Two reps quote the same customer. One puts 7 kW of panels on a 7 kW inverter. The other puts 9.5 kW. Both quotes look fine to the customer. Only one of them is a system your engineers would sign off.
What the ratio is
The DC-to-AC ratio compares the panels’ combined DC capacity with the inverter’s AC rating:
| Panels (DC) | Inverter (AC) | Ratio |
|---|---|---|
| 7.0 kW | 7 kW | 1.00 |
| 8.26 kW | 7 kW | 1.18 |
| 9.5 kW | 7 kW | 1.36 |
Why installers oversize
Panels rarely deliver their full rated output. Heat, angle, dust and the time of day all reduce it. A slightly larger array keeps the inverter working closer to its rating for more of the day, which can raise the energy a system produces.
The cost is clipping: at the sunniest moments the panels could produce more than the inverter can convert, and that extra is lost. Go too far and you also risk exceeding the inverter’s DC input limits, or breaking a utility’s or a subsidy scheme’s rules on capacity.
Different systems, different limits
There is no single correct ratio. Grid-tied string-inverter systems are commonly designed with some oversizing. Off-grid and micro-inverter systems are usually sized quite differently. What matters is that your business decides the limit for each system type, based on the equipment you sell and the rules where you work, and that every quote follows it.
Put the limit on the quote, not in a manual
A sizing policy in a document does not stop a rep under pressure. Put it where the quote is built:
- Set a maximum per system type. For example, a limit for on-grid and hybrid systems and a tighter one for off-grid, by AC capacity range if needed.
- Check on save. A quote above the limit should not save as normal.
- Allow exceptions, with a reason. Some sites justify more. Let a person with permission go above the limit, and record who, when and why.
- Keep old quotes stable. If you change the limit, quotes already saved should keep the rule they were checked against.
- Check the strings too. Panels per string must sit inside the inverter’s voltage window. A quick check on the quote catches impossible designs early.
How AIMS IQ handles it
In AIMS IQ, sizing rules are set for each system type (on-grid, hybrid, off-grid, micro-inverter, or types you add), with a maximum DC oversize by AC range. When the panels on a quote, or from a panel layout, go over the limit, the save is stopped. Someone with the override permission can save it with a reason, and the override is recorded on the quote. The rule is stored with the quote at the time it was checked, and a string design check shows panels per string against the inverter’s voltage window on the quote and the sales order.