| Coefficient | Value (solar heating) |
| a0 | −1.420431356898 × 10−3 |
| a1 | 2.103778934489 × 10−3 |
| a2 | 3.051129914167 × 10−5 |
| a3 | −1.593109661544 × 10−7 |
| a4 | 2.808978144742 × 10−10 |
| a5 | −1.011443517559 × 10−13 |
| a6 | −1.814326134579 × 10−16 |