Solar makes power when the sun shines. Your family needs it at 7 p.m., at 2 a.m. when the well pump kicks on, and on the fourth gray day of a January storm. Storage and backup are how sunshine becomes a reliable home.
This is where off-grid systems are won or lost. Too little battery and you're running a generator every evening. Too much and you've spent thousands on capacity you'll never use. The right answer is a balance of batteries, a backup, and a few habits.
- 1Explain the difference between battery capacity and usable capacity.
- 2Choose between lithium iron phosphate and lead-acid batteries.
- 3Size a battery bank for your days of cloudy weather.
- 4Know where batteries can legally and safely be installed.
- 5Plan a backup that keeps you going through the worst week of winter.
Capacity, and the part you can actually use
Batteries are rated in two ways. Amp-hours (Ah) at a voltage, or kilowatt-hours (kWh) of energy. They connect with one formula:
A 48-volt, 100 amp-hour battery holds 4,800 watt-hours, or 4.8 kWh. But you can't use all of it. Draining a battery completely shortens its life, so every chemistry has a safe depth of discharge: how much of the label you can really use, day after day.
Use about 80–90%
3,000 to 5,000+ cycles, no maintenance, built-in protection. Costs more up front, often less over its life.
Use about 50%
Roughly 300 to 800 cycles, heavy, and flooded types need watering and venting. Cheaper to buy, shorter lived.
Sizing for the storm
The key planning question is days of autonomy: how many days your batteries should carry the home with little or no sun. Sunny climates with a backup generator often plan 1 to 2 days. Cloudy, remote places with no backup may plan 3 or more. Each extra day adds a full day's worth of battery.
The Lesson 4 homestead uses about 4,600 watt-hours a day. Planning 2 days with lithium iron phosphate:
4,600 × 2 ÷ (0.8 × 0.9) ≈ 12.8 kWh
With lead-acid at 50 percent, the same home needs about 20.4 kWh of label capacity, and much more weight and space.
How much battery?
Bank size = daily Wh × days ÷ (usable depth × 0.9 for inverter losses). Planning numbers only.
Store energy in forms other than batteries. A full water tank uphill is stored pumping. A wood stove is stored heat. A freezer packed with frozen water jugs rides through a night with the power off. Every one of these shrinks the battery bank you need.
Where batteries can live
California's residential codes set limits on home battery systems, called energy storage systems, by where they're installed. Systems must be listed to safety standards and installed to the manufacturer's instructions.
| Location | Limit per system | Notes |
|---|---|---|
| Inside the home | Up to 40 kWh | Utility closets, basements and storage rooms only; never bedrooms, closets off bedrooms, or living spaces |
| Garage, outbuilding, outdoor wall or ground mount | Up to 80 kWh | Outdoors, keep at least 3 ft from doors and windows into the home |
| Combined | Higher totals across locations | The State Fire Marshal has said locations can be combined; confirm the total with your building department |
Rules for attached garages can be more complicated than for detached ones, including fire detection. Ask early.
- Stays between about 40°F and 85°F
- Dry, clean and away from the kitchen and bedrooms
- Lockable, out of reach of children
- Clear space in front for service, with a fire extinguisher nearby
- Ventilated if you use flooded lead-acid batteries
A small insulated power shed, near the panels and away from the house, solves almost every rule at once: a stable temperature, separation from sleeping rooms, a short wire run, and a tidy place for the inverter, charge controller and spare parts.
Backup: generators and beyond
Almost every off-grid home needs a backup for long cloudy stretches, a broken part, or a week of heavy use. Sizing batteries for the worst week of the year is expensive; a backup for that one week is not.
Generator
Charges the batteries through the inverter on cloudy days. Propane stores for years without going bad.
Fuel, noise, maintenance and carbon monoxide. Run it outside, well away from the home.
More panels
Panels are now inexpensive. Adding extra array often beats extra batteries for winter.
Doesn't help when it's dark for days; needs space.
Non-electric backups
Wood heat, a gas range, gravity-fed water and oil or LED lanterns keep the essentials going with no power.
Each needs its own safe installation and habits.
California's generator rules are changing
Under the state air board's small-engine rules, new portable generators sold in California must be zero-emission starting in 2028. The rule applies to sales of new units; generators you already own can still be used. Larger permanently installed standby generators fall under different rules, so ask your county and air district before you buy.
Good backup habits
- Shift heavy loads to sunny hours. Laundry, pumping water and power tools at midday use solar directly and spare the batteries.
- Have a storm mode. Decide in advance what turns off when batteries run low: the second fridge, the dryer, the shop.
- Exercise the generator. Run it under load every month or so, so it starts the day you need it.
- Watch the monitor. Most systems show battery level on a phone or screen. Glancing at it becomes a family habit, like checking the weather.
Permits and inspections
Battery systems and permanently connected generators are electrical work and need permits. The inspector checks that the batteries are listed and installed as designed, that disconnects and labels are in place, and that a generator can't send power the wrong way.
Questions to ask the building department
- Which battery locations and sizes will you accept on my plans?
- Do you require a specific listing for the battery system?
- Are there smoke or heat alarm requirements for the battery location?
- Does a permanently installed generator need its own permit, or an air district permit?
- What clearances do you require around batteries, the generator and fuel tanks?
The power budget weekend
For one day, your family lives on a battery. You'll set a daily energy budget from your Lesson 4 numbers, turn it into tokens, and "spend" them on everything that uses power. By evening you'll know exactly what matters, and what you never needed.
You'll need
- Your load list from Lesson 4
- Paper tokens or coins (each one = 100 watt-hours)
- A jar labeled "battery"
- Your home's electric meter, or a plug-in meter
- Lanterns or flashlights for the evening
Do it together
- Set the budget. Pick a daily budget, such as 4,000 watt-hours. That's 40 tokens in the battery jar.
- Price your appliances. Using your load list, mark what each use costs: an hour of TV, a load of laundry, a pot of coffee.
- Spend tokens all day. Every time something is used, a token comes out of the jar.
- Read the meter. Note the electric meter in the morning and again at night to see the real number.
- Handle a "cloudy day." Halve the budget for the second day. What goes first?
- Talk it over. What was easy to give up? What would your family insist on keeping?
Battery keepers
- Guard the jar and hand out tokens
- Count how many tokens are left at lunch and dinner
- Pick one fun thing to save tokens for
System managers
- Price each appliance from watts × hours
- Compare the token count to the real meter reading
- Design the family's "storm mode" list
Don't turn off the main breaker if anyone in the home depends on medical equipment, and keep the fridge and freezer running. The point is to notice power, not to go without what you need.
Resilience is a family habit
The strongest off-grid homes aren't the ones with the biggest battery banks. They're the ones where everyone knows how full the battery is, saves the laundry for a sunny morning, keeps the woodbox stocked, and treats a stormy evening as a reason to light the lanterns and play a game. Storage is hardware. Resilience is how you live.
Next we put every system together: water, waste and power, in the order that gets you through inspections smoothly and into your home.
Your Lesson 5 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:
- MathUsing formulas, multiplying and dividing decimals, and percents (Common Core 6.RP.A.3, 6.EE.A.2, 7.RP.A.3).
- ScienceStoring and converting energy, and how temperature affects materials (NGSS 4-PS3-4, MS-PS3-3).
- EconomicsBudgeting a limited resource and comparing cost over the life of a purchase.
- Life skillsEmergency preparedness and generator and battery safety.
Standards mapping is a draft. If you use charter or ESA funds, confirm it with your education specialist.
Sources for this lesson
- Sustainable Energy Action Committee — California residential energy storage code requirements
- Enphase — California fire code expands residential energy storage
- PowMr — lead-acid vs LiFePO4 for solar
- California Air Resources Board — small off-road engine regulations
- Steve Garlock Equipment — California generator regulations for 2028