Template · Engineering
Solar PV system sizing calculator with monthly production
Panel count, system size, monthly production against use, bill savings, payback and 25 years of output.
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XLSXCSVODSXMLNumbers| Solar PV system sizing calculator | ||||||
| Example data — replace the blue input cells with your own. | ||||||
| SYSTEM SIZE (KW DC) | PANELS | ANNUAL PRODUCTION (KWH) | SIMPLE PAYBACK (YEARS) | |||
| 7.98 | 19 | 11,055 | 13.5 | |||
| Roof needed: 36 m² | Panel count, rounded up | Installed kW x sun energy x losses | Net cost after incentives / year-1 savings | |||
| Monthly production against use (kWh) | ||||||
| Month | Production (kWh) | Use (kWh) | Net (kWh) | Production bar | Use bar | |
| Jan | 531 | 1,000 | -469 | ████████ | ███████████████ | |
| Feb | 627 | 880 | -253 | ██████████ | █████████████ | |
| Mar | 899 | 850 | 49 | ██████████████ | █████████████ | |
| Apr | 1,048 | 800 | 248 | ████████████████ | ████████████ | |
| May | 1,225 | 830 | 395 | ███████████████████ | █████████████ | |
| Jun | 1,285 | 920 | 365 | ████████████████████ | ██████████████ | |
| Jul | 1,307 | 1,010 | 297 | ████████████████████ | ███████████████ |
Showing the first 16 of 38 rows and 7 of 7 columns. Cells with formulas show the formula on hover.
| Site, tariff and monthly load | ||||
| Blue cells are inputs. Use and sun hours are example values; replace them with your own data. | ||||
| Site and tariff | ||||
| Site name (fictional) | Example house, Riverton | Replace with the site or household name. | ||
| Analysis year | 2027 | Sets the days in each month, leap years included. | ||
| Electricity price ($ per kWh) | $0.17 | Retail price for the energy the array supplies directly. | ||
| Export credit ($ per kWh) | $0.08 | Net-metering credit for power sent to the grid, from your utility tariff. | ||
| Monthly electricity use and sun hours | ||||
| Month | Days | Use (kWh) | Peak sun hours (kWh/m² per day) | Use bar |
| Jan | 31 | 1,000 | 2.6 | ████████████████████ |
| Feb | 28 | 880 | 3.4 | █████████████████ |
| Mar | 31 | 850 | 4.4 | █████████████████ |
| Apr | 30 | 800 | 5.3 | ████████████████ |
| May | 31 | 830 | 6.0 | ████████████████ |
Showing the first 16 of 28 rows and 5 of 5 columns. Cells with formulas show the formula on hover.
| Array sizing, cost and 25-year output | ||||||
| Sizing from annual use, sun hours and losses; cost, incentives, payback and year-by-year output. | ||||||
| Inputs | ||||||
| Panel rated power (W) | 420 | Typical residential module rating. Use the datasheet value. | ||||
| Panel area (m²) | 1.90 | Module outline area, used for the roof area. About 1.9 m² for a 420 W module. | ||||
| Target offset of annual use (%) | 100% | Share of the year's electricity use the array should cover. 100% sizes for an annual balance. | ||||
| System losses (%) | 14% | Wiring, soiling, mismatch, shading and similar losses. 14% is close to the NREL PVWatts default of 14.08%. | ||||
| Inverter efficiency (%) | 96% | AC output as a share of DC input at typical operation. | ||||
| Installed cost ($ per W) | $2.90 | Installed price per watt of DC capacity. Use a quote from your installer. | ||||
| Incentive rate (% of installed cost) | 0% | Rebates or tax credits that apply to you. The US federal residential credit (Section 25D, 30% under the Inflation Reduction Act) ended for expenditures after December 31, 2025 (Public Law 119-21). | ||||
| Output decline per year (%) | 0.5% | Annual loss of module output, used in the 25-year table. | ||||
| Roof area available (m²) | 60 | Usable roof area. Row 22 compares it with the area the array needs. | ||||
| Sizing, cost and payback | ||||||
| Annual electricity use (kWh) | 10,680 |
Showing the first 16 of 59 rows and 7 of 7 columns. Cells with formulas show the formula on hover.
| Battery sizing for days of autonomy | |||
| Usable and nameplate capacity, Ah at the nominal voltage and the battery cost. | |||
| Inputs | |||
| Energy to cover each day (kWh) | 9.0 | Evening and overnight load the battery supplies. Use your own daily figure. | |
| Days of autonomy | 1.0 | Days the battery can carry the load with no sun. | |
| Depth of discharge (%) | 90% | Share of nameplate capacity used each cycle. The battery warranty sets the limit. | |
| Round-trip efficiency (%) | 90% | Energy out as a share of energy in, for one charge and discharge. | |
| Nominal battery voltage (V) | 51.2 | 51.2 V is the nominal voltage of a 16-cell lithium iron phosphate pack. | |
| Battery cost ($ per kWh nameplate) | $420 | Installed price per kWh of nameplate capacity. Use a quote. | |
| Sizing | |||
| Usable energy needed (kWh) | 9.0 | ||
| Nameplate capacity (kWh) | 10.0 | Usable energy / depth of discharge. | |
| Energy drawn from the array to store it (kWh per day) | 10.0 | Usable energy / round-trip efficiency. | |
| Capacity at nominal voltage (Ah) | 195 |
Showing the first 16 of 26 rows and 4 of 4 columns. Cells with formulas show the formula on hover.
| Monthly production, use and bill savings | |||||||||||
| Each month: production, use, self-supplied energy, exports and savings, on one bar scale. | |||||||||||
| Production, use and savings by month | |||||||||||
| Month | Days | Peak sun hours | Production (kWh) | Use (kWh) | Self-supplied (kWh) | Exported (kWh) | Net (kWh) | Offset of use (%) | Self-supplied share (%) | Bill savings ($) | Production bar |
| Jan | 31 | 2.6 | 531 | 1,000 | 531 | 0 | -469 | 53% | 53% | $90 | ████████ |
| Feb | 28 | 3.4 | 627 | 880 | 627 | 0 | -253 | 71% | 71% | $107 | ██████████ |
| Mar | 31 | 4.4 | 899 | 850 | 850 | 49 | 49 | 106% | 100% | $148 | ██████████████ |
| Apr | 30 | 5.3 | 1,048 | 800 | 800 | 248 | 248 | 131% | 100% | $156 | ████████████████ |
| May | 31 | 6.0 | 1,225 | 830 | 830 | 395 | 395 | 148% | 100% | $173 | ███████████████████ |
| Jun | 30 | 6.5 | 1,285 | 920 | 920 | 365 | 365 | 140% | 100% | $186 | ████████████████████ |
| Jul | 31 | 6.4 | 1,307 | 1,010 | 1,010 | 297 | 297 | 129% | 100% | $195 | ████████████████████ |
| Aug | 31 | 6.0 | 1,225 | 990 | 990 | 235 | 235 | 124% | 100% | $187 | ███████████████████ |
| Sep | 30 | 5.2 | 1,028 | 870 | 870 | 158 | 158 | 118% | 100% | $161 | ████████████████ |
| Oct | 31 | 4.1 | 837 | 790 | 790 | 47 | 47 | 106% | 100% | $138 | █████████████ |
Showing the first 16 of 24 rows and 12 of 15 columns. Cells with formulas show the formula on hover.
| Solar PV system sizing calculator |
| Notes: what this workbook does, how to use it and the method behind it. |
| What it does |
| Sizes a residential solar photovoltaic (PV) array from a year of electricity use and monthly peak sun hours, then shows monthly production against use, bill savings, simple payback and 25 years of output. |
| The Site and load tab holds the tariff and the twelve monthly values. Array sizes the system and calculates cost, incentives and payback. Battery sizes storage for a number of days of autonomy. Monthly lays out production, self-supplied en… |
| The example household uses about 900 kWh a month. The sun hours are example values for a mid-latitude US site. Replace both with your own data. |
| How to use it |
| 1. On Site and load, enter the site name, the analysis year, the electricity price and the export credit. |
| 2. Replace the twelve monthly use values with your own bills, and the peak sun hours with monthly averages for your location from NREL PVWatts or NASA POWER. |
| 3. On Array, enter the panel rating and area, the target offset, the losses, the inverter efficiency, the installed cost per watt and the roof area you have. |
| 4. Read the system size, panel count, production and payback from the Dashboard tiles. Check the roof and sizing lines under Checks. |
| 5. On Battery, enter the daily energy to cover, the days of autonomy, the depth of discharge, the round-trip efficiency and the nominal voltage. |
| Formulas and method |
Showing the first 16 of 37 rows and 1 of 1 columns. Cells with formulas show the formula on hover.
What does this template do?
This workbook sizes a residential solar photovoltaic (PV) array from a year of electricity use and monthly peak sun hours. It is for homeowners comparing quotes, installers checking a first estimate, and students learning how the inputs fit together. The Dashboard shows the system size, panel count, annual production, simple payback, a monthly chart of production against use, and cumulative bill savings every five years.
The required DC size is annual use times the target offset, divided by the annual sun energy per square meter times (1 minus system losses) times the inverter efficiency. Panels are rounded up to a whole count. Monthly production is installed kW times peak sun hours times days in the month, times (1 minus losses) times inverter efficiency. Self-supplied energy is the lower of production and use, and exports earn the export credit. The 25-year table applies an annual output decline of 0.5 percent.
The example household is fictional. It uses about 900 kWh a month, and its sun hours are example values for a mid-latitude US site. Replace the blue cells with your own bills and with sun hours from NREL PVWatts or NASA POWER.
What’s inside
- Required DC size and panel count from annual use, sun hours, losses and the target offset
- Monthly production, self-supplied energy, exports and bill savings on one bar scale
- Net cost after any rebate or tax credit you enter, and simple payback in years
- 25-year output table with an annual decline, cumulative savings and bars for each year
- Battery block that sizes usable and nameplate capacity for a number of days of autonomy
- Roof check that flags an array needing more roof area than you enter
Which tabs does the workbook have?
| Tab | What it holds |
|---|---|
| Dashboard | System size, panels, annual production and payback, with monthly and 25-year charts. |
| Site and load | Site name, analysis year, tariff, and the twelve monthly use and sun-hour values. |
| Array | Panel and cost inputs, the sizing calculation, incentives, payback and the 25-year table. |
| Battery | Usable and nameplate capacity, Ah at nominal voltage, and battery cost for days of autonomy. |
| Monthly | Production, use, self-supplied energy, exports, net and bill savings for each month. |
| Notes | What the workbook does, the formulas, assumptions and limits. |
What formulas does this template use?
This template holds 518 formulas in 782 cells across 6 tabs, so 66% of its cells calculate. They use 19 distinct functions; the longest formula is 1,359 characters and 171 of them read from another tab.
| Function | Uses | What it does |
|---|---|---|
IF | 209 | one result when a test is true, another when false |
MAX | 162 | largest value |
ROUND | 156 | rounds to a number of digits |
MIN | 131 | smallest value |
REPT | 118 | repeats text |
OR | 94 | true when any test is true |
SUM | 39 | adds numbers |
ISNUMBER | 16 | true for a number |
INDEX | 15 | value at a position in a range |
CHOOSE | 12 | picks the nth value from a list |
DAY | 12 | day of a date |
EOMONTH | 12 | last day of a month, months later |
The 12 most used of 19 functions. Counted from the workbook itself. Only functions that Excel, LibreOffice Calc, Google Sheets and Apple Numbers evaluate the same way are used, so the formulas survive every download format.
How do you use it?
- On Site and load, enter the site name, the analysis year, the electricity price and the export credit.
- Replace the twelve monthly use values and peak sun hours with your own bills and NREL PVWatts or NASA POWER data.
- On Array, enter the panel rating and area, the target offset, losses, inverter efficiency, cost per watt and roof area.
- Read the system size, panel count, production and payback from the Dashboard tiles, and check the Checks block.
- Use Battery to size storage for the number of days of autonomy you need.
What is it good for?
- Checking a solar installer's panel count against your own bills
- Comparing a full-offset array with a smaller one
- Estimating payback before requesting quotes
- Sizing battery storage for a number of days without sun
- Teaching how array size follows sun hours and losses
Questions about this sheet
Does this replace a site design?
No. The workbook does arithmetic on the inputs. A licensed installer or engineer should confirm the design, wiring, structure and permits.
Where do I get peak sun hours for my site?
NREL PVWatts and NASA POWER publish monthly values for many locations. Enter the monthly averages on the Site and load tab.
Does it include the federal solar tax credit?
The incentive rate is 0% by default because the US federal residential credit under Section 25D ended for expenditures after December 31, 2025. Enter any state, utility or other incentive that applies to you as a share of the installed cost.
What does simple payback leave out?
It divides net cost after incentives by year-one bill savings. It ignores rate changes, maintenance, and inverter or battery replacement.