What this calculator does: You list the devices you want to run and how many hours a day each one runs. The calculator adds up your daily energy use, then estimates the solar panels, charge controller, inverter and battery bank needed to cover it. An optional “Adjust system assumptions” section lets you change the panel size, battery voltage, overall system efficiency, battery type, days of autonomy, inverter efficiency and how many devices run at the same time.
Who it’s for: Homesteaders, cabin builders and anyone planning a stand-alone (off-grid) solar system who wants a realistic starting number before pricing equipment or talking to an installer.
The answer it produces: Your daily energy use in kWh, a panel count and charge controller size for both summer and winter, an inverter size in kW, and the rated battery bank size in kWh for the days of autonomy you choose. The starting assumptions are 400 W panels, a 48 V battery bank, 75% overall system efficiency, a lithium battery, 2 days of autonomy and 90% inverter efficiency. If the winter sun estimate falls below 2 peak sun hours, it adds a low-winter-sun warning.
Important limitation: This is a planning estimate, not a system design. It allows for real-world losses with one overall efficiency figure (75% unless you change it) and gives the rated battery capacity you shop for, but it does not model shading, panel tilt and direction, temperature effects, the controller’s maximum panel voltage or motor start-up surges. It also sizes panels to cover your full daily use in each season, which for the winter figure can mean a large array. The sections below show how to check each one.
Off-Grid Solar Sizing Calculator
For a more location-specific estimate, enter your latitude below. If you leave it blank, we’ll provide a generalized average estimate.
How the Calculation Works
Every time you press Calculate, the tool checks your entries and then runs the same five steps. It is plain arithmetic that you can check with a pencil, and the math runs in your browser.
- Daily energy use. For each device, watts × hours used per day = watt-hours (Wh). Add every device together and divide by 1,000 for kWh per day. This is the same load-analysis method the U.S. Department of Energy describes: multiply each appliance’s wattage by the hours it runs each day.
- Peak sun hours. One peak sun hour is one hour of sunlight at full strength (1,000 W per square meter), so “4 peak sun hours” means the day’s total sunlight adds up to four hours of full-strength sun. If you enter a latitude, the tool estimates the sunlight on a flat, horizontal surface for the June and December solstices from solar geometry (sun angle and day length), then multiplies by a fixed clearness factor of 0.55 to allow for typical atmosphere and cloud. If you leave latitude blank, it takes the peak sun hours you entered (default 5) and multiplies by 1.25 for summer and 0.75 for winter.
- Solar panels. Panels = daily Wh ÷ (panel watts × peak sun hours × overall system efficiency), rounded up to the next whole panel. The defaults are 400 W panels and 75% efficiency. It is calculated separately for summer and winter.
- Charge controller. Amps = (panels × panel watts ÷ battery voltage) × 1.25, rounded up. In other words, total array watts divided by the battery bank voltage, with a 25% margin on top. The battery voltage defaults to 48 V.
- Inverter and battery bank. Inverter kW = the combined wattage of every device you listed × the share running at once (100% by default) × 1.3 for 30% headroom. Battery kWh = daily kWh × days of autonomy ÷ (usable depth of discharge × inverter efficiency). The defaults are lithium at 90% usable, 90% inverter efficiency and 2 days, so this is the rated capacity to shop for, not just the energy you can use.
Inputs and Assumptions
These are the fields you fill in:
| Input | What to enter | Notes |
|---|---|---|
| Device name | Any label you like | For your reference only. It does not affect the math. |
| Wattage (W) | Running watts for the device | Use the nameplate or, better, a plug-in watt meter. Every device you list adds to the inverter size, even one with 0 hours, so list only what you actually plan to run. |
| Hours used daily | Average hours per day the device draws that wattage | Quarter-hour steps are allowed, up to 24. For cycling loads like refrigerators, use real run time, not 24 hours. |
| Peak sun hours | Your area’s average, default 5 | Used only when latitude is left blank. Must be between 1 and 12. |
| Latitude (optional) | Decimal degrees, such as 36.85 | Use a negative number in the Southern Hemisphere. Degrees-and-minutes formats are not accepted, and a value outside -90 to 90 shows an error. |
| Solar panel size (W) | Default 400 | Under Adjust system assumptions. 50 to 1,000. |
| Battery system voltage | 12, 24 or 48 V, default 48 | Used for the charge controller current. |
| Overall solar system efficiency (%) | Default 75 | 30 to 100. Covers dirt, shading, wiring, heat, the charge controller, battery round-trip losses and the inverter. |
| Battery type | Lithium (about 90% usable) or lead-acid (about 50%), starts on lithium | Sets the usable depth of discharge. |
| Days of battery autonomy | Default 2 | More than 0 and up to 10, in half-day steps. |
| Inverter efficiency (%) | Default 90 | 50 to 100. |
| Share of devices running at the same time (%) | Default 100 | 10 to 100. 100 is the safe choice for sizing an inverter. |
And these are the assumptions built into the tool. The first five can be changed in the assumptions section, and the rest are fixed:
| Assumption | Value used |
|---|---|
| Panel size | 400 W each (editable) |
| Battery bank voltage (for controller amps) | 48 V (editable) |
| Overall system efficiency | 75% (editable) |
| Battery type and autonomy | Lithium at 90% usable, 2 days, with 90% inverter efficiency (all editable) |
| Devices running at once | 100% (editable) |
| Inverter headroom | Listed watts × share running at once × 1.3 (fixed) |
| Charge controller margin | 25% (fixed) |
| Clearness factor (latitude mode) | 0.55 (fixed) |
| Seasonal swing (no latitude) | Summer +25%, winter −25% around your average (fixed) |
| Not included | Shading, panel tilt and direction, temperature effects, the controller’s maximum panel voltage, motor start-up surge |
What the Result Means
- Total daily usage (kWh). The energy your system must generate and store every day. Every other number scales from this one, so it is the most important figure to get right.
- Summer and winter panel counts. How many panels of your chosen size are needed to cover your full daily use in each season, after the overall system efficiency. Buy against the winter figure, because a system sized for summer runs short in the darkest months.
- Estimated peak sun hours. The sunlight the tool assumed for each season. If these look far too high or too low for your region, check them against measured data (see the sources below).
- Charge controller (amps). The current the array would push toward your battery bank, with a 25% margin, rounded up. It is a starting point for comparing controllers, not a final selection, because it says nothing about the controller’s maximum panel voltage.
- Inverter (kW). A continuous AC rating that could carry the share of your listed devices you said run at once, with 30% to spare.
- Battery bank (kWh). The rated capacity to shop for, covering your days of autonomy after allowing for the usable depth of discharge and inverter efficiency (see the example below). The line above it in the results says which assumptions it used.
- Low winter sun warning. A yellow box appears if the winter estimate is under 2 peak sun hours. It means covering all your winter use with panels alone takes a very large array.
- Red error messages. A latitude outside -90 to 90, an assumption outside its allowed range, hours over 24, or no device with a wattage and hours stops the calculation and names the problem.
Example Calculation
Here is a small-cabin load list run through the calculator at 36.85° N latitude, the Norfolk, Virginia example used in the tool. The devices and wattages are illustrative, not a recommendation, so measure your own.
| Device | Watts | Hours per day | Wh per day |
|---|---|---|---|
| Refrigerator | 150 | 8 | 1,200 |
| LED lighting | 60 | 5 | 300 |
| Laptop | 60 | 6 | 360 |
| Well pump | 750 | 1 | 750 |
| Internet router | 12 | 24 | 288 |
| Total | 1,032 | 2,898 |
The tool then works out the following (it displays peak sun hours rounded to one decimal, but the math uses the unrounded values):
| Output | Result | How it is calculated |
|---|---|---|
| Daily usage | 2.90 kWh | 2,898 Wh ÷ 1,000 |
| Inverter | 1.34 kW | 1,032 W × 1.3 |
| Battery bank (2 days, lithium) | 7.16 kWh | 2.898 kWh × 2 ÷ (0.9 × 0.9) |
| Summer (6.4 peak sun hours) | 2 panels, 21 A | 2,898 ÷ (400 × 6.38 × 0.75) = 1.51, rounded up to 2 panels; 800 W ÷ 48 V × 1.25 = 20.8 A, rounded up |
| Winter (2.4 peak sun hours) | 5 panels, 53 A | 2,898 ÷ (400 × 2.36 × 0.75) = 4.09, rounded up to 5 panels; 2,000 W ÷ 48 V × 1.25 = 52.1 A, rounded up |
Adjusting the example for the real world. The 75% overall efficiency is what turns the winter answer from 4 panels into 5. Four panels would produce about 3,780 Wh on an average December day (400 W × 4 × 2.36), roughly 30% above the 2,898 Wh needed, but only about 2,835 Wh after 25% losses, just under what the cabin needs. The 75% figure is our own planning allowance, not a published one, and NREL’s PVWatts tool asks for an overall system-losses percentage for the same reason. If you set the efficiency to 100%, the winter answer drops to 4 panels and the controller to 42 A, so change it only if you have measured data.
The battery figure now includes the depth of discharge. Two days of this cabin’s use is 5.80 kWh of energy. With lithium at 90% usable and a 90% efficient inverter, the rated capacity to shop for is 5.80 ÷ 0.81 = 7.16 kWh. Choose lead-acid at 50% usable and it becomes 12.88 kWh, and asking for 3 days of autonomy on lithium raises the lithium figure to 10.73 kWh. Check your own battery’s datasheet for the depth of discharge it allows.
The 1.34 kW inverter figure covers the running load of everything listed. If you know some devices never run together, lower the share running at once, for example to 60%, and it becomes 0.80 kW, but the well pump is a motor, and motors can draw several times their running wattage for a moment at start-up, so check the pump’s nameplate and the inverter’s surge rating before you choose one.
Battery voltage changes the controller current, not the panel count. The five winter panels (2,000 W) on a 24 V bank need 105 A and on a 12 V bank 209 A, against 53 A at 48 V. If your panels are 300 W instead of 400 W, the winter answer becomes 6 panels (1.8 kW) and 47 A on a 48 V bank.
How to Use the Result in a Real System
- Cut loads before you size. Every watt-hour you avoid saves panels, battery and inverter. The Department of Energy makes the same point: efficiency improvements first mean a smaller, cheaper system. Our guide to energy-efficient off-grid appliances is a good place to start.
- Size panels from the winter number and check the efficiency. The tool already applies a 75% overall efficiency. NREL’s PVWatts tool asks for an overall system-losses percentage for the same reason, and an installer can refine the figure for your equipment and site.
- Check the battery figure against the datasheet. The tool’s kWh is a rated capacity based on the battery type you chose. Confirm the depth of discharge your battery allows and the bank voltage you plan to use.
- Check the inverter two ways. The continuous rating should meet the calculator’s kW figure, and the surge rating should cover your largest motor start-up. Running big loads one at a time lets a smaller inverter do the job, so lower the share running at once only when you are sure of it.
- Match the charge controller to your actual array. Use the amp figure, which already includes a 25% margin, to shortlist controllers, then check each datasheet for maximum panel input voltage and maximum panel power. Panel voltage rises in cold weather, so the wiring layout of your panels matters here, and the calculator does not cover it.
- Cross-check your sun hours against measured data. Compare the tool’s estimate with NREL’s solar data for your location (links below).
- Plan for cloudy stretches. Two days of battery is the starting point, and you can change the days of autonomy. Decide whether a generator or extra battery makes sense for your climate, and see how long your bank will actually last with our battery bank runtime calculator.
- Get the wiring and protection right. Wire sizes, fuses, breakers and disconnects must follow the National Electrical Code and your local rules. Have a qualified electrician or solar installer review the design before you buy or connect anything.
Important Variables That Can Change the Result
- Latitude and local climate. Sunlight varies with location, season and weather, and the tool applies one fixed clearness factor everywhere. A foggy coastline and a dry high desert at the same latitude will not produce the same energy.
- Panel tilt and direction. The tool never asks about either. A fixed panel angled toward the sun captures different energy than the flat-surface estimate the calculator uses.
- Shading. Trees, roof edges and even a chimney can cut a panel string’s output sharply, well beyond the area of the shadow.
- Temperature. Hot panels produce less, and cold panels produce higher voltage, which matters for charge controller compatibility.
- Panel wattage. The tool starts at 400 W panels and lets you enter a different size. Bigger panels mean fewer of them for the same array watts.
- System voltage. Controller amps are calculated for the voltage you choose, 48 V by default. The same array on a 24 V bank carries about double the current, and on a 12 V bank about four times.
- Days of autonomy. The tool uses two days unless you change it, and the battery figure scales in direct proportion.
- Battery chemistry and depth of discharge. This decides how much rated capacity you need for a given usable figure, and lead-acid needs about 1.8 times the rated capacity of lithium.
- Conversion losses. Wiring, the charge controller, the inverter and battery charging and discharging all lose some energy along the way. The tool uses one overall efficiency for the panels and a separate inverter efficiency for the battery.
- How your household uses power. A winter heating fan, a summer cooling load or a well pump that runs more in dry weather can shift your real daily use away from the average you entered.
Common Mistakes
- Guessing wattage and hours. A plug-in watt meter over 24 hours gives you real numbers for a refrigerator or freezer, which cycle on and off. Entering 24 hours at full wattage overstates it, and forgetting standby draw understates it.
- Sizing to the summer figure. Summer output is the easy case. Winter is when systems fail.
- Leaving lithium selected when you are buying lead-acid. The default assumes lithium at 90% usable. Lead-acid at 50% usable needs about 1.8 times as much rated capacity.
- Setting the system efficiency to 100%. Without a loss allowance, a system that looks adequate on paper often comes up short in winter.
- Lowering the share running at once without checking. If two big loads overlap, the inverter will be too small.
- Forgetting motor start-up surge. Pumps, compressors and power tools can trip an undersized inverter even when their running wattage looks small.
- Confusing peak sun hours with daylight hours. Twelve hours of daylight is not twelve peak sun hours. Typical values are a small fraction of daylight length, so entering daylight hours will badly undersize your system.
- Entering latitude the wrong way. Use decimal degrees (36.85, not 36°51′), and a negative number south of the equator.
- Listing seasonal loads year-round. Air conditioning does not run in January. Size for the season that drives your worst case, and use separate runs of the calculator if your loads change a lot.
- Leaving out electric heat. Resistance heating can dwarf every other load. Plan heat separately with our off-grid heating calculator rather than adding it to a solar system by default.
When This Calculator Should NOT Be Used
- For a grid-tied or net-metered system. Those systems are sized around your utility bill and utility rules, and usually have no battery.
- As final design, permit or purchasing documents. It is a planning tool. Code compliance, wiring and protection need a qualified designer.
- At high latitudes. From about 41° N or S, the tool’s winter estimate falls below 2 peak sun hours and it shows a low-winter-sun warning, and it never returns fewer than 0.5. Above about 50° the December estimate is around one peak sun hour or less. Winter panel counts there become very large and very uncertain, so use measured local data instead.
- For life-safety or medical equipment. Anything that must never lose power needs professional design, redundancy and backup.
- For heavy-load properties. Welders, electric heat, EV charging and large motors need a proper engineering load calculation.
- For DC-direct systems with no inverter. The panel, controller and battery figures still apply, but ignore the inverter size.
- For hybrid systems with a generator. The tool sizes for solar and batteries alone.
Related Honey Hen Calculators
- Battery Bank Runtime Calculator: how long a battery bank runs a given load.
- Off-Grid Budget Calculator: estimate the total cost once your system is sized.
- Off-Grid Heating Calculator: estimate heating fuel so heat does not land on your solar system.
- Off-Grid Home Cooling: cooling options, since air conditioning is a major power draw.
- All Homestead Calculators: the full set of planning tools in one place.
Related Honey Hen Articles
- Solar Battery Backup for Off-Grid Homesteads: A Guide
- DIY Off-Grid Solar Installation: A Step-by-Step Guide
- Hidden Costs Solar Installation: What to Avoid
- Energy-Efficient Off-Grid Appliances: 10 Picks for Homesteads
- Energy Independence Off-Grid Living: Power Options
Authoritative Sources
- U.S. Department of Energy: Planning for Home Renewable Energy Systems. Covers stand-alone versus grid-connected systems and the wattage-times-hours load analysis.
- U.S. Department of Energy: Solar Radiation Basics. Explains how location, season and weather change the sunlight available to panels.
- NREL National Solar Radiation Database. Solar and weather data by location, useful for checking the calculator’s sun-hour estimate.
- NREL PVWatts Calculator. Estimates panel output for a location. It is built for grid-connected systems, but its production estimates and loss input are a useful cross-check.
- Sandia National Laboratories: Stand-Alone Photovoltaic Systems, A Handbook of Recommended Design Practices. A 1987 design handbook covering load analysis, arrays, batteries, charge controllers and inverters. It is dated, so confirm current equipment specifications.
- Sandia National Laboratories: Photovoltaic Power Systems and the National Electrical Code, Suggested Practices. A 2001 guide to conductor sizing, overcurrent protection and disconnects. Code editions change, so follow the version your local authority enforces.
Off-Grid Solar Calculator FAQ
How many solar panels do I need to go off-grid?
Divide your daily energy use in watt-hours by the panel wattage times your peak sun hours, using the winter figure, then add a margin for losses. In the example above, a cabin using 2.9 kWh a day at 36.85° N needs 5 panels of 400 W at the calculator’s default 75% overall efficiency. It would need 4 if you set the efficiency to 100%, so keep the loss allowance unless you have measured data.
What size battery bank do I need?
The calculator gives the rated capacity for the days of autonomy you choose, 2 by default, after allowing for the battery’s usable depth of discharge and the inverter’s efficiency. For the example cabin that is 7.16 kWh of lithium or 12.88 kWh of lead-acid. Adjust the days of autonomy for your climate and how much risk you are willing to accept.
Why does the calculator show both a summer and a winter result?
Sunlight changes through the year, so the number of panels needed to cover the same daily use changes too. Winter is the limiting case for an off-grid system, so buy panels using the winter figure.
How accurate is the peak sun hour estimate?
It is a geometric estimate with one fixed clearness assumption, not measured weather data. It gives a reasonable starting point and shows the seasonal swing, but local cloud cover, elevation and terrain can move the real number. Compare it with NREL’s solar data for your location before you buy equipment.
Do I need a bigger system for cloudy climates?
Yes. Fewer peak sun hours means each panel produces less energy per day, so you need more panels, more battery, or a backup source such as a generator to cover the same usage. If you leave latitude blank, lower the peak sun hours field to match your climate.
What if I use 24 V or 12 V batteries, or panels that are not 400 W?
Open Adjust system assumptions and enter your panel wattage and choose 12, 24 or 48 V. The panel count and controller amps update to match. The lower the voltage, the higher the current.
Why is the system efficiency set to 75%?
Real panels rarely deliver their full rating. Dirt, shading, wiring, heat, the charge controller, battery round-trip losses and the inverter all take a share. The 75% figure is our planning allowance for an off-grid system with batteries, not a published number, and you can change it under Adjust system assumptions if you have measured data.
Can this calculator size a grid-tied system?
No. It is built around off-grid sizing, with panels, batteries, an inverter and a charge controller for full self-sufficiency. Grid-tied systems have different sizing rules and typically do not need battery storage.
Want the full playbook? Practical Off-Grid Solar, our complete plain-English guide, walks through everything on this page in depth — sizing, wiring, safety, and a full real-world case study from our own system — available now in ebook and paperback.
Building a DC-direct system with no central inverter? Our book No Inverter Needed covers the 48V wiring side.
This calculator provides general planning estimates, not an engineering design. Actual system requirements depend on your specific location, equipment efficiency, wiring, code requirements, and installation conditions — consult a qualified solar installer or electrician before purchasing equipment.
