DIY solar installation

A DIY solar installation at the 1–5 kW scale can be completed by a mechanically competent homeowner for roughly $0.80–$1.50 per watt installed — compared to $2.50–$3.30 per watt for a professional installation. The savings come from your own labor and direct component purchasing. The risk comes from skipping the load audit, undersizing wire, or omitting required overcurrent protection. Both failure modes are avoidable.

This page walks through the sizing and installation sequence in order: load audit, panel count, battery sizing, controller selection, wire sizing, fusing, and physical installation. Read solar basics first to understand the panel specifications and system vocabulary this page builds on.

Load audit worksheet

Every sizing decision flows from a load audit. Work through this before pricing any hardware.

List every critical electrical device you intend to power. For each, record the wattage (from the nameplate or measured with a plug-in watt meter) and the daily hours of use. Multiply to get daily watt-hours.

Device	Watts	Hours/day	Wh/day
LED lighting (8 bulbs × 10W)	80 W	5 h	400 Wh
Chest freezer (average draw)	60 W	24 h	1,440 Wh
Laptop	45 W	4 h	180 Wh
Phone and radio charging	30 W	3 h	90 Wh
CPAP machine	30 W	8 h	240 Wh
Well pump (500W @ 15 min/day)	500 W	0.25 h	125 Wh
Daily critical load total			2,475 Wh

Field note

Appliance nameplates list maximum rated wattage, not average draw. A 150W refrigerator compressor runs intermittently — at a 40% duty cycle, average draw is 60W. Use a plug-in power meter (affordable from any hardware store) to measure actual average consumption for refrigerators, freezers, and pumps. Sizing from nameplate alone overestimates loads by 50–100% and results in an oversized, expensive system.

Add a 20–25% overhead factor to cover the inverter efficiency loss (typically 90–95%), wiring losses, and controller losses:

2,475 Wh × 1.25 = 3,094 Wh/day net required production

Panel sizing formula

Panel sizing converts your daily load requirement into a panel array size, using your location's worst-month peak sun hours (PSH).

Formula:

Array watts = (Daily Wh ÷ worst-month PSH) ÷ derate factor

Worked example:

Daily net required production: 3,094 Wh
Worst-month PSH (Northeast US, December): 2.5 h/day
Derate factor (temperature, soiling, mismatch): 0.80

Array watts = (3,094 ÷ 2.5) ÷ 0.80 = 1,237 ÷ 0.80 = 1,547W

Round up to the next standard array size: 4 panels × 400W = 1,600W array.

For a Midwest location with 3.5 PSH worst-month, the same load requires only 1,104W — three 400W panels. Worst-month PSH is the single most important input; do not use annual averages.

Battery sizing

Battery sizing is determined by your autonomy requirement — how many days of critical loads must the battery cover without any solar input.

The preparedness standard is 3 days of autonomy. Size for the critical loads from your audit, not total household consumption.

Battery capacity (Wh) = Daily load × autonomy days ÷ usable DoD

For LiFePO4 chemistry at 90% depth of discharge (DoD), using 3 days of autonomy:

2,475 Wh × 3 days ÷ 0.90 = 8,250 Wh

Add a 15% buffer for aging and inverter losses:

8,250 Wh × 1.15 = 9,488 Wh — round up to 10 kWh

For a 48V system: 10,000 Wh ÷ 48V = 208 Ah at 48V. Two 48V 100Ah LiFePO4 rack batteries (200 Ah total) meets this requirement.

See batteries for chemistry comparison (LiFePO4 vs. lead-acid), BMS requirements, and detailed maintenance schedules.

Charge controller selection

MPPT vs. PWM

maximum power point tracking (MPPT) controllers are the correct default for any system with series-wired panels or total array power above 100–200W. They convert excess panel voltage to additional current, recovering 20–30% more energy than pulse width modulation (PWM) in cold weather and when panel Vmp exceeds battery voltage. MPPT controllers are also required when panel strings run at 60–100V+ (common in 48V systems) — PWM cannot handle this.

PWM controllers connect panels directly to batteries. They are only appropriate for very small, cost-sensitive systems where all panels are wired in parallel and panel Vmp closely matches battery voltage — typically 12V systems under 100W. For any preparedness or off-grid system, this is a narrow use case.

Controller sizing

Size the charge controller to handle 125% of the array's Isc per National Electrical Code (NEC) 690.8:

Controller amperage = (Isc per string × number of parallel strings) × 1.25

For our 1,600W example (four 400W panels, each with Isc = 10.1A, wired as two parallel strings of two series panels):

Each string carries 10.1A Isc Two parallel strings: 10.1A × 2 = 20.2A 20.2A × 1.25 = 25.3A — use a 30A or 40A MPPT controller

Also verify the controller's maximum PV input voltage is above your string Voc at lowest expected temperature. At 20°F (-7°C), silicon panel voltage increases approximately 10–15% above STC Voc. For two 400W panels in series with Voc = 49.5V each: 49.5V × 2 = 99V at STC; at cold temperature, up to 110V. Choose a controller rated for at least 150V PV input for this configuration.

Wire sizing

Off-grid solar wiring diagram showing solar panels, 30A fuse, MPPT charge controller, battery bank, 200A fuse, pure sine inverter, and AC load panel with wire gauge callouts

Undersized wiring is the leading cause of field failures and fires in DIY solar systems. The goal is to keep voltage drop below 3% on any circuit, with conductors rated for at least 125% of maximum expected current.

Use only solar-rated PV wire or USE-2 cable for panel connections. Standard THWN wire lacks UV resistance and is not rated for outdoor DC solar circuits under NEC 690.31.

DC wiring voltage drop reference (3% drop maximum):

Circuit	Length (one way)	Amps	Minimum wire gauge
Panel string to controller	Up to 50 ft (15 m)	20A	10 AWG (6 mm²)
Panel string to controller	50–100 ft (15–30 m)	20A	8 AWG (10 mm²)
Controller to battery bank	Up to 10 ft (3 m)	60A	4 AWG (21 mm²)
Battery bank to inverter	Up to 5 ft (1.5 m)	200A	2/0 AWG (67 mm²)

Always calculate voltage drop for your specific run length. Tools are available free at online wire sizing calculators — input your voltage, current, wire length, and acceptable voltage drop percentage.

DC current does not behave like AC

DC arcs do not self-extinguish the way AC arcs do. An undersized or loose DC connection generates heat proportional to the square of the current. At high DC currents (100A+), a poor connection or undersized wire can sustain an arc and start a fire before any fuse responds. Use ferrules or ring terminals on all DC connections, torque terminals to spec, and keep cable runs as short as physically possible.

Fusing rules

Every conductor leaving the positive battery terminal must be protected by a fuse or breaker within 18 inches (46 cm) of the terminal. This protects against short-circuit faults — the highest-risk condition in any DC system.

Fuse sizing per NEC 690.8: multiply the circuit's maximum current by 1.25 for the minimum fuse rating.

Circuit	Max current	Minimum fuse rating
Panel string (per string)	Isc × 1.25	e.g., 15A for 10.1A Isc panels
Battery to MPPT controller	40A	50A ANL or breaker
Battery to inverter	200A	250A ANL fuse or DC breaker

Use only fuses rated for DC interruption and marked with a DC voltage rating equal to or greater than your system voltage. Standard automotive fuses and AC breakers are not rated for DC fault current and can fail explosively. Use ANL fuses, MIDI fuses, or UL 489B-listed DC breakers.

No fuse = no protection

Battery banks can deliver thousands of amps under a dead short — enough to vaporize wire and ignite insulation in less than a second. Any unfused conductor between the battery and a load, controller, or inverter is a live hazard. Install fusing before energizing any circuit. Verify fuses are present and properly rated at every commissioning and maintenance inspection.

Step-by-step installation sequence

This sequence assumes a standalone off-grid DC system with a 48V battery bank, MPPT controller, and battery-based inverter. Follow this order to avoid energizing unfused conductors.

Mount panels on roof or ground frame. Do not connect any wiring yet. Confirm panel orientation is within 30° of true south and tilt angle matches your latitude.
Run conduit and wiring from panels to the battery/controller location. Use PV wire or USE-2 for all outdoor runs; transition to THWN or larger inside conduit when entering the building. Maintain minimum 3-foot (0.9 m) conduit burial depth outdoors in non-traffic areas (12 inches / 30 cm in conduit for residential circuits in most jurisdictions — verify local code).
Install the battery bank in its final location. Do not connect cables to the battery terminals yet.
Mount the MPPT charge controller within 3–10 feet (1–3 m) of the battery bank to minimize controller-to-battery cable length and resistive loss.
Mount the inverter as close to the battery bank as practical. Every additional foot of 2/0 cable at 200A costs you voltage drop and adds cost.
Install all fusing and disconnects. This includes the battery-side fuse or breaker for each output circuit, the DC disconnect between the panel array and the charge controller, and the AC breaker in the inverter output panel.
Connect battery to fuses and disconnects — but leave the main fuse open. Verify polarity at every terminal before energizing.
Connect charge controller to battery. Close the battery-to-controller fuse. Verify the controller powers up and displays correct battery voltage. Configure the charging profile for your battery chemistry before connecting any panels.
Connect inverter to battery. Close the battery-to-inverter fuse. Run a test load. Confirm AC output voltage is correct.
Connect panel wiring to the charge controller. Panels in full sun will immediately begin delivering voltage. Use a multimeter to verify string Voc matches calculations before plugging MC4 connectors into the controller. Verify the controller enters bulk charging.
Install grounding. Ground the array frame and all metallic components to the grounding electrode system. A dedicated grounding rod (5/8 inch / 16 mm diameter, minimum 8 feet / 2.4 m long, driven vertically) is required if the system is not bonded to an existing building grounding system.

Crimping MC4 connectors

MC4 connectors are the standard weatherproof connector used between solar panels and array wiring. Each connector consists of a pin or socket contact, a barrel, and an outer housing with an IP65-rated seal.

Crimping requires a dedicated MC4 crimping tool — do not substitute generic crimpers, pliers, or wire strippers. Improper crimps cause resistance buildup, heat, arcing, and eventual connector failure.

Strip wire insulation 11–13 mm (approximately 1/2 inch) from the end
Insert the bare wire fully into the contact; no copper should show outside the contact barrel
Crimp firmly with the MC4 crimp tool; the contact should not pull free from the wire under manual tension
Slide the contact into the connector housing; push until you hear and feel a click
Tighten the cable retention gland finger-tight; do not overtighten and crack the housing
Test the mated pair by attempting to pull apart by hand — a properly mated MC4 pair requires a MC4 disconnect tool to release

Permitting and inspection

Permit requirements vary significantly

Most jurisdictions require permits for any grid-connected solar installation and many standalone systems above a certain wattage threshold (commonly 200W or 600W). A permit typically costs $400–$1,000 USD in total fees. Unpermitted installations can result in fines of $500–$10,000, forced removal, insurance denial, and complications when selling your home.

For off-grid systems in a rural outbuilding or not connected to the utility grid, many jurisdictions have fewer requirements — but verify before you build.

What a permit review typically requires: - Single-line electrical diagram showing panel array, controller, battery bank, inverter, and all fusing and disconnects - Panel spec sheets for the specific model being installed - Site plan showing panel location relative to roof ridge and property lines - Completed application form with system size and equipment list

Most homeowners can prepare permit drawings using free tools such as AutoCAD LT, Lucidchart, or even a hand-drawn schematic. The NEC 690 code requirements covered in this page form the basis of what an inspector will verify during final inspection.

DIY solar installation checklist

For a fully independent home system, the decisions around system architecture — grid-tied vs. hybrid vs. off-grid — and whole-home design are covered in off-grid solar systems. For battery chemistry comparison, BMS requirements, and maintenance, see batteries.