Generation
Solar Panels
The array is sized by how much energy you use over a year, not by how much power you draw at once. Rigid residential modules are typically 400-500W each.
What to compare
- Dollars per watt — not dollars per panel
- Temperature coefficient (how much output it loses when hot)
- Degradation rate, typically 0.4-0.5% a year
- Warranty length and who honours it
How to size it
Start from annual consumption, not from peak draw. Take your yearly kWh from twelve utility bills, divide by 365, then divide by the peak sun hours your location averages — roughly 4 in the Pacific Northwest, 5.5 in the Midwest, 6 or more in the Southwest. That gives the kW of array you need before losses. Add 20-25% on top for inverter conversion, wiring, soiling, heat and panel degradation, because the rating on the box is measured in laboratory conditions your roof will never reproduce.
Mistakes that cost money
- Sizing by panel count instead of by watts
- Panels ranging from 100W to 500W are all called 'a panel'. Twelve 400W modules and twelve 250W modules differ by 1.8 kW — enough to change whether the system covers your usage.
- Ignoring the temperature coefficient
- A panel loses roughly 0.3-0.4% of output per degree above 25 C. On a black roof in August, cell temperature can reach 65 C, which is a 12-16% loss on the number you sized against. In hot climates this is not a rounding error.
- Buying panels before checking the inverter window
- Every inverter and charge controller has a voltage range it will accept. Panels that fall outside it will not work, and panels whose cold-weather open-circuit voltage exceeds the ceiling can destroy the input. Check the equipment first, then buy panels to match.
- Assuming a bigger array fixes shade
- Cells wired in series mean a shadow across one corner cuts output out of all proportion to the area covered. Adding panels in the same shaded spot compounds the cost without fixing the problem. Move the array or trim the tree.
Options
No solar panels listed yet. We only list components we can price and specify accurately, and we are still onboarding retailers for this category.
The guides below cover how to choose one in the meantime — they are what we would tell you before recommending any specific product.