Go Off-Grid: Size a Solar Power System — Project Guide

🧑‍🔧 Go Off-Grid: Size a Solar Power System

Field: Renewable Energy / Solar

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Every calculator used here is tested against independent reference values · how PhDino checks its numbers

Size the panels, batteries, charge controller and inverter for an off-grid solar setup, in the order a real system gets designed.

An off-grid (or backup) solar system is really four components sized together: enough panels to cover a daily energy need, enough battery capacity to ride out days without sun, a charge controller sized for the array feeding into the battery bank, and an inverter sized for the loads it must run. This guide works through the first three in the order they depend on each other, then sizes the inverter as its own check against the loads that run at once.

This is a planning sanity-check, not a permit-ready electrical design. Battery chemistry, wiring gauge, fusing, disconnects and grounding all need to be worked out with your equipment's actual datasheets (and likely a licensed installer, depending on system size and your jurisdiction) before anything gets wired.

🧮 Solar Array Sizing 🧮 Battery Bank Sizing 🧮 Charge Controller Sizing 🧮 Inverter Sizing

Four parts, one energy budget

An off-grid system is four machines that share one budget. The panels turn sun into energy during the day. The battery holds it for the night and for cloudy spells. The charge controller sits between them and manages how the panels charge the battery. The inverter turns the battery's direct current into the alternating current your appliances expect. Each is sized from the same starting point, the number of kilowatt-hours you use per day, and most of the mistakes in this field are mistakes about that one number.

That is why the order matters. Array and battery both come from the daily energy figure. The controller comes from the array and the battery voltage. The inverter is different: it is sized by power, from what runs at the same moment, not by energy, and it also adds to the energy figure through its own idle draw.

How to do the energy audit

The audit is dull and it is the whole design.

List every load with its watts and the hours per day it runs, and multiply to get watt-hours. Divide the total by 1,000 for kilowatt-hours per day.
Use measured or nameplate figures, and remember that an appliance with a compressor or heating element runs only part of the time. A plug-in energy meter over a day or two beats any table.
Add the inverter's idle draw. An inverter that is switched on uses power even with nothing connected, often tens of watts around the clock.
Design for your worst month. If the system must work in winter, the peak sun hours that matter are the winter ones, which can be a third of the summer figure or less.

System voltage and battery chemistry

Battery capacity is quoted in amp-hours, which means nothing without the voltage: watt-hours are volts times amp-hours. Higher system voltage means lower current for the same power, so thinner cables and cooler connections. Small systems often use 12 V, cabins commonly 24 V, and larger houses 48 V, and the inverter and controller must match the bank.

Chemistry sets how much of the nameplate you can use. Lead-acid batteries are normally limited to about half their capacity to keep them healthy, while lithium iron phosphate batteries are commonly used to 80 percent or more. The same energy stored costs noticeably more capacity in lead-acid, as the example shows.

Steps

1🔆Solar Array Sizing
Start from your daily energy need: add up the appliances you want to run and roughly how many hours a day each one draws power, or start from a past utility bill divided by 30, and do not forget the inverter's own idle draw. Use the peak sun hours of the darkest month you must cover, not the yearly average. This returns the array size and panel count needed. Hit "📋 Copy Values" once you have a result: the daily energy need and the array wattage feed the next two steps.
🧮 Open the Solar Array Sizing calculator
2🪫Battery Bank Sizing
"📥 Paste Values" fills in the same daily energy need from Step 1. Add how many days of autonomy you want (two to five is typical for a residential backup system), your battery chemistry's usable depth of discharge, and your system voltage, and this returns the battery capacity needed in amp-hours at that voltage.
🧮 Open the Battery Bank Sizing calculator
3🎛️Charge Controller Sizing
"📥 Paste Values" fills in the array wattage from Step 1. That is the wattage the calculation required. If you round up to whole panels, use the installed wattage (panel count × panel watts) instead, because that is what the controller will see. Add your battery bank's voltage and controller type (MPPT is more efficient than PWM, especially when the array's voltage is meaningfully higher than the bank's) to get the required controller current rating.
🧮 Open the Charge Controller Sizing calculator
4🔌Inverter Sizing
Separately from the array and battery sizing above, add up the loads the system must run at once for the continuous rating, and identify the single largest motor-driven appliance for the surge rating. A well pump, refrigerator compressor or power tool are the usual culprits. This is an independent check against your actual loads, not something the panel and battery numbers determine on their own.
🧮 Open the Inverter Sizing calculator

Worked example: a cabin that uses 3.6 kWh a day

An off-grid cabin runs a refrigerator, lights, electronics, a water pump and a few small appliances. The audit gives 1.0 + 0.3 + 0.8 + 0.3 + 0.6 = 3.0 kWh a day. The inverter will idle at about 25 W around the clock, which is 25 × 24 ÷ 1,000 = 0.6 kWh a day that is easy to forget, so the daily need is 3.6 kWh. The design month has 4 peak sun hours, the panels are 450 W, and the battery is a 24 V lithium iron phosphate bank with two days of autonomy.

Step 1: Panels

You enter	Value
Daily Energy Need	3.6 kWh
Peak Sun Hours	4 hr
System Losses (wiring, inverter, heat)	25 %
Panel Wattage	450 W

The calculator returns	Value
Required Array Size	1,200 W
Panels Needed	3

The calculator derates the sun: 4 peak sun hours × (1 − 25 percent) = 3.0 effective hours a day. We use 25 percent losses rather than the default 20 because everything here passes through a battery and an inverter. The array then has to produce 3.6 kWh in those hours, so 3.6 × 1,000 ÷ 3.0 = 1,200 W.

In whole 450 W panels that is 3, an installed 1,350 W. Notice that rounding up gave a little headroom, and that we will need it.

Step 2: Battery bank

You enter	Value
Daily Energy Need	3.6 kWh
Days of Autonomy	2 days
Max Depth of Discharge	80 %
System Voltage	24 V

The calculator returns	Value
Required Battery Capacity	375 Ah

The energy to store is 3.6 kWh × 2 days = 7.2 kWh. Dividing by the 24 V system voltage gives the amp-hours you would use, and dividing again by the 80 percent depth of discharge gives the capacity to buy: 375 Ah. As a check, 375 Ah × 24 V is 9.0 kWh of nameplate, of which 80 percent is the 7.2 kWh we need.

The same energy in lead-acid batteries, limited to about 50 percent depth of discharge, would need 600 Ah. Chemistry is a lever on the size and cost of the bank, not a detail.

Step 3: Charge controller

You enter	Value
Array Wattage	1,350 W
Battery Bank Voltage	24 V
Controller Type	MPPT
Continuous-Duty Safety Margin	25 %

The calculator returns	Value
Required Controller Current	72.5 A

The Copy Values button carries the required 1,200 W. We enter the installed 1,350 W instead, because that is what the controller will actually see. At 24 V that is 56.25 A of charge current, and the calculator allows for conversion losses and a 25 percent continuous-duty margin: 72.5 A.

Controllers are sold in steps, and the next size above 72 A is an 80 A unit. A PWM controller would need 83 A for the same array, which is one reason MPPT is the usual choice. The calculator does not check the controller's maximum input voltage, which must exceed the array's open-circuit voltage on the coldest morning, so read the datasheet before you wire panels in series.

Step 4: Inverter

You enter	Value
Total Continuous Load	1,110 W
Largest Motor — Running Watts	500 W
Motor Starting Surge Multiplier	3
Safety Margin	20 %

The calculator returns	Value
Continuous Rating Needed	1,332 W
Surge Rating Needed	2,110 W

The inverter is sized by what runs at the same moment, not by the day's energy. The refrigerator, lights, electronics, water pump and a small appliance together draw 1,110 W, and the pump is the largest motor at 500 W. With a 20 percent margin the continuous rating needed is 1,332 W.

When the pump starts at 3 times its running watts (1,500 W) while the rest keeps running, the surge reaches 2,110 W. A 2,000 W inverter with a surge rating above that covers both, provided it is a 24 V model to match the battery bank. Its idle draw is the 0.6 kWh a day we already counted in step 1.

What it adds up to

The system that came out has 3 panels of 450 W, a 375 Ah bank at 24 V, an MPPT controller of at least 72 A (an 80 A unit in practice), and a 2,000 W inverter. Each part was sized from a different number: energy for the array and battery, current for the controller, and power at one moment for the inverter.

There is a catch that the chain does not show on its own. In the design month the array makes 4.05 kWh a day against a 3.6 kWh need, a surplus of only 0.45 kWh, so a bank drawn down through 2 cloudy days (7.2 kWh) would take about 16 days to refill. Adding a fourth panel raises production to 5.4 kWh and cuts that recovery to about 4 days. Sizing for autonomy without sizing for recovery is one of the commonest ways an off-grid system disappoints, and the honest fix is more array, or a generator for the darkest weeks.

Notes & limitations

This assumes a simple single-voltage DC bus. Larger systems sometimes run the array at a higher voltage than the battery bank specifically to reduce wiring losses over a long run, which changes some of these numbers, and the charge controller's maximum input voltage then becomes a check of its own.
Depth of discharge and inverter surge multipliers both depend heavily on the specific equipment chosen. Treat the defaults here as a starting planning estimate, then re-run each step with your actual battery and appliance datasheets once you've picked real hardware.

Mistakes that leave an off-grid system short

These are the errors behind most under-delivering systems.

Leaving out the inverter's own consumption. Here it is 17 percent of the entire daily need, and it runs day and night.
Using the yearly average sun hours. A system sized for the average runs short in the darkest months, exactly when you most want it to work.
Sizing for autonomy but not recovery, as the example shows. The battery has to be refilled by a surplus, not by the bare daily need.
Skipping the cable check. At 24 V, the same 3 percent voltage drop is only 0.72 V, so long runs between the array, the controller and the battery need thick conductors; use the Voltage Drop calculator on each one.
Ignoring temperature. A panel's voltage rises in the cold, which is when a series string can exceed the controller's maximum input, and a battery loses capacity and may not charge safely below freezing.
Treating the controller check as complete. The calculator sizes the current only; the voltage limit and the panels' short-circuit current are on the datasheets.

When to bring in a licensed installer or electrician

A battery bank stores a large amount of energy and can deliver very high current into a short circuit, which is a fire and burn hazard. The system needs DC-rated fuses or breakers on every battery and array conductor, a battery disconnect, correct grounding and, in most places, a permit and an inspection.

Even a fully off-grid building is normally still subject to the building and electrical codes and to your insurer's rules. A licensed installer can check the design against your equipment's datasheets, which decide most of what these calculators can only estimate.

Frequently asked questions

Why size the array for the worst month rather than the average?

Because an off-grid system has no other supply to fall back on. If it covers the load in the darkest month, it covers every other month too, with a surplus. The cost is a larger array that is oversized for most of the year, and many owners accept that in exchange for a system that never runs out in winter.

Do I need an MPPT or a PWM charge controller?

MPPT is the usual choice. It can convert a higher array voltage down to the battery voltage efficiently, which lets you wire panels in series and use thinner cable, and it typically harvests more energy. PWM is cheaper and can suit a small system where the array and battery voltages already match.

How many days of autonomy should I choose?

The number reflects how long you can tolerate no sun and no other power source. Two to three days covers typical weather in many places, and each extra day adds a full day of storage to the bank. A generator that can charge the bank lets you choose fewer days.

Why is battery capacity in amp-hours, and why does voltage matter?

An amp-hour counts charge, not energy. Energy is amp-hours times volts, so a 375 Ah bank is 9 kWh at 24 V but would be 18 kWh at 48 V. The same energy is fewer amp-hours at higher voltage, which is why the battery step asks for the system voltage.

Should I include the battery's round-trip losses?

Yes, through the system losses input. Energy that passes through a battery, a charge controller and an inverter loses a fifth or more on the way, so 25 percent is a more realistic figure than 20 for a battery system.