Whole-home off-grid energy system design

Educational use only

This page is for educational purposes only. Hands-on skills should be learned and practiced under qualified supervision before relying on them in emergencies. Use this information at your own risk.

A whole-home off-grid energy system produces, stores, and delivers every watt your household consumes without any utility connection. The design decisions that determine whether it works reliably — or requires a generator rescue every January — are made before the first component arrives. Most off-grid failures trace back to one of three mistakes: undersizing the battery bank because the load audit was lazy, mismatching panel array capacity to actual winter sun hours, or selecting components from different voltage tiers that don't integrate cleanly. This page walks through the complete design sequence in the order it has to happen.

Step 1 — Audit every load in your home

The load audit is the foundation every other calculation rests on. A number pulled from a general rule of thumb ("the average home uses 30 kWh per day") will produce a system that is either wildly oversized and over-budget or embarrassingly undersized.

How to conduct a complete load audit:

Walk every room and list every electrical device — lights, appliances, tools, electronics, HVAC equipment, well pump, water heater, phone chargers, entertainment systems. Leave nothing out.
For each device, record: nameplate watts (from the label, usually on the back or bottom), average daily hours of use (be honest — two hours a day is not three), and frequency (daily, or three times per week, or seasonal only).
Calculate Wh/day for each device: watts × hours per day. For devices used less than daily, calculate weekly Wh and divide by seven.
Separate the list into two columns: summer and winter. Air conditioning runs in summer; space heating fans and long lighting hours run in winter. Your off-grid system must be designed for the harder season at your location.
Sum the total Wh/day for each season. This is your baseline household load.

Sample partial audit table:

Device	Watts	Hours/day	Wh/day
Refrigerator (Energy Star)	60 W avg	24 h	1,440
Chest freezer (medium)	40 W avg	24 h	960
LED lighting (8 fixtures)	60 W	5 h	300
Well pump (1/2 HP)	750 W	0.5 h	375
Laptop × 2	90 W	6 h	540
CPAP machine	30 W	8 h	240
Phone and device charging	40 W	3 h	120
Furnace blower (ECM motor)	150 W	4 h	600
Critical load daily total			~4,575 Wh
Electric oven	2,400 W	0.5 h	1,200
Clothes dryer	5,000 W	0.5 h	2,500
Full household daily total			~8,275 Wh

The gap between 4,575 Wh (critical loads) and 8,275 Wh (full household) is your discretionary load buffer — the appliances you shed when production is low.

Field note

A smart plug with energy monitoring costs very little and measures actual watts consumed, not nameplate maximums. Nameplates list worst-case draw, not average draw. A refrigerator labeled 150 W typically averages 40-70 W because the compressor cycles. Measure for a week before finalizing your audit numbers — the difference between nameplate and actual is often 30-50%.

Step 2 — Separate loads into three tiers

Before sizing any component, explicitly categorize each load. This classification drives every subsequent design decision.

Critical loads (size the system around these): - Medical equipment: CPAP, oxygen concentrators, insulin refrigerators - Refrigerator and chest freezer (food safety requires continuous power) - Well pump (potable water) - Basic LED lighting (safety and navigation) - Communications: router, radio, phone charging - Furnace or heat pump controls (blower motor, not burner) in cold climates

Essential loads (designed in, but can shed for 24-48 hours): - Hot water heater (if electric — consider propane or solar thermal instead) - Laptop and work equipment - Washing machine

Discretionary loads (shed immediately during low production): - Electric oven, microwave, and range - Clothes dryer — air-dry instead - Window air conditioning units - Power tools and workshop equipment - Entertainment systems

Design your battery bank and solar array to reliably power critical loads indefinitely and essential loads for most days. Discretionary loads run on surplus.

Step 3 — Size the battery bank

Battery sizing translates your critical load requirement into amp-hours of storage at a specific system voltage. Work through these four steps:

Step 3a — Choose your target autonomy window. Autonomy is how many consecutive days the battery bank can power critical loads with zero solar or wind input.

2 days: minimum reasonable floor; fine in the American Southwest or other high-sun regions
3 days: standard design target for most US climates
4–5 days: appropriate for the Pacific Northwest, northern Great Lakes, or any location with frequent multi-day cloud cover

Step 3b — Calculate raw capacity needed.

Battery capacity (Wh) = Critical daily load (Wh) × Autonomy days ÷ Usable DoD

Using the example above: 4,575 Wh critical load, 3-day autonomy, LiFePO4 at 90% usable DoD:

4,575 Wh × 3 days ÷ 0.90 = 15,250 Wh (15.25 kWh)

For AGM lead-acid at 50% usable DoD:

4,575 Wh × 3 days ÷ 0.50 = 27,450 Wh (27.45 kWh) — nearly double the nameplate capacity

This is why LiFePO4 wins on usable capacity despite the higher upfront cost. See the batteries page for a full chemistry comparison and lifetime cost analysis.

Step 3c — Add a buffer for losses and aging.

Add 15–20% for inverter conversion losses (typically 5–8%) and capacity aging (LiFePO4 loses roughly 20% capacity over 10 years of daily cycling):

15,250 Wh × 1.18 = 17,995 Wh — round up to 20 kWh

Step 3d — Select system voltage and convert to amp-hours.

For any whole-home system above 3 kW, design at 48V. The reasons are physical: a 3,000W inverter draws 62.5A from a 48V bank and 250A from a 12V bank. Quarter the current, quarter the required cable cross-section, and dramatically reduce resistive heat losses throughout the system.

20,000 Wh ÷ 48V = 416 Ah at 48V

Two 48V 200Ah server-rack LiFePO4 batteries (400 Ah combined) come within 4% of this target — close enough. A 48V 100Ah unit is a common modular size, so four units in parallel covers this bank at a total of 400 Ah.

Never mix battery chemistries in one bank

LiFePO4 and lead-acid batteries have different charge voltage profiles. A charger configured for LiFePO4 (absorption at 57.6V for 48V bank) will overcharge and damage AGM batteries. A charger configured for AGM will chronically undercharge LiFePO4, causing cell imbalance. Same chemistry, same capacity, same age — within one bank, no exceptions.

Step 4 — Size the solar array

Solar array sizing determines whether the system can replenish the battery bank within a typical production day. The critical variable is peak sun hours (PSH) — not total daylight hours, but the hours of sunlight intense enough to operate panels at rated output.

PSH by region at worst month (typically December/January in North America):

Region	Worst-month PSH
Southwest US (AZ, NM, NV)	4.5–6.5
Southeast US (FL, GA, TX coast)	3.5–4.5
Midwest (IL, OH, MI)	2.5–3.5
Pacific Northwest (OR, WA)	1.5–2.5
Northeast (NY, MA, VT)	2.5–3.5
Rocky Mountains (CO, UT)	4.0–5.5

Array sizing formula:

Array watts = Daily production target (Wh) ÷ Worst-month PSH ÷ 0.80 derate

The 0.80 derate accounts for real-world losses: panel temperature derating (panels lose about 0.35% output per degree Celsius above 77°F / 25°C standard test condition), soiling, wiring losses, and MPPT charge controller efficiency.

For 4,575 Wh critical load plus 20% recharge margin = 5,490 Wh daily production target, at 3.0 PSH worst-month (Midwest):

5,490 Wh ÷ 3.0 h ÷ 0.80 = 2,288W nameplate panel capacity — minimum

For a system designed to run full household loads (8,275 Wh) plus recharge margin:

8,275 Wh × 1.20 ÷ 3.0 h ÷ 0.80 = 4,138W panel array

At 400W per modern panel, that is approximately 10–11 panels for the critical-load-only design and 10–11 panels for the full-load design in a favorable climate, rising to 14–16 panels in the Midwest worst month.

Field note

Size the array for winter, not summer. A system that barely covers loads in January produces large surpluses in June and July — and that's fine. Surpluses can dump into a water heater element (resistive heating is a perfect dump load), charge tools, or power seasonal high-draw tasks. A system sized for summer comfort will leave you cold and dark from November through February.

Step 5 — Evaluate and integrate supplemental sources

Solar alone leaves two blind spots: night and winter cloud cover. Strategically adding a second generation source changes the risk profile of the entire system.

Wind integration

Wind and solar are naturally complementary in many climates: winter storms bring both clouds and strong wind, and nighttime wind compensates for zero solar production. If your site averages 10–12 mph (4.5–5.4 m/s) or higher at hub height, a small wind turbine can meaningfully reduce battery drawdown during solar's worst periods.

A 1 kW wind turbine on an adequate wind site produces roughly 150–250 kWh per month — enough to cover 30–50% of the critical loads in the example above. See the small wind turbines page for site assessment methodology and the DOE 10 mph (4.5 m/s) minimum threshold.

Wind and solar share the same battery bank and charge controller bus. Most modern MPPT charge controllers handle both sources simultaneously via separate input terminals. A hybrid controller with separate PV and wind inputs simplifies the wiring.

Micro-hydro integration

If your property includes a stream or creek with year-round flow and at least 10 feet (3 m) of vertical head, micro-hydro is the most powerful supplemental source available. Unlike solar and wind, micro-hydro produces continuous power — 24 hours a day, every day, regardless of weather.

Power output calculation:

Output (W) = Head (ft) × Flow (GPM) ÷ 10 (simplified — assumes ~60% system efficiency)

A modest stream with 20 feet (6 m) of head and 50 GPM of flow:

20 × 50 ÷ 10 = 100W continuous = 2.4 kWh/day

A site with 50 feet (15 m) of head and 100 GPM:

50 × 100 ÷ 10 = 500W continuous = 12 kWh/day — more than the critical load in the example above

The tradeoff is seasonal reliability: summer drought can reduce stream flow to a fraction of spring levels. A micro-hydro system on a stream that reliably flows year-round can dramatically reduce the required solar array size and battery bank. See micro-hydro power for site qualification, turbine selection, and penstock sizing.

Generator backup

Every whole-home off-grid system needs a generator for three conditions: 1. Extended low-production events (3+ consecutive days of cloud cover with no wind) 2. Deep battery bank recovery after a multi-day event 3. High-surge loads (welders, large air compressors, stump grinders) that would otherwise require a much larger inverter

Generator sizing rule: Generator continuous output must equal or exceed the inverter-charger's maximum AC charging input plus simultaneous critical loads.

For an inverter-charger charging at 5,000W (100A at 48V) with 1,500W of critical loads running simultaneously:

Generator minimum: 5,000 + 1,500 = 6,500W — specify a 7,500W unit with 15% headroom

A propane-primary dual-fuel generator fed from a 500-gallon (1,893-liter) tank provides indefinite backup fuel with no degradation. Propane stores without stabilizer, whereas gasoline in a generator fuel tank starts degrading within 3–6 months. A 500-gallon (1,893-liter) propane tank holding enough fuel to run a 7,500W generator at 50% load for roughly 250–300 hours gives substantial winter backup coverage.

Step 6 — Wiring architecture overview

Off-grid systems have two distinct wiring circuits that must remain completely separate throughout the system: the DC bus and the AC distribution.

DC bus (48V): All generation sources — PV array, wind turbine, micro-hydro turbine — connect via MPPT charge controllers to the 48V battery bank bus. The inverter-charger also connects to this bus on its DC input side. This bus operates at 48V and high DC current. NEC Article 690 governs PV source circuit wiring, requiring appropriate wire types (USE-2 or PV wire for outdoor exposed runs), conduit fill requirements inside structures, and a DC disconnect within sight of the inverter.

Key DC wiring rules: - Use copper conductor only; aluminum corrodes at low-voltage battery terminals - Fuse every source circuit and every battery output circuit as close as practicable to the positive terminal — industry practice targets within 18 inches (46 cm) - Keep DC cable runs short — every foot of cable adds resistance and losses - Use torque specifications on all battery and busbar terminals; under-torqued connections arc and fail

AC distribution: The inverter-charger output connects to a critical-load subpanel — a dedicated breaker box fed exclusively by the inverter. This subpanel powers critical and essential loads. Discretionary loads remain on a separate conventional panel that is either fed from a generator transfer switch or simply left off-grid when grid connection is absent.

The critical-load subpanel approach has three advantages: 1. The inverter only powers what it needs to — smaller inverter, lower idle losses 2. Discretionary high-draw loads (dryer, oven) can never accidentally overload the inverter 3. In a hybrid setup with grid connection, the inverter seamlessly transitions critical loads to battery without affecting the rest of the house

See inverters for pure-sine wave requirements, continuous vs. surge sizing, and installation sequence including the mandatory DC fuse placement.

Separate AC and DC wiring inside the structure

NEC 690 requires that DC PV conductors and AC inverter output conductors be run in separate raceways or conduits inside a building or structure. Running 48V DC and 120/240V AC conductors in the same conduit creates a code violation and a shock hazard. Label all DC conduits clearly — DC arc faults are more dangerous than AC faults at equivalent voltage because DC current does not self-extinguish at current zero-crossings.

Step 7 — System monitoring and data logging

An off-grid system without monitoring is flying blind. You need to know battery state of charge, daily production, daily consumption, generation source contributions, and any fault conditions — in real time and historically.

Minimum monitoring for a whole-home system:

Shunt-based battery monitor (Victron BMV-712 or equivalent): measures state of charge, current in/out, amp-hours consumed since last full charge. Mount the shunt in the negative cable between the battery bank and all loads/chargers.
Charge controller display or data port: shows daily PV production (kWh) and battery charging state.
Inverter display: shows current AC load watts and any fault/alarm conditions.

Recommended: networked monitoring platform. Victron VRM (free with any Victron inverter), Midnite Solar CLASSIC monitor, or Schneider Electric InsightHome all log production, consumption, and battery trends accessible from a phone app. Set these automatic alerts:

Battery SoC drops below 30% → generator start alert
Battery SoC drops below 20% → inverter low-voltage alarm (automatically starts generator if auto-start is wired)
AC load exceeds 80% of inverter continuous rating → overload warning
Battery temperature exceeds 95°F (35°C) or drops below 40°F (4°C) → temperature alarm

Data logging matters for long-term health. A monthly log of: highest SoC reached (ideally 100% or near), lowest SoC reached, generator runtime hours, and any battery alarms tells you whether the system is healthy or drifting. A bank that used to reach 100% SoC every sunny day but now only reaches 85% is losing capacity — visible only when you have historical records to compare against.

Step 8 — Maintenance calendar

A whole-home off-grid system requires regular maintenance at each component tier. Deferred maintenance compounds quickly: a corroded terminal leads to a voltage drop, which trips the inverter, which discharges the batteries overnight.

Monthly: - [ ] Inspect all battery terminal connections for corrosion, heat discoloration, or looseness. Re-torque if any doubt. - [ ] Verify BMS display shows all cells within 50 mV of each other at full charge (LiFePO4) - [ ] Check inverter event log for any faults or alarms during the past month - [ ] Verify charge controller shows daily production consistent with expected seasonal output - [ ] For flooded lead-acid: check electrolyte levels and top off with distilled water only

Quarterly: - [ ] Run a known-load runtime test: discharge the battery bank to 20% SoC under a measured load, record Ah delivered, compare to calculated baseline - [ ] Inspect all DC and AC cable insulation for cracking, chafing against conduit edges, or UV degradation on exterior runs - [ ] Test generator: run under full charging load for 30 minutes; verify output voltage (120/240V ± 5%) and that auto-start/stop cycles correctly - [ ] Inspect PV panels for bird droppings, lichen, shading from new tree growth, or cracked glass. Clean with soft brush and water if soiled. - [ ] Check micro-hydro inlet screen and penstock intake for debris (if applicable) - [ ] Inspect wind turbine tower for loose guy wire tension and check turbine rotation and braking (if applicable)

Annually: - [ ] Full battery capacity test: discharge to minimum SoC under measured load; compare Ah delivered to nameplate. Below 80% of nameplate signals end-of-life planning. - [ ] Torque all battery terminal bolts and busbar connections to specification - [ ] Test all DC and AC breakers and fuses by manually tripping and resetting; replace any that don't reset cleanly - [ ] Change generator engine oil, air filter, and spark plugs (typical interval: 100–200 hours of runtime or 12 months) - [ ] Inspect roof penetrations and conduit seals for water infiltration - [ ] Review system logs: compare actual winter and summer production to design targets; identify any drift

Step 9 — Expansion planning

Design your initial system to accommodate growth without rebuilding from scratch. Off-grid systems scale in two directions: more generation (more panels or sources) and more storage (more battery capacity).

Expandable design choices to make from day one:

Oversize the inverter-charger. An inverter-charger running at 50% of its rated output runs cooler, more efficiently, and lasts longer than one running at 95%. Leaving headroom allows you to add loads without replacing the inverter.
Install a combiner box for the PV array with room for additional strings. Adding two more panels should require connecting two wires to an available terminal, not rewiring the whole array.
Size the battery enclosure for twice your initial bank. LiFePO4 server rack batteries add in parallel — physically room to double matters.
Plan the critical-load subpanel with spare breaker slots. A subpanel that's full on day one is a subpanel you'll need to replace when you add a load.
Document everything. A wiring diagram taped inside the electrical enclosure with component labels, cable gauge notation, and fuse ratings is worth hours of troubleshooting three years from now.

Step 10 — Cost modeling and payback

A whole-home off-grid system is a significant investment. Understanding what drives cost and where realistic payback occurs helps you design to a budget rather than discover the number at the end.

Component cost tiers by system scale (DIY labor only):

System scale	Solar array	Battery bank	Inverter-charger	Total components
Critical loads only (~5 kWh/day)	2–3 kW	10–15 kWh	3–5 kW	Moderate–significant investment
Modest full household (~10 kWh/day)	4–6 kW	15–25 kWh	5–8 kW	Significant investment
Larger household + workshop (~15 kWh/day)	8–12 kW	25–40 kWh	8–12 kW	Major investment
Full homestead (~20+ kWh/day)	12–20 kW	40–80 kWh	10–15 kW	Substantial capital project

Professional installation adds significant cost on top of components — typical rates run per-watt on the array and hourly on balance-of-system work. DIY installation is achievable for homeowners comfortable with electrical work, but requires understanding NEC 690 (PV wiring), conductor sizing, grounding, and labeling requirements. The DIY solar installation page covers the permitting and code compliance sequence.

Payback context. For properties already connected to the grid, the payback calculation compares system cost against displaced electricity bills. At average US residential electricity rates and typical system costs, grid-connected solar with battery backup (hybrid systems) commonly reaches payback in 8–15 years depending on local rates and incentive programs. Verify current federal and state incentive programs at energy.gov before finalizing your cost model — incentive programs change and can significantly affect effective system cost.

For properties where utility connection would cost a significant investment to extend service across a rural parcel, the payback comparison changes entirely: the off-grid system competes against a one-time utility extension cost, and the answer is often immediate.

Where off-grid energy systems create non-financial returns: - Energy independence from utility rate increases and outages - No grid extension cost for remote parcels - Ability to power water pumps, refrigeration, and medical equipment indefinitely - Foundation for homestead economics — not just a backup, but the primary infrastructure

System design checklist

A well-designed whole-home off-grid system begins with the individual components but succeeds as an integrated system. The solar-offgrid page covers off-grid solar architecture decisions and autonomy sizing in depth, batteries covers chemistry selection and cell-level wiring safety, inverters covers pure-sine sizing and installation sequence, small wind turbines covers site assessment and integration, and micro-hydro power covers run-of-river qualification for properties with streams. The combination of all five gives you every resource needed to move from load audit to commissioned system.