How the Math Works
Every off-grid solar system has four sized components: panels, batteries, charge controller, and inverter. Get any of them wrong and the whole system underperforms. Oversize unnecessarily and you have thrown money away. The math is not hard, but the inputs take honest effort.
The calculator above uses the standard engineering approach used by every professional solar designer:
- Panel wattage: Daily Wh ÷ (Peak Sun Hours × 0.75 derate) × 1.25 safety margin.
- Battery amp-hours: (Daily Wh × Autonomy Days) ÷ (System Voltage × Usable DoD).
- Charge controller amps: Panel Wattage ÷ Battery Voltage × 1.25 safety margin, rounded up to a standard size.
- Inverter VA: Peak instantaneous load × surge factor, rounded up to a standard size.
The 0.75 panel derate covers real-world losses: wiring resistance, inverter inefficiency, panel degradation, soiling, temperature coefficient, and MPPT conversion losses. In perfect lab conditions you would get 100 percent of rated output. In a real system on a real roof, you get 70 to 80 percent. The 1.25 multipliers are the safety margins the NEC and seasoned installers apply so the array can still charge the bank on a marginal day and so the controller never runs at its absolute ceiling.
Worked Example (Step by Step)
Let us size a real small-cabin system by hand so you can see exactly what the calculator does. Assume a couple living off-grid part-time in Colorado: 3,000 Wh per day, 4.5 peak sun hours, 2 days of autonomy, a 24V LiFePO4 bank (80% usable), and mixed loads (1.5x surge).
1. Panel array. First, recover the energy the loads consume, accounting for losses: 3,000 Wh ÷ (4.5 sun hours × 0.75 derate) = 3,000 ÷ 3.375 = 889 W of "delivered" capacity. Apply the 1.25 design margin: 889 × 1.25 = 1,111 W. The calculator rounds up to the nearest 10 W, so it reports 1,120 W of panels — call it six 200W panels or twelve 100W panels.
2. Battery bank. Store two full days of energy: (3,000 Wh × 2 days) = 6,000 Wh of usable energy needed. Because only 80% of a LiFePO4 bank is usable, the nameplate energy is 6,000 ÷ 0.8 = 7,500 Wh = 7.5 kWh. Convert to amp-hours at 24V by dividing the daily-times-days energy by volts and usable DoD: (3,000 × 2) ÷ (24 × 0.8) = 6,000 ÷ 19.2 = 312.5 Ah, which the calculator rounds up to 315 Ah at 24V. That is roughly four 100Ah modules at this voltage, or a pair of 200Ah units wired appropriately.
3. Charge controller. The array can push 1,110 W into a 24V bank: 1,110 ÷ 24 = 46.3 A. Add the 25% margin: 46.3 × 1.25 = 57.8 A. Round up to the next standard MPPT size: 60 A MPPT.
4. Inverter. We estimate peak draw at 20% of the daily total: 3,000 × 0.20 = 600 W. Apply the 1.5x mixed-load surge factor: 600 × 1.5 = 900 VA, rounded up to the next standard size: 1,000 VA inverter. If this cabin ran a well pump or air conditioner, bumping the surge factor to 2.0x would push it to a 1,500 VA unit.
That is the entire calculation. Everything the console shows is one of those four formulas plus standard-size rounding. The bill of materials then translates those four numbers into a real parts list, including the wire gauge needed to carry the controller and inverter currents safely.
Step 1: Daily Watt-Hours — The Input That Matters Most
Your daily watt-hour number drives everything else. Get this wrong and your entire system is wrong. The good news: it is not hard. List every device that will draw power, multiply wattage by hours of use per day, and add them all up. Our load / appliance calculator does this tally for you appliance by appliance.
| Device | Typical Watts | Hours/Day | Wh/Day |
|---|---|---|---|
| LED lighting (house total) | 40-80 | 5 | 200-400 |
| Efficient refrigerator | 60-120 | 24 (cycling) | 700-1,200 |
| Laptop | 45-65 | 6 | 270-390 |
| Starlink dish (standard) | 45-75 | 24 | 1,080-1,800 |
| Starlink Mini | 20-40 | 24 | 480-960 |
| Water pump (well, on-demand) | 800-1,500 | 0.5 | 400-750 |
| Mini-split heat pump (cooling) | 400-900 | 8 | 3,200-7,200 |
| Washing machine | 400-1,200 | 0.5 (run-time) | 200-600 |
| TV 50" LED | 80-120 | 4 | 320-480 |
| Coffee maker | 900-1,200 | 0.2 | 180-240 |
| Microwave (small) | 900-1,200 | 0.1 | 90-120 |
| Phone chargers x2 | 10 | 4 | 40 |
For a small off-grid cabin used by one or two people with a propane range, a propane hot water heater, efficient LEDs, a laptop, a small fridge, Starlink, and modest entertainment, daily consumption typically lands between 2,000 and 4,000 Wh. A full family homestead with an electric refrigerator, a washing machine, a well pump, Starlink, computers, and a mini-split hovers around 6,000 to 15,000 Wh/day.
Measure, do not guess. If you already have grid power, a $25 Kill A Watt meter plugged into each major device for a week will give you airtight numbers. If you are planning from scratch, add 25 percent to whatever you estimate. You will always find loads you forgot.
Step 2: Peak Sun Hours Are Not Daylight Hours
This confuses almost every beginner. A "peak sun hour" is one hour of solar irradiance averaging 1,000 watts per square meter. A 100W panel produces 100 Wh of energy per peak sun hour. A location that gets "5 peak sun hours" might technically have 14 hours of daylight — the 5 hours measures energy-equivalent output, not time.
US regional averages (annual):
- Southwest (AZ, NM, southern CA, southern NV): 5.5 to 6.5 PSH
- Southern Plains (TX, OK): 4.8 to 5.8 PSH
- Southeast (FL, GA, AL, SC): 4.5 to 5.2 PSH
- Midwest / Mid-Atlantic: 4.0 to 4.8 PSH
- Pacific Northwest (WA, OR): 3.2 to 4.0 PSH
- Northeast (NY, VT, ME): 3.5 to 4.2 PSH
- Alaska: 2.5 to 3.5 PSH (annual, with huge seasonal swing)
Critical point: use winter sun hours, not annual averages, if you plan to run the system year-round without a generator. Winter values are often half of summer. A Vermont cabin might get 4.0 PSH on average but only 2.3 PSH in December. Size for the worst month or you will be running the generator all winter.
NREL provides free, accurate per-location sun hour data through their PVWatts tool. For calculator inputs, use the December value if you want a worst-case design, or the annual average if you have generator backup.
Step 3: Battery Autonomy — How Long Without Sun?
Autonomy is the number of days your batteries can power the house with zero solar input. It is the single biggest lever on battery cost. To design the bank in detail — string configuration, parallel limits, BMS choice — use our battery bank calculator and the DIY battery bank guide.
- 1 day: Minimal. Only works if you have reliable sun and a generator. Smallest, cheapest battery bank.
- 2 days: Standard for hybrid off-grid with generator backup. Covers most overcast stretches.
- 3 days: Sweet spot for most full off-grid systems. Rarely leaves you needing the generator.
- 5 days: Heavy autonomy. Only needed in Pacific Northwest, Alaska, or locations with frequent week-long overcast.
- 7+ days: Rarely cost-effective — you are paying $1,000+ per extra day for a battery bank that rarely gets used. Add a generator instead.
Step 4: System Voltage — 12V, 24V, or 48V?
Higher voltage means thinner wire, less loss, and cheaper components for larger systems. Lower voltage means cheaper small batteries and more DC appliance options. Rules of thumb:
| System Size | Best Voltage | Why |
|---|---|---|
| Under 1,000W | 12V | Abundant DC gear (RV, marine), cheap batteries, simple wiring |
| 1,000-3,000W | 24V | Thinner wire than 12V, lower cost, good inverter options |
| 3,000W and up | 48V | All modern hybrid inverters are 48V; cuts current by 75% vs 12V; best efficiency |
Full deep dive: 12V vs 24V vs 48V Solar Systems.
Reading Your Bill of Materials
The parts list above the article turns your four sizing numbers into real components. A few notes on how each line is derived:
- Panels: We divide your total array wattage by 200W (a common, cost-effective panel size) to suggest a panel count. You can substitute 100W or 400W panels — just match the total wattage and confirm the string voltage stays inside your controller's input window.
- Batteries: The count assumes 100Ah modules at your chosen voltage. Wiring three 100Ah batteries in parallel gives 300Ah; check your BMS parallel limits and balance the bank.
- Charge controller: The amperage shown is the calculator's rounded MPPT rating. Confirm the controller's maximum PV input voltage exceeds your panel string's open-circuit voltage (Voc) at the coldest expected temperature.
- Inverter: Sized to your continuous VA. For pure motor or AC loads, verify the inverter's surge rating (often 2x) covers startup inrush.
- Battery-to-inverter cable & fuse: This is the highest-current circuit in the system. The gauge is sized to the inverter's maximum continuous DC current (roughly VA ÷ voltage ÷ 0.9 efficiency), and the fuse is sized just above that current and below the cable's ampacity. Always verify gauge against run length and voltage drop with our wire sizing guide or the wire size calculator.
Example Off-Grid Setups (Calculator Presets)
Small Cabin (weekend + light full-time)
Inputs: 2,500 Wh/day, 4.5 PSH, 2 days autonomy, 24V, LiFePO4. Calculator output: ~930W of panels, ~265Ah at 24V, 50A MPPT, 1000VA inverter. Budget: $2,500 to $4,000 for gear.
RV / Van Life
Inputs: 1,500 Wh/day, 5 PSH, 1.5 days autonomy, 12V, LiFePO4. Calculator output: ~500W of panels, ~235Ah at 12V, 60A MPPT, inverter sized to your peak (bump the surge factor up if running a microwave or AC). Budget: $2,000 to $3,500. See our van life solar guide for real build breakdowns.
Tiny Home (off-grid full-time)
Inputs: 4,000 Wh/day, 4 PSH, 3 days autonomy, 24V, LiFePO4. Calculator output: ~1,670W of panels, ~625Ah at 24V, 100A MPPT. See the full tiny home solar system guide.
Full Homestead (family, year-round)
Inputs: 10,000 Wh/day, 4 PSH, 3 days autonomy, 48V, LiFePO4. Calculator output: ~4,170W of panels, ~785Ah at 48V (about 38 kWh), 120A MPPT. Budget: $18,000 to $35,000 DIY. Related: off-grid cabin solar guide.
Weekend Getaway (minimal)
Inputs: 800 Wh/day, 5 PSH, 1 day autonomy, 12V, LiFePO4. Calculator output: ~270W of panels, ~85Ah at 12V, 30A MPPT. Perfect starter rig.
Common Sizing Mistakes That Sink Off-Grid Systems
1. Using Annual Average Sun Hours Instead of Winter
Your system must work in December, not in June. Always size panels to the worst month if you want year-round off-grid operation with no generator.
2. Forgetting Phantom Loads
Routers, chargers, microwaves with clocks, inverter idle draw, and smart devices draw power 24/7. 30 to 80W of constant parasitic load is normal — that is 720 to 1,920 Wh/day on its own.
3. Oversizing the Inverter
A 6kW inverter idling at 40W wastes 960 Wh/day. Size the inverter for your peak load, not your daily average. Two stacked inverters or a 48V high-efficiency unit is better than one huge wasteful one.
4. Lead-Acid in 2026
LiFePO4 now costs less per usable kWh over its lifetime than lead-acid and lasts 3 to 5 times as long. Use lithium unless you have a specific reason not to. Full comparison in our best LiFePO4 batteries guide.
5. Panels Too Small for the Charge Controller
Charge controllers need a minimum panel voltage to start working. A "60V max input" MPPT paired with a single 20V panel will underperform. Always check the controller spec sheet.
6. Under-Sizing the Battery Cable and Fuse
The battery-to-inverter cable carries the most current in the entire system. Undersized cable overheats and drops voltage; a missing or oversized fuse leaves a battery short unprotected. Size both off the inverter's maximum DC current, not its rated watts.
Real Gear That Matches Calculator Output
Once the calculator has given you a number, you need real hardware. Here are widely used options across system sizes:
Complete Small-System Kits
The Renogy 400W 12V Premium Kit (4x100W panels, 40A MPPT, mounting, cables) matches almost exactly what a small cabin or tiny home needs. It is one of the most popular starter kits on the market.
Single 100W Panel (Add to Existing System)
The Renogy 100W Monocrystalline Panel is the workhorse panel for 12V and 24V systems — compact, high-efficiency, long-running track record.
LiFePO4 Batteries
The Power Queen 12V 200Ah LiFePO4 gives you 2,560 Wh of usable storage per battery, and most owners stack two or four of them for 24V/48V systems. Excellent value in the "proven brand, reasonable price" tier.
For premium builds, the Victron Smart 12.8V 200Ah LiFePO4 integrates with the Victron ecosystem (BMS, shunt, charge controller) with bluetooth monitoring that is unmatched.
Plug-and-Play Solar Generators (All-in-One)
If wiring a DIY system sounds like too much, a solar generator packages the battery, inverter, charge controller, and outlets into one box. The EcoFlow DELTA 2 Max with 220W solar panel (2,048 Wh LiFePO4) is a solid fit for cabins and mid-size RV setups.
For larger homestead backup, the Anker SOLIX F2000 (2,048 Wh + 400W panel) is a strong value pick, and the EcoFlow DELTA Pro with 400W panel (3,600 Wh) expandable to 25 kWh is the best whole-home-capable all-in-one.
Full roundup: Best Solar Generators 2026.
Frequently Asked Questions
How do I calculate what size solar system I need for off-grid?
Start with your daily energy usage in watt-hours. Divide by average peak sun hours for your location, then divide by an efficiency factor of about 0.75. That gives you the panel wattage needed. Multiply daily Wh by autonomy days and divide by system voltage and usable depth of discharge to get battery amp-hours. The calculator above does this automatically.
What is a peak sun hour?
A peak sun hour is an hour when solar irradiance averages 1,000 watts per square meter. Most US locations get 3 to 6 peak sun hours per day depending on latitude and season. The Southwest gets 5 to 6, the Northeast gets 3 to 4, and winter values are often half of summer.
How many days of battery backup do I need?
Two to three days is standard for off-grid systems. One day is only enough with a backup generator. Three to five days gives comfortable margin for extended cloudy stretches. More than five days is rarely cost-effective — add a generator instead.
What size charge controller do I need?
Divide total panel wattage by battery bank voltage, then add a 25 percent safety margin. For 400W on 12V: 400/12 x 1.25 = 42 amps, so you would pick a 50A MPPT controller. Always round up to a standard size.
What wire gauge do I need between the battery and inverter?
Size the battery-to-inverter cable to the inverter's maximum continuous DC current, which is roughly inverter VA divided by system voltage divided by about 0.9 efficiency. A 2000VA inverter on 12V can pull near 185A, requiring 2/0 AWG copper for a short run; the same inverter on 48V pulls under 50A and needs only 6 AWG. Always add a DC fuse or breaker close to the battery positive terminal, sized just above the inverter's continuous current. Confirm with our wire sizing guide.
Is 12V, 24V, or 48V better for off-grid?
12V is fine for small systems under 1kW (RV, van, weekend cabin). 24V is the sweet spot for 1 to 3kW (cabin, tiny home). 48V is best for systems over 3kW because it cuts wire costs, reduces losses, and is standard for modern high-end inverters.
Should I use lithium or lead-acid batteries?
Use LiFePO4 (lithium iron phosphate) in almost every case in 2026. It is safer, lasts 4,000 to 8,000 cycles vs 500 for lead-acid, gives you 80-100 percent usable capacity vs 50 percent, and the cost-per-usable-kWh over the lifetime is lower. Only consider lead-acid for very low-use emergency backup.
Does the calculator account for shading and tilt?
It uses a 0.75 overall derate factor that averages expected real-world losses (wire, MPPT conversion, temperature, soiling, mild shading, panel degradation). If you have significant shade or a suboptimal roof angle, oversize the panel output by another 15 to 25 percent, or use NREL PVWatts for a location-specific derate.
What does "VA" mean for inverter sizing?
VA stands for volt-amperes and is slightly larger than watts for reactive loads (motors, compressors, pumps). Sizing by VA instead of raw watts gives headroom for the inrush current that motors draw on startup. A well pump rated 800W can require 2,000 to 3,000 VA of startup surge capacity.
Next Steps
Once you have your sizing numbers, the natural next moves are:
- Pick a complete solar kit sized to your output.
- Choose the right LiFePO4 batteries for your Ah target.
- Select an MPPT charge controller sized for your array.
- Size your wire and fuses for each run.
- Study wiring diagrams for your specific system voltage.