Off-Grid Power Calculator
Size your off-grid solar power system. Calculate panels, battery bank, and system cost based on daily energy usage and sun hours.
About this calculator
This calculator sizes an off-grid solar system from your daily electricity habit rather than sheet-metal nameplate numbers, and that distinction shapes every downstream figure. It first inflates your entered daily kWh usage by dividing by a 0.75 system-efficiency factor — the roughly 25% lost to inverter conversion, wiring resistance, panel heat, and partial shading that never shows up on a panel's spec sheet — to get the true daily production the array must deliver. Panels needed is that inflated figure divided by what one panel actually produces in a day (its wattage times your peak sun hours, expressed in kWh), rounded up to a whole panel. Battery sizing follows a different, more conservative logic: it targets two full days of autonomy at 80% depth-of-discharge (typical for lithium LFP chemistry), dropping to 1.5 days if you've specified a backup generator, since the generator is assumed to cover the gap during extended cloudy stretches.
Your currently-installed battery capacity is evaluated separately to show how many days it would actually last today, independent of the recommendation. Costs are 2024-2025 market averages — $0.70/W installed solar, $300/kWh for LFP batteries, $250/kW for hybrid inverters, a flat 30% markup for installation and balance-of-system, and $650/kW plus 500 assumed annual run-hours for a backup generator — so real quotes will vary by region and installer. Because the model doesn't know your roof orientation, tilt, or local weather variability, treat Panels Needed as a starting point for installer conversations, not a final purchase order.
Inputs
Results
Panels Needed
10
Recommended Battery Bank
28.1 kWh
≈ 28 loads of laundry
Estimated System Cost
$19,159.00
≈ 10 gaming PCs
How to Use This Calculator
- Enter Daily kWh Usage from your current electricity bills or a load audit of planned appliances.
- Input Peak Sun Hours (4–6 for most US locations) and Panel Wattage for the solar panels you are considering.
- Enter Battery Capacity kWh and Backup Generator kW to size the storage and backup portions of the system.
- Review Panels Needed and Total Solar Capacity (kW) to request quotes from solar installers.
- Check Recommended Battery Bank (kWh) for 2 days of autonomy and Current Autonomy (days) with your current batteries.
- Use Estimated System Cost and Days of Autonomy to decide on the right balance of solar, battery, and generator backup.
How the result changes with Daily Energy Usage
| Daily Energy Usage | Panels Needed | Recommended Battery Bank | Estimated System Cost |
|---|---|---|---|
| 7.5 | 5 | 14.1 kWh | $11,204.00 |
| 11 | 8 | 20.6 kWh | $15,246.00 |
| 23 | 16 | 43.1 kWh | $27,973.00 |
| 38 | 26 | 71.3 kWh | $43,882.00 |
What each input means
- Daily Energy Usage
- Average daily electricity consumption (US average: 30 kWh; off-grid efficient: 10-20 kWh).
- Peak Sun Hours
- Average daily peak sun hours for your location (check NREL maps).
- Panel Wattage
- Wattage rating of each solar panel.
- Current Battery Capacity
- Total battery bank capacity you have or plan to install.
- Backup Generator
- Backup generator capacity for cloudy periods.
What each result means
- Panels Needed
- Number of solar panels required.
- Total Solar Capacity
- Total solar array size in kilowatts.
- Recommended Battery Bank
- Battery capacity needed (reduced with backup generator).
- Current Autonomy
- Days your current batteries can power without sun.
- Generator Cost
- Estimated cost for backup diesel generator (~$650/kW).
- Annual Fuel Cost
- Estimated annual generator fuel cost (~500 run hours/year).
- Estimated System Cost
- Total cost: panels, batteries, inverter, installation, and generator.
How this is calculated
Worked example, using the default values
- Identify Input Parameters5 parametersDaily Energy Usage = 15 kWh, Peak Sun Hours = 5 hrs, Panel Wattage = 400 W, Current Battery Capacity = 30 kWh, Backup Generator = 5 kW = 5 input(s) provided
- Calculate Daily Production NeededDaily Usage / System Efficiency15 / 0.75 = 20 kWh/day
- Calculate Panels Needed⌈Production Needed / (Panel W / 1000 × Sun Hours)⌉⌈20 / (400 / 1000 × 5)⌉ = 10 panels (4 kW)
- Calculate Recommended Battery BankDaily Usage × Autonomy Days / DOD15 × 1.5 / 0.8 = 28.1 kWh
- Calculate Current AutonomyBattery Capacity × DOD / Daily Usage30 × 0.8 / 15 = 1.6 days
- Calculate System Cost(Solar + Battery + Inverter) × (1 + Install Factor) + Generator($2800 + $8438 + $1000) × 1.3 + $3250 = $19,159
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator ask for less than my full daily kWh usage when sizing panels?
It doesn't — it actually inflates your entered usage before sizing anything. The daily kWh you enter is divided by a 0.75 system-efficiency factor to get the true daily production the array must deliver, since roughly 25% of what the panels generate is lost to inverter conversion, wiring resistance, panel heat, and partial shading before it ever reaches your appliances. Panels Needed is based on that inflated number, not your raw entry.
Why does adding a backup generator shrink the recommended battery bank?
The battery sizing targets full days of autonomy at 80% depth-of-discharge — 2 days with no generator, but only 1.5 days once you specify a backup generator, because the generator is assumed to cover the remaining gap during extended cloudy stretches. It's a design tradeoff: more generator capacity lets you carry a smaller, cheaper battery bank for the same reliability.
What's the difference between Recommended Battery Bank and Current Autonomy?
Recommended Battery Bank is what the calculator suggests you install, sized to the 1.5-2 day autonomy target described above. Current Autonomy is a separate, independent calculation that takes the battery capacity you actually entered and reports how many days it would last today at your usage rate — it doesn't change the recommendation, it just tells you where your current setup stands relative to it.
Why might my real installer quote differ from the Estimated System Cost?
The cost model uses flat national averages — $0.70/W for solar, $300/kWh for lithium batteries, $250/kW for inverters, a 30% installation markup, and $650/kW for a backup generator — none of which account for your roof orientation, local labor rates, permitting fees, or regional equipment pricing. Treat the estimate as a ballpark for budgeting, then get 2-3 real installer quotes for your specific site.
Why does Panels Needed always round up to a whole number?
You can't buy a fraction of a solar panel, so once the calculator determines the daily production needed and divides it by what a single panel produces (its wattage times your peak sun hours), it rounds that count up to the next whole panel. This means your actual installed capacity will always be slightly higher than your bare minimum requirement, which also builds in a small buffer for real-world losses the model doesn't otherwise capture.
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