Most solar advice starts with panels. Ours starts with your life. Off grid, you don't buy electricity, you budget it — and the honest way to size a system is to know exactly what you use before you spend a dollar on hardware.
This lesson is written from the field. The difference between an off-grid system that quietly works for twenty years and one that leaves you in the dark every January comes down to three things: an honest load list, an honest sun number, and respect for the motors that surge when they start.
- 1List every load in your home and find its watts and daily watt-hours.
- 2Look up your real winter sun hours.
- 3Size a solar array honestly, with losses included.
- 4Size an inverter that can start your well pump.
- 5Choose between roof and ground mounts, and know what the permit requires.
Watts, watt-hours and your load list
Two numbers run everything. Watts is how hard something pulls power right now. Watt-hours is how much energy it uses over time. A 100-watt light left on for 5 hours uses 500 watt-hours. A thousand watt-hours is one kilowatt-hour (kWh), the unit on a power bill.
The loads that matter
- Heat is the budget-breaker. Space heaters, electric water heaters, electric dryers and electric ranges can each use as much as the rest of the house. Off grid, heat usually comes from wood, propane, the sun or a very efficient heat pump.
- Motors surge. Well pumps, refrigerators and power tools pull about twice their running watts for a moment when they start. That decides your inverter size.
- Phantom loads add up. Chargers, TVs on standby and routers that run all day can quietly use hundreds of watt-hours.
- Small, efficient, always-on beats big and occasional. An efficient fridge running all day uses less than an hour of a hair dryer.
The honest load calculator
Edit the watts and hours to match your home. Use the label on each appliance, or measure it with a plug-in meter.
| Load | Watts | Hrs/day | Wh/day |
|---|---|---|---|
| Daily total |
Planning numbers only. Array = daily Wh ÷ (sun hours × 0.75). The 0.75 covers heat, dust, wiring and battery losses. Have a licensed professional finalize any design.
Every watt-hour you don't need is solar you don't have to buy, mount or maintain. Efficient appliances, LED lighting, a propane or wood range, and a sunny kitchen often cut an off-grid system's size in half. The most beautiful homes are also the most efficient ones.
Sun hours: the number that matters
A "peak sun hour" is one hour of sunlight at full strength, 1,000 watts per square meter, the condition panels are rated in. A summer day may have 14 hours of daylight but only 6 peak sun hours, because morning and evening light are weak.
California ≈ 5.6
peak sun hours a day, averaged over the year. Oregon is about 4.2, Washington 3.8, Texas 5.3, Missouri 4.6, Tennessee 4.4.
Your worst month
Winter sun can run 25 to 50 percent below the yearly average. A grid-tied home can average it out. An off-grid home has to live through December.
Find your own numbers with NREL's free PVWatts calculator: enter your address and a test system size, and it shows expected output for each month. Use the lowest winter month, or plan a generator for the darkest weeks (Lesson 5).
Shade beats everything
Even a little shade on part of an array can drop its output sharply. Winter sun is low in the sky, so a tree that's harmless in July can shade your panels all of December. Check your site in winter, or use a sun-path app, before you choose where panels go.
Sizing the array and the inverter
The 0.75 is honesty. Real systems lose roughly a quarter of their rated output to heat, dust, wiring, the charge controller and the batteries. Marketing math leaves it out. Your winter will not.
A small, efficient homestead uses about 4,600 watt-hours a day (the starting numbers in the calculator). With 3 winter sun hours:
4,600 ÷ (3 × 0.75) ≈ 2,050 watts of panels
That's six 400-watt panels, rounding up. In summer the same array makes far more than you need, which is when to run the washer, pump the tank full and charge the tools.
The inverter: sized by your biggest motor
The inverter has two ratings. Continuous watts is what it can run all day. Surge is the brief extra it can give a motor starting up. Well pumps set the bar:
| Well pump | Running watts | Starting surge | Suggested inverter |
|---|---|---|---|
| ½ hp | about 370 W | about 740 W | 1,500 W or more |
| ¾ hp | about 500 W | about 1,000 W | 2,000 W or more |
| 1 hp | about 750 W | about 1,500 W | 2,500 W or more |
| 1½ hp | about 1,125 W | about 2,250 W | 3,000–3,500 W |
Typical figures; check your pump's label. A "soft start" pump or a variable-speed drive can cut the surge sharply. Choose a pure sine wave inverter for motors and electronics.
Design choices that last
Ground mount
Aim and tilt it perfectly for winter, sweep snow and dust off by hand, and keep the roof free of holes.
Takes yard space, needs a trench for wiring, and should be fenced from animals and kids.
Roof mount
Uses space you already have, and the wiring run is short.
You live with the roof's angle and direction, and roof arrays have added shutdown rules for firefighters.
Barn or carport roof
A sunny outbuilding roof can carry the array and shade a car, tools or firewood.
Needs a longer wire run to the house, sized for voltage drop.
- Tilt for winter. Off grid, tilt panels steeper than your latitude, often about latitude plus 15 degrees, to catch low winter sun and shed snow.
- Go higher voltage. 48-volt battery systems need thinner wire and lose less power than 12- or 24-volt systems, for anything bigger than a cabin.
- Leave room to grow. Pick an inverter and charge controller with spare capacity, so adding panels later doesn't mean starting over.
- Put the equipment somewhere cool and dry. A clean, ventilated room or shed makes batteries and electronics last longer and keeps them away from children.
Permits and inspections
Solar is electrical work, and electrical work needs a permit almost everywhere that has a building department. That's a good thing: the inspection catches the mistakes that start fires.
- Correct wire sizes and overcurrent protection
- Proper grounding and labeling
- Disconnects you can reach in an emergency
- Batteries installed to the manufacturer's rules
- Roof arrays that meet firefighter shutdown rules
New homes generally have to include solar under the state energy code, a requirement written around homes connected to the grid. Ask your building department how it applies to an off-grid home on your parcel. As an owner-builder you can do the electrical work on your own home with a permit and inspections, or hire a licensed electrical (C-10) or solar (C-46) contractor.
Questions to ask the building department
- What do you need on a permit for an off-grid solar and battery system?
- How does the state solar requirement apply to a home with no grid connection?
- Are there battery location or fire-separation rules I should plan around?
- Do you prefer a ground mount, and what setbacks apply to it?
- Can I do the work as an owner-builder, and which inspections will you schedule?
Become energy detectives
For one week your family will find out where every watt goes. You'll measure real appliances, hunt down phantom loads, track the sun across your land, and finish by sizing a solar system for the home you actually live in.
You'll need
- A plug-in watt meter (about $20–30 at a hardware store)
- A notebook for the load list
- A straight stick about 3 ft long and some stones
- Your last power bill, if you have one
- A computer for PVWatts
Do it together
- Meter the fridge. Plug it through the watt meter for 24 hours and read the kWh. Most surprise you.
- Read the labels. List every appliance with its watts, and estimate hours a day.
- Hunt phantoms. Meter the TV, router and chargers when they're "off."
- Check the bill. Divide the month's kWh by the days. How close was your list?
- Track the sun. Push the stick in the ground in a sunny spot. Mark the tip of its shadow with a stone at 9, 12 and 3 o'clock.
- Size your system. Look up your winter sun hours on PVWatts and run your numbers in the calculator above.
Little detectives
- Read the numbers on the watt meter out loud
- Find every glowing standby light in the house
- Place the shadow stones and watch the shadow move
Older detectives
- Build the full load list and total the watt-hours
- Graph the shadow length through the day
- Work out how many panels your home would need in winter and in summer
Repeat the shadow stick on the first day of winter if you can. The long winter shadow shows exactly which trees and buildings would shade your panels.
Living within the light you're given
Off-grid power changes how a family lives, and almost always for the better. You notice the sun. You run the washer on bright mornings and read by lamplight on long winter evenings. You stop wasting what you used to take for granted. The system is not just hardware; it's a rhythm.
Next we follow the power into storage: batteries, backup generators, and how to keep the lights on through a week of storms.
Your Lesson 4 worksheet
Work through these together. Your answers save on this device as you type, and print cleanly for your records.
Progress record
What this lesson practices:
- MathMultiplying and dividing multi-digit numbers and decimals, unit rates, and using formulas (Common Core 5.NBT.B.5, 6.RP.A.3, 6.EE.A.2).
- ScienceEnergy transfer and conversion, and how the sun's path changes with the seasons (NGSS 4-PS3-2, 5-ESS1-2, MS-PS3-3).
- DataMeasuring, recording and graphing real-world data over time.
- Life skillsBudgeting a household resource and electrical safety.
Standards mapping is a draft. If you use charter or ESA funds, confirm it with your education specialist.