Sizing off-grid solar for a shed, workshop, or tiny home depends entirely on whether you are running intermittent power tools or continuous loads like a refrigerator. A basic storage shed may only need a 100W panel and a small 12V battery for LED lighting, while a full-time tiny home requires a 1,000W+ array and a robust 24V or 48V battery bank. The key is calculating your daily watt-hours accurately and sizing your inverter to handle the surge wattage of your heaviest tool or appliance.
Scope: Scaling Down the Math
The sizing principles for a shed or tiny home are identical to sizing a full off-grid house, but the scale and margin for error are much smaller. Because you have fewer panels and smaller batteries, a single miscalculation—like leaving an inverter turned on overnight—can completely drain the system.
You must differentiate between energy capacity (Watt-hours per day) and power output (instantaneous Watts). A workshop might use very little energy overall because you only use a saw for 5 minutes a day, but it requires massive power to start the saw's motor.
Typical Load Profiles
- The Storage Shed (100W Solar | 50Ah 12V Battery): Used purely for visibility and convenience. Loads include LED strip lights (10–30W) and charging 18V power tool batteries (50W). Total daily use is usually under 200 Wh/day.
- The Hobby Workshop (400W Solar | 100Ah 12V Battery | 2000W Inverter): Used on weekends. Loads include lights, a radio, and intermittent use of heavy tools like a miter saw, drill press, or air compressor. Total daily use might be 500–800 Wh/day, but requires a large inverter for tool surges.
- The Full-Time Tiny Home (1,200W+ Solar | 400Ah+ 12V or 24V Battery): Lived in daily. Loads include a 12V or highly efficient 120V fridge, water pump, lights, fans, laptops, and internet routers. Total daily use is typically 1,500–3,500+ Wh/day.
Critical Factors Often Overlooked in Small Systems
Many beginner guides treat a shed system like a toy, but failing to account for these factors will leave you in the dark:
- Motor Surge (LRA): Power tools (saws, compressors) and refrigerators have electric motors. When they start, they pull a "surge" or "Locked Rotor Amps" that is 3 to 5 times their running wattage. A 1,500W table saw might pull 4,500W for a split second. If your inverter isn't sized for that surge, it will instantly shut down and flash an overload error.
- Inverter Standby Drain: An inverter consumes power just by being turned on, even if nothing is plugged into it. A typical 2,000W inverter might have a standby draw of 20W. Left on 24/7, that consumes 480 Wh per day—which could be more energy than your actual tools use!
- Winter Shading: Sheds and tiny homes are often placed under or near trees. While a location might get great sun in July, the lower angle of the winter sun combined with bare tree branches can cast devastating micro-shadows across the panels, cutting winter production by 80%.
Illustrative Example: Sizing a Weekend Workshop
Note: This is an illustrative calculation for a workshop used primarily on weekends.
1. Calculate Daily Loads (Saturday Usage):
- LED Shop Lights: 40W x 4 hours = 160 Wh
- Radio/Bluetooth Speaker: 10W x 4 hours = 40 Wh
- Miter Saw (1,500W running): Used for a total of 15 minutes (0.25 hours) = 375 Wh
- Tool Battery Charger: 60W x 2 hours = 120 Wh
- Inverter Inefficiency (add 15% to AC loads): (160+40+375+120) * 1.15 = 800 Watt-hours (Wh) total daily use.
2. Size the Solar Array: Assuming 4 peak sun hours:
- 800 Wh / 4 hours = 200 Watts needed.
- Add 25% buffer for system losses: 200W x 1.25 = 250W minimum array. (Two 150W panels would be perfect).
3. Size the Battery Bank: Since this is a weekend shop, you don't need many "days of autonomy" because the system has all week to recharge. We will size for 1 day of use using a LiFePO4 battery (80% DoD).
- 800 Wh / 12V = 66 Ah.
- Account for 80% DoD: 66 Ah / 0.8 = 83 Ah minimum battery. (A standard 100Ah 12V lithium battery is ideal).
4. Size the Inverter (The Catch): Even though the daily energy use is low (800 Wh), the miter saw pulls 1,500W running and likely surges to 3,500W on startup. You must buy an inverter rated for at least 2,000W continuous and 4,000W surge, otherwise, the saw will not spin up.
Practical Checklist for Small Off-Grid Systems
- [ ] Do a Load List: Guessing doesn't work. Write down every item, its wattage, and how many hours you use it. Use the WattSizing calculator to do the math.
- [ ] Choose the Right Voltage: Stick to 12V for basic sheds. If your tiny home requires more than 1,000W of solar or a 3,000W+ inverter, step up to a 24V system to keep wire thickness and charge controller costs down.
- [ ] Install an Inverter Remote Switch: If your inverter is hard to reach, install a remote on/off switch by the door. Turn the inverter off when you leave the shed to eliminate standby battery drain.
- [ ] Buy an MPPT Controller: Even for a small 200W system, an MPPT charge controller will harvest significantly more power in cloudy conditions than a cheap PWM controller.
Frequently Asked Questions (FAQs)
Can I run a table saw or air compressor on off-grid solar? Yes, but it requires a large inverter. Heavy power tools have massive startup surges. A typical 15-amp table saw requires an inverter capable of at least 3,000W to 4,000W of surge power. You also need a battery bank capable of delivering that instantaneous high current without experiencing extreme voltage drop (usually requiring at least two 100Ah lithium batteries wired in parallel).
What size solar system do I need for a tiny home? A full-time tiny home with a refrigerator, water pump, lights, and electronics typically requires between 1,000W and 2,000W of solar panels, paired with a 300Ah to 600Ah battery bank (often at 24V or 48V). If you plan to run electric heating or air conditioning, the system size and cost will triple.
Should I leave my shed inverter on all the time? No. Unless you are running a continuous load like a refrigerator or a security camera system, you should turn the inverter off when you leave the shed. Inverters consume "standby power" (often 10W to 30W) continuously, which can completely drain a small battery bank over a few days of bad weather.
Is a 12V or 24V system better for a workshop? If your workshop only runs lights and small chargers (under 1,000W total draw), 12V is cheaper and simpler. However, if you are running heavy power tools requiring a 2,000W+ inverter, a 24V system is significantly better. It cuts the amperage in half, allowing you to use thinner, cheaper wiring and reducing the stress on the battery terminals during high-surge tool startups.
Can I use a portable "solar generator" (like a Jackery or EcoFlow) for my shed? Yes, portable power stations are excellent for small sheds. They combine the battery, inverter, and charge controller into one plug-and-play box. However, they are generally more expensive per watt-hour than building your own component system, and if one internal part breaks, the whole unit must be repaired.
How do I heat my off-grid tiny home or shed? Do not use off-grid solar for space heating. Electric heaters consume massive amounts of power (1,500W continuous). Running a space heater for just 4 hours requires 6,000 Wh of energy, which would require an enormous and expensive solar array. Use propane, a wood stove, or a diesel heater for off-grid heating instead.
