Quick answer: start with 650 W of solar
Welding uses surprisingly little energy and a lot of power. About 20 minutes of arc time a day is only 1,400 Wh, roughly 650 W of solar, but while the arc is lit the welder can pull 4,000 W or more, so the inverter, the battery current rating, and the cabling are the real design.
Planning result
At 200 W per module, the array target is 4 panels. That count assumes four peak-sun-hours, 75% end-to-end solar yield, 90% conversion efficiency, and a 20% planning margin. Welding is a weather-independent shop load; the seasonal effect is fewer sun-hours to recharge after a long winter project.
How a MIG welder actually uses power
Planning case: a home-shop MIG welder planned at the 240 V class, drawing about 4,000 W from the supply while the arc is struck, for about 30 minutes of welding a day at 70% arc-on time, roughly 20 minutes of actual arc. Miller rates its Millermatic 211 at 16.76 A from 240 V (about 4.0 kW) while welding at 160 A, and recommends a generator of 8,000 W or more for full output.
A welder draws heavy power only while the arc is struck; between beads, while you reposition or change wire, it idles with its cooling fan. The duty-cycle rating on the nameplate, a percentage at a stated output current, says how many minutes in every ten it can weld before it must cool: it is a thermal limit, not the share of the day it uses energy.
Load calculation from running watts and duty cycle
The appliance-class estimate is 4,000 W running for 0.5 h/day at a 70% duty cycle, an average of about 2,800 W across those hours:
Then losses and reserve raise the energy the system must collect to about 1,867 Wh/day.
Typical manufacturer nameplate/manual range for the MIG welder class, triangulated with Lincoln Electric: Easy-MIG 140 specification sheet and Miller Electric: Millermatic 211 PRO, then reduced to a conservative planning midpoint; this is not a product specification. Verify the exact unit's nameplate and manual before sizing.
Read the nameplate's rated input current at your usual output setting and time the real arc-on minutes on a project day. A clamp meter on the supply cord shows the actual input current while welding.
Solar and battery sizing table
| Design item | Planning value | Basis |
|---|---|---|
| Appliance energy | 1,400 Wh/day | Running W × hours × duty cycle |
| Solar collection target | 650 W | 4 peak-sun-hours, 75% yield, 20% margin |
| 200 W panel count | 4 | Rounded up; real modules must satisfy controller voltage/current limits |
| One-day battery allowance | 2,400 Wh | 90% conversion efficiency, 80% LiFePO4 usable fraction, 20% margin |
| 12.8 V LiFePO4 equivalent | 190 Ah | Nominal planning conversion |
| 25.6 V LiFePO4 equivalent | 95 Ah | Nominal planning conversion |
Short, intense loads like this are about battery discharge current more than capacity: the battery must deliver the welder's input power through the inverter without hitting its current limit or low-voltage cutoff.
Build the closest passing wiring-diagram preset
The MIG Welder Planning System preset is an engine-passing starting point whose array, battery bank, continuous inverter rating, and surge rating each meet or exceed this page's stated planning allowances. Adjust it only from measured load data and verified equipment limits.
Inverter and startup current
Use at least 5,000 W continuous with 8,800 W surge for this planning case. Inverter-type welders draw a charging inrush when switched on, and transformer welders can draw a large magnetizing inrush and a heavy current while welding. The surge allowance here is 8,800 W; check the welder manual for its guidance on generator or inverter supply.
The 8,000 W startup estimate is not a substitute for the manual's locked-rotor, compressor-start, or motor-start requirements, and an inverter's surge rating usually lasts seconds, not minutes. A battery-to-inverter cable that is too long or too small can pull the DC input below the inverter's low-voltage threshold during that start even when the battery has plenty of energy, so use the diagram machine's real run length.
The common mistake
Ways to cut the load
- Weld at the lowest output that gives good penetration; input power rises with output current.
- If it can do the work, a 120 V welder on a 20 A circuit is far easier to supply off-grid than a 240 V machine.
- Batch welding into the sunniest hours of the day.
- Keep the supply cord short and heavy so voltage drop does not destabilize the arc.
What to verify before buying equipment
- The exact running watts, startup current, voltage, frequency, and required circuit from the appliance label and manual.
- Actual watt-hours over a representative day; seasonal heating or cooling loads need a design-day measurement.
- Local peak-sun-hours for the worst operating season, plus shade and array orientation.
- Controller maximum input voltage at the coldest expected temperature and maximum output current.
- Battery discharge current, series/parallel permission, terminal torque, low-temperature controls, and overcurrent protection.
- Conductor ampacity, voltage drop, terminal temperature ratings, disconnecting means, grounding, local amendments, and inspection requirements.
For a whole-system load list, use the load and appliance calculator. For the system context around this appliance, continue to the Shed, workshop, and garage planning hub.
Frequently asked questions
How much solar do I need for a MIG welder?
For the planning case on this page (a 240 V MIG welder with about 20 minutes of arc time a day), allow about 650 watts of solar at four peak-sun-hours per day. That covers an estimated 1,400 Wh daily load plus conversion, temperature, and recharge margin. The array is rarely the limit for welding; the inverter and battery current are.
What size inverter should I use for a MIG welder?
Plan on at least 5,000 W continuous and 8,800 W surge. Confirm the welder's input current at your output setting against the inverter's continuous rating, and check whether the welder manual permits inverter or generator supply.
What battery size keeps a MIG welder running for one day?
The one-day planning allowance is about 2,400 Wh nominal usable design capacity, roughly 190 Ah at 12.8 V or 95 Ah at 25.6 V for LiFePO4. Battery discharge current, not capacity, is usually the constraint for a welder.