Solar Critical
This page is not available for printing. Please visit solarcritical.com.
Solar Critical Members

Off-grid solar, sized right and explained plainly

Solar Critical walks you through what a solar power system needs, from a 100 W shed light to an 8,000 W whole-house setup, and what every box on the wall actually does.

See the 8,000 W install Compare 100 W to 8,000 W
Introduction

Welcome to Solar Critical

In a nuclear reactor, "critical" is the point where the system sustains itself. That is the goal here too: a solar system that keeps your lights, well pump and freezer running on its own, day after day.

Solar Critical was started by Joe Miller, a nuclear engineer who designed and installed his own off-grid system at his ranch. The site gathers what he learned into one plain-language guide, built around five parts every system has: the charge controller, the inverter, the shunt, the battery and the transfer switch.

Every number on this page comes from the same sizing method, so you can see how a system grows from 100 W to 8,000 W and why the wire and fuses grow with it.

6system sizes compared, 100 W to 8,000 W
4.5 hof full sun per day assumed for sizing
46 kWhof battery storage in the 8,000 W system
125%rule: every fuse sized at 1.25 × the current
Solar arraypanels wired in strings Charge controllerMPPT Battery + busbarClass T fuse on positiveshunt on negative InverterDC to 120 or 240 V AC Transfer switchpicks one AC source House loadspanel or subpanel 1 PV breaker 2 fuse 4 AC breaker from generator or grid 3 heaviest cable + fuse

Every system on this site follows this layout. The four numbered runs each get their own fuse or breaker; run 3, battery to inverter, carries the most current.

Step by step

Installing an 8,000 W system

This is a whole-house, 48 V system with 120/240 V split-phase output. It produces about 36 kWh on a sunny day and stores about 46 kWh. The parts below are Victron examples; equivalent brands work the same way.

Solar array8,000 W, 6 strings (3 per controller)
Charge controllers2 × SmartSolar MPPT 250/85
Inverter/chargers2 × MultiPlus-II 48/5000 120 V, split-phase
Battery48 V LiFePO4, 900 Ah (about 46 kWh)
ShuntSmartShunt 500 A
Transfer2-pole 50 A switch + critical-loads subpanel

  1. Plan the loads and pull a permit

    List what must run: well pump, refrigerator, freezer, lights, and anything 240 V. Add up daily kWh and the biggest starting surge. Most counties require a permit and inspection for a system this size, and many utilities require approval before anything connects near the grid.

  2. Mount the solar array

    Install racking facing true south, tilted about equal to your latitude. Bond every panel frame and rail to a grounding conductor.

    • Wire panels into 6 strings, 3 strings for each controller.
    • Check that each string's cold-weather open-circuit voltage stays under the controller's 250 V limit.

  3. Wire the strings through a combiner

    Each group of 3 strings lands in a combiner box with a 20 A fuse per string. From the combiner, run 4 AWG copper through a 70 A DC-rated PV breaker to each controller. This is run 1.

  4. Set the battery bank and busbars

    Install the 900 Ah, 48 V lithium bank on a level, ventilated, non-combustible surface.

    • Put a 250 A Class T fuse within 7 inches of the battery positive.
    • Run 250 kcmil copper from the battery to positive and negative busbars.
    • Install the SmartShunt in the negative cable so all current passes through it.

  5. Mount and connect the two charge controllers

    Connect each controller to the busbars with 2 AWG cable through a 110 A fuse. This is run 2. Set the charge voltages to the battery maker's lithium settings in the VictronConnect app.

  6. Mount and connect the two inverter/chargers

    Each MultiPlus-II gets 1 AWG cable through a 125 A fuse from the busbars. This is run 3, the heaviest current in the system. Program the pair as a split-phase set (one unit on L1, one on L2) with VE.Configure before any AC is connected.

  7. Wire the AC side and the transfer switch

    The inverter output runs through a 2-pole 45 A breaker on 8 AWG copper to a critical-loads subpanel. This is run 4. The generator or grid feeds the MultiPlus AC input through a 2-pole 50 A transfer switch, so the two sources can never connect to each other.

  8. Add monitoring

    A Cerbo GX (or similar) ties the controllers, inverters and shunt together, so you can see charge level, solar input and loads on one screen or your phone.

  9. Power up in order

    • Torque every lug to spec and check polarity with a meter.
    • Close the battery fuse, then the controller fuses, then the PV breakers.
    • Confirm charging, then close the inverter fuses and turn on the inverters.
    • Close the AC breaker and test each circuit, then test the generator transfer.

  10. Inspect and recheck

    Have the system inspected. After the first month, re-torque the battery and busbar lugs, since copper settles under heavy current.

Comparison

Every size from 100 W to 8,000 W

As systems grow, the battery voltage goes up to keep the current, and the cable size, under control. The run numbers match the diagram above.

System size100 W500 W1,000 W2,000 W4,000 W8,000 W
System
Battery voltage12 V12 V24 V24 V48 V48 V
Solar energy, sunny day0.45 kWh2.25 kWh4.5 kWh9 kWh18 kWh36 kWh
AC output120 V120 V120 V120 V120/240 V120/240 V
Main parts
Charge controller (MPPT)10 A50 A50 A85 A85 A2 × 85 A
Inverter300 W500 W1,000 W2,000 W4,000 W8,000 W
Battery bank (LiFePO4)100 Ah
1.3 kWh		300 Ah
3.8 kWh		300 Ah
7.7 kWh		500 Ah
12.8 kWh		500 Ah
25.6 kWh		900 Ah
46.1 kWh
Shunt500 A500 A500 A500 A500 A500 A
Main battery fuse (Class T)35 A70 A70 A125 A125 A250 A
Transfer switchNoneNonePlug-in automatic30 A manual or inverter relay2-pole 30 A or interlock2-pole 50 A + critical-loads panel
Wiring
1 · PV strings111236
1 · PV breaker / wire10 A / 14 AWG15 A / 14 AWG25 A / 10 AWG45 A / 10 AWG70 A / 4 AWG70 A / 4 AWG
2 · Controller fuse / cable15 A / 14 AWG70 A / 4 AWG70 A / 4 AWG110 A / 2 AWG110 A / 2 AWG110 A / 2 AWG
3 · Inverter fuse / cable35 A / 8 AWG60 A / 6 AWG60 A / 6 AWG125 A / 1 AWG125 A / 1 AWG250 A / 250 kcmil
4 · AC breaker / wire15 A / 14 AWG15 A / 14 AWG15 A / 14 AWG25 A / 10 AWG2-pole 25 A / 10 AWG2-pole 45 A / 8 AWG
Example Victron parts
ControllerMPPT 75/10MPPT 100/50MPPT 100/50MPPT 150/85MPPT 250/852 × MPPT 250/85
InverterPhoenix 12/375Phoenix 12/800Phoenix 24/1200MultiPlus-II 24/30002 × MultiPlus-II 48/30002 × MultiPlus-II 48/5000

Sizing assumes 4.5 peak sun hours a day and lithium batteries used to 80%. Fuses are 125% of continuous current, solar circuits 156% of panel short-circuit current (NEC 690.8), wire from NEC 310.16 copper at 75 °C. For 8,000 W, runs 1 and 2 are per controller. Have a licensed electrician confirm sizes for your site.

The parts

What each part does

Five parts make up every system on this site. Here is each one in a sentence, and then in a little more detail.

What is a charge controller?

The charge controller is the gatekeeper between the solar panels and the battery.

Panels put out a voltage that rises and falls with the sun. The controller converts it to exactly what the battery needs and stops charging when the battery is full, so the battery is never overcharged. An MPPT controller also finds the panels' best operating point minute by minute, gathering up to 30% more energy than an older PWM type.

Rated by
Output amps and maximum input volts (an MPPT 250/85 accepts up to 250 V and delivers up to 85 A)
How to size
Solar watts ÷ battery charging voltage, plus 10% headroom

What is an inverter?

The inverter turns the battery's DC power into the 120 V or 240 V AC power your house uses.

Batteries store direct current; your outlets, pump and appliances run on alternating current. The inverter makes that conversion. An inverter/charger, such as a MultiPlus-II, also charges the battery from a generator or the grid and has a built-in transfer relay.

Rated by
Continuous watts, surge watts for motor starts, and output voltage
How to size
Larger than everything that runs at once, with surge room for pumps and compressors

What is a shunt?

The shunt is the battery's fuel gauge.

It is a precise, low-resistance link in the battery negative cable. Every amp going in or out passes through it, so it can count exactly how much energy has been used and how much is left, shown as a percentage on your phone or monitor. Without one, battery voltage alone gives only a rough guess, especially with lithium.

Rated by
Maximum amps (a SmartShunt 500 A covers every size on this site)
Where it goes
Between the battery negative and everything else, so nothing bypasses it

What is a battery?

The battery stores the sun's energy so you have power at night and on cloudy days.

Solar systems today mostly use lithium iron phosphate (LiFePO4) batteries. They last thousands of cycles, can be used to 80% or more of their capacity, need no watering, and are far safer than other lithium types. Banks are built at 12, 24 or 48 volts; bigger systems use higher voltage so the current and cable stay manageable.

Rated by
Amp-hours at a voltage; amp-hours × volts ÷ 1,000 = kWh
How to size
One sunny day's solar production, divided by the usable fraction (80%)

What is a transfer switch?

The transfer switch picks which power source feeds the house: solar inverter, generator or grid, never two at once.

It protects your equipment and protects line workers, because power can never flow backward into the utility lines. Manual switches use a handle; automatic ones change over by themselves when a source fails. Small systems that only run plug-in loads don't need one.

Rated by
Amps and poles (2-pole for 240 V split-phase)
By size
None at 100–500 W; plug-in automatic at 1,000 W; 30 A at 2,000–4,000 W; 2-pole 50 A at 8,000 W

Before you energize anything