Solar & battery
Grid-Tie Solar Calculator
Sizes a grid-tied system, where the tariff and the interconnection rules set the limits.
Result
Grid-tied systems are sized on annual averages because the grid covers the gaps. This ignores interconnection limits, which some utilities set relative to your historical usage, and any requirement for a production meter or a separate solar tariff. A standard grid-tied inverter also shuts down in an outage.
Get a Free Business AuditHow the calculation works
Annual consumption against production, with the export and import split and the resulting bill under the applicable tariff. Grid-tied design is bounded by the interconnection rules as much as by the roof, since utilities limit system size relative to consumption and the equipment must meet interconnection and code requirements.
What moves the number
- Annual consumption and the production target.
- Utility limits on system size relative to usage.
- Export credit terms under the tariff.
- Interconnection and equipment requirements.
Where this one goes wrong
Many utilities limit system size relative to historical consumption, which means a system sized for future loads may not be approved as designed, and that is worth establishing before the design is finalised rather than at application. Grid-tied systems without storage also shut down during a grid outage by design, for the safety of line workers, so a customer expecting backup from a grid-tied system without a battery has been misled.
Before you quote
- Check the utility's size limit relative to consumption first.
- Confirm equipment meets interconnection requirements.
- Be clear that grid-tied without storage gives no outage backup.
Where the rules come from
Questions
About this calculator
Will a grid-tied system work during a power cut?
No, not without storage and the right configuration. Grid-tied inverters disconnect during an outage by design, for the safety of line workers, so a customer expecting backup from a grid-tied system without a battery has been misled.
Can I install any size system I want?
Many utilities limit system size relative to historical consumption, so a system sized for anticipated future load may not be approved as designed. Establishing the limit before finalising the design avoids redesigning at the application stage.
What governs the electrical design?
NFPA 70 Article 690 plus the utility's own interconnection requirements and whatever code edition the local authority has adopted. Equipment listing and rapid shutdown provisions are part of that, and the inspector applies the adopted text.
The formula
kW = annual kWh needed / (peak sun hours x derate x 365)
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How to use this calculator
- Monthly consumption (kWh). Enter your own figure; the page starts at 950.
- Peak sun hours, annual average. Enter your own figure; the page starts at Mid-Atlantic (4.5).
- Target offset (%). Enter your own figure; the page starts at 90%.
- System derate (%). Enter your own figure; the page starts at 80%.
- Installed cost per watt ($). Enter your own figure; the page starts at $2.9.
- Incentives (%). Enter your own figure; the page starts at 30%.
The result updates as you type, so you can try a figure, see what it does to system size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Monthly consumption (kWh) | 950 |
| Peak sun hours, annual average | Mid-Atlantic (4.5) |
| Target offset | 90% |
| System derate | 80% |
| Installed cost per watt | $2.9 |
| Incentives | 30% |
What comes out
| Result | Value |
|---|---|
| System size main result | 7.8 |
| Annual production | 10,260 |
| Cost before incentives | $22,644 |
| Net cost | $15,851 |
| Cost per annual kWh | $1.54 |
A 7.8 kW system produces about 10,260 kWh a year and costs $15,851 net. That is $1.54 for each annual kWh, which is the figure to compare against other systems and against the grid.
How sensitive the answer is
Holding everything else steady and changing only installed cost per watt, this is what happens to system size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Installed cost per watt ($) | System size |
|---|---|
| $1.45 | 7.8 |
| $2.17 | 7.8 |
| $2.9 example above | 7.8 |
| $4.35 | 7.8 |
| $5.8 | 7.8 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Monthly consumption (kWh)
- A number, between 0 and 200000, starting at 950.
- Peak sun hours, annual average
- Choose from Pacific Northwest (3.5), Northeast, Midwest (4.0), Mid-Atlantic (4.5), Southeast, coastal California (5.0), Texas, inland California (5.5), Arizona, Nevada (6.0).
- Target offset (%)
- A percentage, between 0 and 130, starting at 90%.
- System derate (%)
- A percentage, between 40 and 100, starting at 80%.
- Installed cost per watt ($)
- A money amount, starting at $2.9.
- Incentives (%)
- A percentage, between 0 and 90, starting at 30%.
What the results tell you
| Result | What it is |
|---|---|
| System size | The headline result. A decimal figure. |
| Annual production | A number. |
| Cost before incentives | A money figure. |
| Net cost | A money figure. |
| Cost per annual kWh | A money figure. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
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