How to Size an Off-Grid Solar System — The Complete 6-Step Method (2026)

Sizing an off-grid solar system is the single most important engineering task in any build. Every other decision — which panels to buy, how big a battery bank to wire, what inverter you need, how thick your cables must be — flows from getting the numbers right. Undersize the system and you sit in the dark every cloudy week, leaning on a generator you bought solar to avoid. Oversize it blindly and you bury thousands of dollars in batteries and panels you will never fully use.

The good news: sizing follows a deterministic, repeatable method. There is no guesswork once you understand the chain of calculations. This guide walks through that chain as a 6-step method, and we carry one realistic example — a small off-grid cabin using 4.5 kWh per day — all the way through, so you see exactly how each number feeds the next. By the end you will be able to size a complete system on paper, or hand any step to the matching free calculator and let it do the arithmetic.

This page is the editorial spine of our calculator suite. Each step links to the tool that automates it — the Load Calculator for Step 1, the Battery Bank Calculator for Step 2, the System Calculator for Steps 3 through 5, and the Wire Size Calculator for Step 6. Read the method once, then use the tools forever.

---

The 6-Step Sizing Method at a Glance

Here is the entire method on one screen. The rest of this guide is just each step in detail, with the math shown.

Notice that everything starts with the load audit. Your daily kilowatt-hours are the seed number that every subsequent step multiplies, divides, and converts. If that number is wrong, every other component is wrong. Spend the most time here.

---

The Running Example: A 4.5 kWh/Day Cabin

To keep the method concrete, we will size a real-world system for a small weekend-and-seasonal off-grid cabin. Throughout the guide, every step builds on the last using these assumptions:

Northern Idaho is deliberately chosen as a "hard" case — its December sun hours are low (around 3.0), which forces honest array sizing. A cabin in Arizona with the same 4.5 kWh/day load would need a noticeably smaller array. We will note how the warmer, sunnier scenario changes each result so you can adapt the method to your own location.

---

Why the Order of These Steps Matters

People new to off-grid solar almost always start at the wrong end — they ask "how many panels do I need?" before they know how much energy they use. That is backwards. Panels are Step 3; they depend on a load number (Step 1) you have not measured yet. Worse, the popular shortcut of copying someone else's "1.5 kW kit" ignores that their loads, climate, and autonomy needs are nothing like yours.

The six steps are ordered because each output is the next step's input:

• Load (Step 1) sets the daily Wh that both the battery bank and the array must serve.
• Battery bank (Step 2) establishes how much energy you must store through the dark hours and cloudy days.
• Array (Step 3) must generate the daily load plus recharge the battery — within your worst-month sun budget.
• Voltage (Step 4) is chosen once you know the system's total power, because it sets the current everything else carries.
• Controller & inverter (Step 5) are sized from the array current and the peak AC load, both of which depend on voltage.
• Wire & fuse (Step 6) come last because conductor gauge and overcurrent ratings depend on the actual currents the finished design produces.

Follow the order and the design converges cleanly. Skip around and you will redo work — for example, picking an inverter before knowing your voltage, then discovering it does not come in a 48V variant.

---

Step 1 — Audit Your Loads (kWh/Day)

A load audit is a line-by-line tally of everything that consumes power, how much it draws, and how long it runs each day. The result — your daily watt-hours (Wh/day) — is the foundation number for the entire system. There is no shortcut around it that does not eventually cost you money.

How to Run a Load Audit

For each device, you need three things: its power draw in watts, the hours per day it runs, and (for honesty) whether that runtime is a daily average or a peak. Multiply watts by hours to get watt-hours per day, then sum every line.

Find watts on the device nameplate, in the manual, or with an inexpensive plug-in meter (a Kill A Watt) for AC devices. For the most accurate audit, measure rather than guess — refrigerators, in particular, cycle on and off, so their nameplate wattage overstates daily consumption. A fridge rated 120 W might only average 30–40 W over 24 hours because the compressor runs perhaps a third of the time.

A few rules that keep audits honest:

• Use real runtimes. A 1,500 W microwave used 10 minutes a day is 250 Wh, not 1,500 Wh.
• Account for phantom loads. Inverters, routers, and standby electronics draw 24/7. A 30 W idle draw is 720 Wh/day — often the biggest line nobody remembers.
• Separate summer and winter. Fans and AC dominate summer; lighting hours and heating-fan loads dominate winter. Size to the harder season.
• Add a 10–20% miscellaneous buffer for the loads you forgot. Everyone forgets some.

Step 1 Worked Example

Here is the cabin's audit. Notice how runtime, not nameplate wattage, dominates the totals — the fridge's modest 40 W average outweighs the 1,500 W kettle because the fridge runs all day.

Rounding the buffered total gives our headline figure: 4.5 kWh/day. That number now carries through every remaining step. (Worth noting: the always-on loads — fridge, Starlink, phantom draw — make up over 2,800 Wh, nearly two-thirds of the total. On off-grid systems, the things that never turn off matter far more than the things you use briefly.)

Takeaway: Your daily Wh is dominated by 24/7 loads, not high-wattage occasional ones. Cut a phantom load and you shrink your whole system. For deeper appliance-level guidance, see our off-grid refrigeration & appliance guide.

---

Step 2 — Size the Battery Bank (Autonomy & DoD)

Your battery bank must store enough energy to run your loads through the night and across cloudy stretches when panels produce little. Two variables drive the size: days of autonomy (how long you can run with no solar input) and usable depth of discharge (how much of the battery's rated capacity you can safely use). The formula is:

Battery capacity (Wh) = (daily Wh × days of autonomy) ÷ depth of discharge

Choosing Days of Autonomy

Days of autonomy is how many days the bank can power your loads with zero solar input. It is your buffer against cloudy weather and your tolerance for running a generator. There is no universal "right" number — it is a cost-versus-resilience trade:

• 1 day — minimal buffer. Fine for sunny climates with a generator on standby, or seasonal/weekend use. Cheapest bank.
• 2 days — the common default for full-time off-grid living. Covers a typical overcast day without a generator.
• 3–5 days — for cloudy regions (Pacific Northwest, Northeast winters) or anyone who refuses to own a generator. Expensive, but resilient.

More autonomy means a bigger, costlier bank, but also fewer generator hours and a gentler discharge cycle that extends battery life. Most well-designed full-time systems land at 2 days and lean on a generator for the rare multi-day storm. Our cabin is full-time in a cloudy region, so we will use 2 days and accept occasional generator use during prolonged December overcast.

Depth of Discharge by Chemistry

Depth of discharge (DoD) is the fraction of rated capacity you can use without damaging the battery. This single factor can double or halve your required nominal capacity, because it sits in the denominator of the formula. Here are the industry-standard usable depths:

The headline lesson: a 100 Ah lead-acid battery gives you only ~50 Ah of usable energy, while a 100 Ah LiFePO4 battery gives you 80–90 Ah. That is why a lead-acid bank must be built nearly twice the nominal size of a lithium bank to deliver the same usable energy — and why LiFePO4 dominates new off-grid builds despite a higher sticker price. For the full breakdown, see our DIY solar battery bank guide.

Step 2 Worked Example

Plugging the cabin's numbers in, with LiFePO4 at a conservative 80% usable DoD and 2 days of autonomy:

Battery Wh = (4,500 Wh × 2 days) ÷ 0.80
           = 9,000 ÷ 0.80
           = 11,250 Wh  (11.25 kWh nominal)

Convert that to amp-hours at the system voltage we will confirm in Step 4 (48V):

Battery Ah = 11,250 Wh ÷ 48 V ≈ 234 Ah at 48V

In practice you would round up to a standard size — for example a 48V 240Ah bank (about 12.3 kWh nominal), commonly built from one or two server-rack LiFePO4 modules. Two stacked 48V 100Ah modules (200Ah) would be slightly tight at 2 days; a 48V 240–280Ah bank gives comfortable margin.

Lead-acid contrast: the same cabin on flooded lead-acid at 50% DoD would need 9,000 ÷ 0.50 = 18,000 Wh nominal — a 375 Ah bank at 48V, roughly 60% larger and far heavier. This is the single clearest argument for lithium in a year-round build.

---

Step 3 — Size the Solar Array (Sun-Hours & Derate)

The solar array must replace the energy you use each day and recharge the battery — all within the limited daylight your location provides in its worst month. The core formula divides your daily energy by your worst-month peak sun hours, then inflates the result by a derate factor to account for real-world losses:

Array watts = daily Wh ÷ (worst-month peak sun hours × system efficiency)

where system efficiency is typically 0.65–0.80 (i.e. you lose 20–35% to the real-world factors in the derate table below).

Peak Sun Hours by Region

Peak sun hours are the number of hours per day during which solar irradiance averages 1,000 W/m². It is not the same as hours of daylight — it is the daily solar energy condensed into full-intensity hours. The critical rule for off-grid design: size to your worst month, not the annual average, because that worst month (usually December in the Northern Hemisphere) is when you produce the least and often need the most.

These are tilted-array averages and vary with local microclimate and panel tilt; treat them as planning ranges, and look up your exact site on a solar-resource map (NREL's PVWatts is the standard reference) before finalizing. Tilting panels steeper in winter — toward your latitude plus ~15° — recovers meaningful December production in northern sites.

Derate Factors That Eat Your Production

A panel's nameplate rating is measured under lab conditions (Standard Test Conditions). In the field, you never get nameplate. These losses stack multiplicatively, and together they explain why honest designers assume 65–80% of rated output:

For a cold-climate site you can use the higher end of efficiency (cold panels actually outperform their rating), but the conservative move for any year-round design is to assume 0.70 unless you have site data.

Step 3 Worked Example

Our cabin uses 4.5 kWh/day, sits in northern Idaho with December peak sun hours of 3.0, and we will plan at 70% system efficiency:

Array watts = 4,500 Wh ÷ (3.0 h × 0.70)
            = 4,500 ÷ 2.1
            ≈ 2,143 W

So the cabin needs roughly 2,150 W of solar to stay self-sufficient through a December day — call it 2,400 W of panels after rounding up to whole modules (six 400 W panels, or eight 300 W panels). That is a lot of array for a 4.5 kWh load, and it is entirely because December delivers only 3 peak sun hours.

Watch how location dominates this step. The same 4.5 kWh/day cabin in Arizona, with 4.5 December sun hours, needs:

Array watts = 4,500 ÷ (4.5 × 0.70) = 4,500 ÷ 3.15 ≈ 1,430 W

— barely 60% of the Idaho array, for identical loads. Your sun-hours number, not your appliances, often decides how many panels you buy. This is exactly why "how many panels do I need?" has no answer without a location.

For help choosing module size, see our 100W vs 200W vs 400W panel comparison.

---

Step 4 — Select System Voltage (12V / 24V / 48V)

System voltage is the nominal DC voltage of your battery bank, and it determines the current that flows through every cable, fuse, and busbar. Higher voltage means lower current for the same power — which means thinner wire, smaller fuses, less voltage drop, and access to more capable inverters. The choice is driven by total system scale:

The deciding equation is simply Power = Voltage × Current. A 3,000 W inverter pulls 250 A at 12V but only 62.5 A at 48V — a fourfold difference that decides whether your battery cables are thumb-thick 4/0 AWG or manageable 6 AWG. For anything beyond a small system, 48V is now the standard, and most modern hybrid inverters and server-rack batteries are built for it.

Step 4 Worked Example

Our cabin has a ~2.4 kW array, an 11+ kWh battery bank, and an always-on full-time load profile that will eventually run a well pump, microwave, and possibly power tools. That puts it squarely in 48V territory:

• The 2,400 W array at 48V charges at a manageable current; at 12V it would demand impractically heavy charge wiring.
• An 11+ kWh battery bank is most cleanly built from 48V LiFePO4 modules.
• 48V opens up the capable hybrid inverters that handle well-pump surge without strain.

We confirm 48V — which is exactly the voltage we already assumed when converting the battery bank to amp-hours in Step 2. (Had this been a 1.2 kWh/day van, we would have landed on 12V; a 3 kWh/day tiny home could go either 24V or 48V.) For the deep comparison, read 12V vs 24V vs 48V solar systems.

---

Step 5 — Size the Charge Controller & Inverter

With array, battery, and voltage fixed, the power electronics fall out of the numbers. The charge controller is sized by the array's output current; the inverter is sized by your largest simultaneous AC load plus motor surge.

Charge Controller Sizing

An MPPT charge controller is rated by its maximum output (battery-side) current. To find the current it must handle, divide the array wattage by the battery voltage, then add a safety margin (NEC requires sizing continuous PV current at 125%):

Controller A (min) = (array watts ÷ battery voltage) × 1.25

You also must check the controller's maximum PV input voltage (Voc) — the open-circuit voltage of your panel string in cold weather must stay below the controller's limit, or you destroy it. Cold mornings push Voc above the rated STC value, so leave headroom. Always choose MPPT over PWM for anything but the smallest 12V systems; MPPT harvests 15–30% more in cold or high-string-voltage conditions. Our charge controller reference covers MPPT vs PWM sizing in depth.

Inverter Sizing & Surge

Size the inverter to your largest simultaneous AC load, not your daily energy. Add up the running watts of everything that could be on at once, then make sure the inverter's surge rating exceeds the startup spike of your biggest motor. Motor-driven loads — well pumps, fridges, compressors, power tools — draw 3–7× their running wattage for a fraction of a second at startup.

• Continuous rating ≥ sum of running watts that run together.
• Surge rating ≥ the largest single motor's locked-rotor / startup draw.
• Always use a pure sine wave inverter for an off-grid home — motors, electronics, and many medical devices misbehave on modified sine.

For more on matching inverter type to loads, see solar inverter types explained and our inverter buyer's reference.

Step 5 Worked Example

Charge controller. Our 2,400 W array on a 48V bank:

Controller A = (2,400 W ÷ 48 V) × 1.25
             = 50 A × 1.25
             = 62.5 A → round up to an 80A MPPT

A 60A controller would handle the steady-state 50 A, but the 125% rule and cold-weather over-production push us to the next standard size, an 80A MPPT (giving room to add a couple more panels later).

Inverter. The cabin's worst-case simultaneous load is the microwave (1,200 W) running while lights, fridge, and Starlink are on (~250 W) — about 1,450 W steady. But the 1/2 hp well pump can surge to roughly 3,000–4,000 W at startup. Sizing for both:

Continuous needed ≈ 1,450 W (round up for headroom)
Surge needed      ≈ 3,000–4,000 W (well-pump start)
→ Choose a 48V 3,000 W pure sine inverter
  with ≥ 6,000 W surge

A common 48V 3,000 W inverter (with a ~6,000 W / 2-second surge) comfortably starts the pump and runs everything else. If the cabin later adds a 240V deep well pump or AC, the design would step up to a 48V hybrid inverter in the 6 kW class.

---

Step 6 — Size the Wire & Overcurrent Protection

The final step protects everything you have specified. Every conductor must be thick enough to carry its current without overheating (ampacity) and without losing too much voltage over its length (voltage drop), and every circuit must have a fuse or breaker that interrupts a fault before the wire or device is damaged.

Wire Sizing Basics

Two independent checks set the gauge, and you must satisfy both — take whichever demands the thicker wire:

1. Ampacity: the conductor's continuous current rating (from NEC ampacity tables) must exceed the circuit's continuous current, with margin. Size conductors at 125% of continuous current.
2. Voltage drop: over the run length, drop should stay under ~3% for critical DC runs (battery-to-inverter, array-to-controller). Long runs at low voltage often need a thicker wire than ampacity alone would suggest.

Low-voltage DC is unforgiving on long runs — this is the same physics that made us choose 48V. A 50 A run at 48V over 15 feet needs modest wire; the same power at 12V (200 A) would need enormous cable. Our solar wire sizing guide has the full ampacity and voltage-drop tables.

Fuse & Breaker Sizing

Every circuit needs overcurrent protection sized to protect the wire, placed as close as practical to the power source (the battery or the controller output). The general rule:

Fuse/breaker rating = continuous current × 1.25
(then round to the next standard size,
 staying below the wire's ampacity)

Critical protection points in any off-grid system: a main DC fuse/breaker between the battery bank and everything else, a fuse on the inverter's DC input sized to the inverter cable, a breaker between the charge controller and the battery, and string/combiner fusing on the PV side when you have multiple parallel strings.

Step 6 Worked Example

The heaviest run in our cabin is the battery-to-inverter cable. The 3,000 W inverter on a 48V bank draws:

Inverter DC current = 3,000 W ÷ 48 V ÷ 0.90 (eff)
                    ≈ 69 A continuous
Size conductor at: 69 A × 1.25 ≈ 87 A

An 4 AWG copper conductor (≈95 A ampacity at 75°C) clears the ampacity requirement with margin, and over the short battery-to-inverter run it easily holds voltage drop under 3%. The matching overcurrent device:

Fuse = 69 A × 1.25 ≈ 86 A → 90A or 100A
       Class-T fuse on the battery positive,
       below the 4 AWG cable's ampacity

On the PV side, the 50 A array current to the controller calls for roughly 6 AWG from controller to battery and an appropriately rated DC breaker; the lower current there makes wiring straightforward — another payoff of choosing 48V. Each remaining circuit (controller-to-battery, branch loads) is sized the same way: find its current, apply 1.25×, pick the conductor that satisfies both ampacity and voltage drop, then protect it.

---

The Full System on One Data Plate

Here is the entire cabin, sized end to end by the 6-step method — every number derived from the 4.5 kWh/day load we audited in Step 1:

That is a complete, code-aware off-grid system on one nameplate. Run your own numbers through the same six steps — or feed them straight into the System Calculator — and you will get the same kind of clean, defensible spec for your site.

---

How Many Solar Panels to Run a House Off-Grid?

It depends entirely on your daily energy use and your location's sun hours — not the square footage of the house. Most full off-grid homes use 15–40 kWh/day, which works out to roughly a 5–12 kW array (about 12–30 modern 400 W panels) plus 20–40 kWh of battery storage. Run the load audit (Step 1), divide daily kWh by your worst-month peak sun hours, and apply a derate factor (Step 3) to get the real number. An efficient cabin like our example needs only ~2.4 kW; a power-hungry home with electric heat and AC can need triple that.

The takeaway: two identical houses can need wildly different arrays depending on whether they heat with propane or electricity, run a well pump, and sit in Arizona or Washington. Always size from your measured load.

---

How Many Batteries Do I Need?

Size the bank from your daily load, your desired days of autonomy, and your chemistry's usable depth of discharge: Battery kWh = (daily kWh × autonomy days) ÷ DoD. Our 4.5 kWh/day cabin at 2 days on LiFePO4 (80% DoD) needs 11.25 kWh nominal — a 48V 240Ah bank. The same design on lead-acid (50% DoD) would need ~18 kWh, about 60% more capacity, because you can only use half of a lead-acid battery's rating without shortening its life. This is the clearest practical reason most year-round off-grid builds now choose LiFePO4.

---

What Size Inverter Do I Need?

Size it to your largest simultaneous AC load plus motor surge, not your daily energy. Add the running watts of everything that could be on together for the continuous rating, then make sure the surge rating clears your biggest motor's startup spike (well pumps and compressors surge 3–7×). A small cabin like ours runs comfortably on a 48V 3,000 W pure sine inverter with ~6 kW surge; a whole home with AC and electric cooking typically needs a 6–12 kW hybrid inverter. Always pick pure sine wave for an off-grid home.

---

Should I Oversize My Array?

Yes — modest oversizing of the array (not necessarily the battery) is standard and worthwhile. Because real arrays deliver only 65–80% of nameplate and winter sun hours are far below summer, sizing the array 20–30% above the bare minimum keeps batteries topped up through cloudy stretches and short December days, cutting generator runtime. Panels are now the cheapest part of the system, so the extra watts are cheap insurance. Oversizing the battery bank, by contrast, gets expensive fast — add autonomy deliberately, but lean on extra panels and a generator before stacking more batteries than you need.

---

Common Sizing Mistakes to Avoid

• Sizing to the annual average sun hours. You will run out of power every winter. Always size to the worst month.
• Forgetting phantom and 24/7 loads. Idle inverters, routers, and standby electronics quietly dominate the daily total.
• Using nameplate panel wattage with no derate. You never get nameplate in the field; plan at ~70% system efficiency.
• Ignoring depth of discharge. A "100Ah" lead-acid battery is really 50Ah of usable energy — half what the label implies.
• Picking the inverter from daily kWh instead of peak watts. A 5 kWh/day cabin can still need a 3 kW inverter for one microwave-plus-pump moment.
• Skipping surge ratings. An inverter that handles your running load can still stall when the well pump kicks on.
• Under-fusing or skipping DC protection. The most dangerous mistake of all — see the safety notes above.

---

Frequently Asked Questions

---

Putting It All Together

You now have the complete method: audit your loads, size the battery from autonomy and depth of discharge, size the array from worst-month sun hours and a realistic derate, choose the voltage that fits the scale, size the controller and inverter from current and surge, and finish with conductors and overcurrent protection that satisfy both ampacity and voltage drop. Done in order, the six steps turn a vague "how much solar do I need?" into a precise, defensible parts list.

From here, hand the arithmetic to the tools and the build to the guides. Start with the Load Calculator to nail your daily Wh, then feed that into the System Calculator to size the array, battery, and voltage together, the Battery Bank Calculator to dial in storage, and the Wire Size Calculator to spec your conductors and fuses. When you are ready to build, the off-grid cabin system guide and tiny home solar guide walk through real installs end to end.