Most contractors who get solar generator sizing wrong make the same two mistakes: they ignore surge watts, and they don’t account for which tools run at the same time. The result is either a 3,000 dollar unit that trips on the first morning or a 6,000 dollar unit they never needed.
This guide gives you the exact sizing methodology, the same logic behind our free Jobsite Solar Generator Calculator, so you understand the math and can apply it to any jobsite configuration.
RV owners face a similar sizing challenge. See RV Solar vs Generator for Boondocking for a full cost and performance comparison tailored to campers.
What Is a Solar Generator?
A solar generator is a battery-based power station paired with solar panels. It stores DC energy in a lithium battery bank and delivers it through an inverter, rated in continuous output watts, peak surge watts, and battery capacity (Wh). Sizing requires three separate calculations: inverter output, peak surge capacity, and battery storage.
Step 1: List Every Tool and Its Wattage
For each tool, you need two numbers: running watts (continuous power draw) and starting watts (startup spike, lasting 1 to 3 seconds).
| Tool | Running Watts | Starting Watts | Surge Multiplier |
|---|---|---|---|
| Circular Saw | 1,400 W | 2,300 W | 1.6x |
| Air Compressor (1.5 HP) | 1,600 W | 4,500 W | 2.8x |
| Table Saw | 1,800 W | 3,500 W | 1.9x |
| Angle Grinder | 900 W | 1,400 W | 1.6x |
| Battery Charger | 350 W | 500 W | 1.4x |
| LED Work Lights | 200 W | 200 W | 1.0x |
| Drill/Driver | 600 W | 900 W | 1.5x |
| HVAC Vacuum Pump | 500 W | 800 W | 1.6x |
Step 2: Identify Which Tools Run Simultaneously
You need the worst-case simultaneous combination. Example for a crew of 3: Air compressor (1,600 W) + LED lights (200 W) + Battery charger (350 W) = 2,150 W continuous output needed. Use the free Jobsite Solar Generator Calculator to flag simultaneous use per tool.
Step 3: Calculate Peak Surge Capacity
Formula: Peak surge = Starting watts of highest-surge tool + Running watts of all other simultaneous tools. Example: Air compressor (4,500 W) + lights (200 W) + charger (350 W) = 5,050 W peak surge required.
Step 4: Calculate Daily Energy Consumption
Formula: Daily energy (Wh) = Sum of (Running watts x Hours used per day).
| Tool | Running W | Hours/Day | Daily Wh |
|---|---|---|---|
| Air Compressor | 1,600 | 2 | 3,200 |
| Circular Saw | 1,400 | 1.5 | 2,100 |
| LED Lights | 200 | 8 | 1,600 |
| Battery Charger | 350 | 3 | 1,050 |
| Total | 7,950 Wh/day |
Step 5: Size Your Battery Bank
Formula: Battery capacity (Wh) = Daily Wh x 1.25 (safety margin) x Autonomy days. Example (1 day): 7,950 x 1.25 = 9,938 Wh. Most single-trade contractors need 5 to 10 kWh.
Step 6: Size Your Solar Array
Formula: Solar array (W) = Battery Wh divided by Peak sun hours x 1.15. Peak sun hours: Southwest 5.5 to 6.5h, Southeast 4.5 to 5.5h, Midwest/Northeast 3.5 to 4.5h. Example (5h): 9,938 divided by 5 x 1.15 = 2,286 W solar array.
The Jobsite Solar Generator Calculator handles all six steps automatically.
Common Sizing Mistakes
- Sizing for running watts only – the inverter trips on startup.
- Assuming all tools run at once – oversizing by 40 to 60%.
- Ignoring temperature derating – LiFePO4 loses 10 to 20% below 32F.
- Forgetting system losses – wiring reduces output by 10 to 15%.
- Buying based on battery capacity alone – inverter surge must match load.
What Size Do Most Contractors Need?
| Crew Type | Continuous Output | Peak Surge | Battery |
|---|---|---|---|
| Solo electrician/plumber | 1,500 to 2,000 W | 3,500 W | 3 to 5 kWh |
| 2-person HVAC crew | 2,000 to 3,000 W | 5,000 W | 5 to 8 kWh |
| 3 to 4 person framing crew | 3,500 to 5,000 W | 7,000 to 9,000 W | 8 to 15 kWh |
| Multi-trade crew (5+) | 5,000 to 8,000 W | 10,000+ W | 15 to 25 kWh |
FAQ
How many watts does a typical jobsite solar generator need?
For a solo contractor: 1,500 to 2,000 W continuous. For a 3 to 4 person crew: 3,500 to 5,000 W continuous and 7,000 to 9,000 W peak surge.
Can a solar generator run an air compressor?
Yes, but surge capacity is critical. A 1.5 HP compressor needs 4,000 to 4,500 W on startup. Mid-range units like EcoFlow Delta Pro handle this with 6,000 to 7,000 W surge ratings.
How long does a jobsite solar generator last on one charge?
A 5 kWh battery at 1,000 W average load lasts 4 to 5 hours. Solar panels recharge during the day.
Size Your Generator in 2 Minutes
Skip the manual math. The free Jobsite Solar Generator Calculator outputs your continuous output, peak surge, battery capacity, and solar array size instantly. No account required.