- Audit your loads first: watts × hours = watt-hours, for every device, honestly. Conservation is the cheapest panel you'll ever buy.
- Design for December, not July — size the array on your worst month's sun hours.
- Array kW = daily kWh ÷ winter sun-hours ÷ 0.75. Battery kWh = daily kWh × 2 days ÷ 0.8.
- Choose LiFePO₄ batteries and a pure sine inverter sized for surge, not just running watts.
- Start with one "sovereign circuit" — fridge, lights, router — before you bet the house.
Electricity is the pillar people are most afraid of, because it arrives through a meter, priced in units nobody visualizes, from a grid nobody understands. Strip the mystique: a solar system is a budget. Panels are income, batteries are savings, appliances are spending. Size the income to the worst month, keep two days of savings, and spend less than you earn. Everything else is shopping.
Part oneThe load audit: one evening, one notebook
Walk your home with a notepad (a $25 plug-in watt meter — Kill A Watt or similar — settles arguments). For each device: its running watts, and the hours per day it actually runs. Multiply. Sum. That's your daily energy budget in watt-hours.
| Load | Watts | Hours/day | Wh/day |
|---|---|---|---|
| Fridge (modern, 10–15 cu ft) | ~90 avg | cycles | 1,000 |
| Chest freezer (7 cu ft) | ~60 avg | cycles | 700 |
| LED lighting, whole house | 60 | 5 | 300 |
| Laptop + phone charging | 70 | 6 | 420 |
| Internet (router / satellite) | 40 | 16 | 640 |
| Well or transfer pump | 750 | 0.5 | 375 |
| Washing machine (cold, 4 loads/wk) | 500 | ~0.4 | 300 |
| Kitchen small appliances | — | — | 400 |
| Total | ~4,100 Wh |
Notice what's missing: electric heat, air conditioning, electric water heating, electric cooking. Resistive heat is where off-grid budgets go to die — a single space heater running 8 hours is 12,000 Wh, three times the entire table. Off-gridders heat with wood or propane, cook with gas or carefully-timed induction, and heat water with propane or solar thermal. Every watt-hour you remove from the audit saves you roughly $1.50–2.50 in system cost. Swapping an old fridge (2,500 Wh/day) for a modern one (1,000 Wh/day) pays for half the fridge before you've bought a panel.
Part twoThe December rule
Panels are rated at their laboratory peak. What you get is peak sun hours — the equivalent hours per day of full-strength sun, which in winter is a fraction of summer's. Phoenix sees about 6 peak hours in December; Seattle sees barely 1. Size for your worst month you intend to rely on, or budget a generator for the gap — pretending it's always June is the classic first-timer's ruin.
The 0.75 derate covers real-world losses — charge efficiency, wiring, dust, temperature, panel aging. The 0.8 is the design depth-of-discharge for LiFePO₄ (it can go deeper; don't plan to). Two days of autonomy handles the average storm; three gets expensive and a small generator handles the tail more cheaply than a third day of battery ever will.
A worked example
Take the 4.1 kWh/day household above, in a mid-latitude climate with 2.5 December sun hours:
- Array: 4.1 ÷ 2.5 ÷ 0.75 = 2.2 kW → round up to 2.4 kW (six 400 W panels).
- Battery: 4.1 × 2 ÷ 0.8 = 10.25 → ~10 kWh (two 48 V / 100 Ah rack batteries).
- Inverter: worst simultaneous moment — pump (750 W) + fridge and freezer both starting + washer + lights ≈ 2,500 W running with motor surges on top → a 3,000 W pure-sine inverter with ~6,000 W surge.
Amps = watts ÷ volts, and copper is priced by the amp. A 3,000 W load at 12 V is a terrifying 250 A (welding-cable territory); at 48 V it's 62 A. Above about 1,500 W of inverter, 48 V systems are cheaper, safer, and what every modern rack battery expects. 12 V belongs in vans.
Part threeThe parts list, priced honestly
| Component | Spec | Cost |
|---|---|---|
| Panels ×6 | 400 W each, ~$0.35–0.50/W | $850–1,200 |
| Rack batteries ×2 | 48 V 100 Ah LiFePO₄ (5.1 kWh each) | $1,900–2,800 |
| All-in-one inverter/charger | 3 kW pure sine, MPPT built in, 48 V | $700–1,400 |
| Ground mount or roof rails | steel/aluminum | $400–700 |
| Copper, breakers, fusing, disconnects, grounding | never skimp here | $400–600 |
| Backup generator (dual-fuel inverter type) | 3–4 kW | $500–900 |
| Total | $4,750–7,600 |
LiFePO₄ deserves its own sentence: 3,000–6,000 cycles (a decade or two of daily use), no maintenance, safe chemistry, and prices that have collapsed to $150–250 per usable kWh. Lead-acid's only remaining argument is a low sticker price, and it loses that argument within five years — it tolerates just 50% discharge and dies in 500–1,200 cycles. Buy lithium iron phosphate or don't buy yet.
The generator is not an admission of defeat — it's the cheapest insurance in the system. Ten gallons of fuel through a small inverter generator covers the one dark week a year that would otherwise demand $3,000 of extra battery. Off-grid isn't "solar only"; it's "grid optional."
Part fourSafety, in four unglamorous rules
- Fuse the battery. A short across a lithium bank releases thousands of amps. A Class-T fuse within inches of the positive terminal is the difference between a blown fuse and a fire.
- Size wire by amps and length — use an ampacity chart, round up, and torque every lug. Loose connections, not components, cause most system fires.
- Ground the system — panels, frames, inverter chassis, one ground rod, bonded once.
- Anything attached to a dwelling should meet local code (in the US, that means NEC rules like rooftop rapid-shutdown) and, ideally, an electrician's one-hour review of your plan. Pride is not a safety device.
Part fiveStart with one circuit
You don't have to take the house off-grid to start. Build the sovereign circuit: a single 800 W panel array, one 2.5 kWh battery, a 1,500 W inverter — about $1,200 — running your fridge, router, a lamp circuit, and phone charging. It's a complete apprenticeship in solar (you'll learn charge curves, load habits, and your own real usage), it's a permanent whole-home backup for the loads that actually matter in an outage, and every component scales up into the bigger system later. The first time a storm kills the street's power and your kitchen hums along, the rest of the build will fund itself with conviction.
AppendixThe commissioning checklist
- Load audit done with a watt meter, not vibes. Total written down.
- Worst-month sun hours looked up (NREL/PVWatts or your national equivalent).
- Array, battery, inverter sized with the three formulas — then rounded up one notch.
- Class-T fuse on the battery; breakers on every leg; one grounding point.
- Every lug torqued, then re-torqued after a week of cycling.
- Generator tested monthly under load, fuel rotated yearly.
- December survived on paper before it's survived in person.