Minimum sqft | Maximum sqft | Base Consumption, kWh | Add on Consumption, kWh/sqft |
|---|---|---|---|---|
| 1 | 500 | 1199 | 15 | 0.01428 |
| 2 | 1200 | 3399 | 25 | 0.00454 |
| 3 | 3400 | 6000 | 35 | 0.01115 |
| 4 | 6001 | 12000 | 45 | 0.012 |
For example, for a property size of 600 sqft:
Annual Consumption = [15 kWh + (600 sqft - 500 sqft) * 0.01428 kWh/sqft] * 365
Annual Consumption = [15 kWh + 1.428 kWh] * 365 = 5996.22 kWh per year
In the example above, the annual consumption model simplifies to:
y = [15 kWh + (x - 500 sqft) * 0.01428 kWh/sqft] * 365
y = (15 + 0.01428x - 7.14) * 365
y = 5.2122x + 2868.9
where m = 5.183 kWh/sqft and b = 2868.9 sqft.
Note: the values for m and b for residential properties vary based on the property size.
The value of m and b for Commercial Projects
For Commercial Projects, b = 0 kWh and the following are the values of m:| Building Type | m, kWh/sq ft |
|---|---|
| Large Office | 13.19 |
| Medium Office | 14.65 |
| Small Office | 14.95 |
| Warehouse | 8.79 |
| Standalone Retail | 21.10 |
| Strip Mall | 20.81 |
| Primary School | 17.58 |
| Secondary School | 19.64 |
| Supermarket | 53.63 |
| Quick Service Restaurant | 175.26 |
| Full Service Restaurant | 140.09 |
| Hospital | 45.43 |
| Outpatient Health Care | 67.41 |
| Small Hotel | 19.93 |
| Large Hotel | 38.69 |
| Midrise Apartment | 11.72 |
For example, for a hospital of 60,000 sqft, the annual consumption is calculated as follows:
Annual consumption = 60,000 sqft * 45.43 = 2,725,800 kWh per year
Note: the values of m for commercial properties vary based on building type.