Sizing an off-grid solar system is the most expensive math you will do as a homeowner because the numbers translate directly into thousands of dollars of panels, batteries, charge controllers, and inverter capacity, plus the risk of waking up to a dead battery bank in week 3 of a January cloud cycle. The default approach taught in most online calculators (multiply nameplate ratings, divide by peak sun hours, done) produces a system that works for the salesperson’s spreadsheet and fails in the field. Real off-grid sizing involves measuring loads, applying generous derating factors, and planning for worst-case weather rather than average conditions. Here is the framework that actually works.
Step 1: measure your loads, do not estimate
Before any panel math, you need a realistic daily kWh consumption figure. The standard tools:
- Kill A Watt P4400 ($25) for individual appliances. Plug it into a fridge for 48 hours and read the kWh consumed.
- Whole-house energy monitor like Emporia Vue ($150 to $350) for circuit-level monitoring.
- Utility bills for at least 12 months if you currently have grid power and plan to off-grid the same loads.
A typical small off-grid cabin (LED lights, modest fridge, water pump, electronics, occasional power tools) consumes 2 to 5 kWh per day. A full-time off-grid home with the same comforts as a grid home (well pump, electric water heater on demand, regular appliance use, no heating loads) consumes 6 to 12 kWh per day. Off-grid homes with heating, AC, or EV charging exceed 25 kWh per day and require a different class of system.
Common loads to budget for that beginners forget:
- Refrigerator standby cycling (kicks compressor 6 to 10 times per day, peak draw 6x running draw for 1 to 2 seconds each cycle)
- Inverter standby (10 to 40 watts continuous if the inverter is always on)
- Phantom loads (TV in standby, garage door opener, security system: 30 to 80 watts continuous in many homes)
- Well pump surge current (3 to 5x running current at startup, requires inverter sized for surge)
- Seasonal variation (heating, AC, longer pump runtime in summer)
Step 2: calculate your peak sun hours honestly
Peak sun hours (PSH) is the number of hours per day that solar irradiance equals 1000W per square meter. It varies wildly by location and season.
| Location | Summer PSH | Winter PSH | Annual avg |
|---|---|---|---|
| Phoenix AZ | 7.5-8.5 | 4.5-5.5 | 6.5 |
| Denver CO | 7.0-8.0 | 4.0-5.0 | 5.5 |
| Seattle WA | 5.5-6.5 | 1.0-2.0 | 3.5 |
| Boston MA | 5.5-6.5 | 2.5-3.5 | 4.2 |
| Anchorage AK | 5.0-6.0 | 0.5-1.5 | 3.0 |
Off-grid sizing uses the worst-case month, not the annual average. For a year-round Seattle system, design around the 1.0 to 2.0 PSH of December and January. For a summer-only Colorado cabin, design around 7.0 to 8.0 PSH. Mismatch this number and the system either overproduces (wasted money) or underproduces (dead batteries).
Step 3: size the solar array
The basic formula:
Solar array size (watts) = daily kWh consumption / worst-case PSH / system efficiency
System efficiency accounts for panel temperature derating (panels lose efficiency above 25C cell temp), MPPT charge controller losses, battery round-trip efficiency, and inverter efficiency. The compound factor is typically 0.65 to 0.75 for a well-designed system.
Example: 5 kWh per day load, Seattle, December, system efficiency 0.70
5 kWh / 1.5 PSH / 0.70 = 4,762 watts of solar
That number assumes the loads must be met by solar alone in December. If you accept a propane generator running 10 hours per month to handle the worst weeks, you can drop the array size by 30 to 40 percent.
Step 4: size the battery bank
Battery bank size is set by autonomy requirement and daily load.
For 3 days of autonomy on a 5 kWh per day load:
5 kWh x 3 days = 15 kWh of usable energy
For lithium iron phosphate (LiFePO4) batteries with 90 percent usable depth of discharge:
15 kWh / 0.90 = 16.7 kWh nominal capacity
For flooded lead-acid with 50 percent usable depth of discharge:
15 kWh / 0.50 = 30 kWh nominal capacity
Convert to amp-hours at your bank voltage:
- 24V system: 16.7 kWh / 24V = 696 Ah lithium, or 1,250 Ah lead-acid
- 48V system: 16.7 kWh / 48V = 348 Ah lithium, or 625 Ah lead-acid
48V banks are standard for systems above 4 kWh per day because they reduce conductor size, reduce charge controller current requirements, and allow longer inverter runs at higher efficiency.
Step 5: size the charge controller
Charge controllers are rated in amps at the battery bank voltage. MPPT (Maximum Power Point Tracking) controllers are the only practical choice for modern systems because they handle voltage mismatch between panel array and battery bank.
Charge controller amps = (solar array watts) / (battery bank voltage) x 1.25 safety factor
For our 4,762W array on a 48V bank:
4,762 / 48 x 1.25 = 124 amps
That requires a 150A controller minimum. Standard options: Victron SmartSolar 150/100 or 250/100 (two controllers split the load), Midnite Classic 200, or EPEver MPPT 150/100.
Step 6: size the inverter
The inverter must handle peak load (not just average load) with margin for surge currents.
Add up the wattage of every device that might run simultaneously, then add 50 percent surge margin for compressors and pumps.
Typical sizing:
- Small cabin (lights + fridge + electronics): 1,500 to 2,500W inverter
- Full off-grid home (with well pump): 3,000 to 5,000W inverter
- Large off-grid home (HVAC, EV charging): 8,000 to 15,000W inverter (often two 8K units in parallel)
Pure sine wave is mandatory for sensitive electronics, well pumps with variable frequency drives, and any inductive load. Modified sine costs less but causes failure or premature wear on many modern devices. See our inverter pure sine vs modified sine guide for the full picture.
Step 7: include a backup generator
Most off-grid systems include a backup generator (propane, diesel, or gas) to handle multi-week cloudy stretches and to charge the battery bank in emergencies. Sizing the generator at roughly 0.5 to 1.0 kW per kWh of daily load is typical. A 5 kWh per day system pairs well with a 3 to 5 kW generator.
The backup generator dramatically reduces the required solar array and battery bank sizes by eliminating the worst-case weather margin. A system designed for solar-only operation in December Seattle might need 4,500W of panels and 20 kWh of batteries. The same system with a generator for emergency use needs 2,500W of panels and 12 kWh of batteries, at roughly half the cost.
Putting it together: a real example
Off-grid cabin in Bend, Oregon, year-round use, with propane generator backup.
- Loads: 6 kWh per day (fridge, lights, well pump, electronics, occasional power tools)
- Worst-case PSH: 2.5 (December)
- Solar array: 6 / 2.5 / 0.70 = 3,400W, round up to 3,600W (9 panels at 400W each)
- Battery bank: 3 days x 6 kWh / 0.90 = 20 kWh LiFePO4 = 416 Ah at 48V
- Inverter: 4,000W pure sine wave (Victron Quattro 48/5000 or Sol-Ark 8K)
- Charge controller: 100A at 48V MPPT
- Backup generator: 5kW propane, autostart connected to inverter
Total system cost in 2026 (DIY install): $18,000 to $26,000. Professional install adds $8,000 to $15,000. For more on how the battery side of this math works, see our guides on lithium vs lead-acid battery storage and battery life cycle and depth of discharge, and our methodology page for the test protocols behind these numbers.
Frequently asked questions
How big a solar array do I need for a typical off-grid cabin?+
For a typical 4kWh per day cabin load (lights, fridge, water pump, occasional power tools), plan on 1500 to 2500 watts of solar in a sunny climate and 2500 to 4000 watts in a cloudy or northern climate. The wide range reflects the difference between summer-only use (smaller array) and year-round use (larger array). Always size for your worst-case month, not the average.
How many days of battery autonomy should I plan for?+
Three to five days of full autonomy is the standard for a remote cabin without a backup generator. Two days is acceptable if you have a propane or gas generator as backup. Seven days is realistic for medical or commercial loads where downtime is unacceptable. Note that lead-acid batteries should not be discharged below 50 percent regularly, so a 3-day autonomy bank needs to be sized for 6 days of nominal capacity.
Why does my off-grid system fall short on cloudy days when the math said it would work?+
Three common causes. First, manufacturer panel ratings (STC, Standard Test Conditions) assume 1000W per square meter of irradiance at 25C cell temperature, which rarely happens in real conditions. Real-world output is typically 75 to 85 percent of rated. Second, battery efficiency losses (round-trip 85 to 95 percent depending on chemistry) eat into stored energy. Third, MPPT charge controllers run at 95 to 98 percent efficiency and inverters at 88 to 95 percent. Each loss compounds. Realistic system math should apply a 30 to 40 percent derating to nameplate values.
Should I use a hybrid grid-tied inverter or a true off-grid inverter?+
True off-grid inverters (Victron Quattro, Schneider XW Pro, Outback Radian) are designed for pure islanded operation with no utility connection. Hybrid inverters (Sol-Ark 15K, EG4 18K, Deye SUN-12K) can run grid-tied, off-grid, or backup mode. For a remote cabin with no grid in sight, a true off-grid inverter is reliable, simpler, and cheaper. For a residential property where the grid is available but you want energy independence, a hybrid is the right call.
What is the biggest mistake people make when sizing their first off-grid system?+
Underestimating loads. People list the wattage of each device and multiply by hours, then forget standby loads, fridge compressor cycling, well pump surge currents, and seasonal variation. The actual daily kWh usage usually exceeds the spreadsheet estimate by 30 to 60 percent. Measure your loads with a Kill A Watt or whole-house energy monitor for at least 2 weeks before sizing. If you cannot measure, double your calculated number for safety.
Morgan Davis
Morgan Davis writes for The Tested Hub.