Easy Solar Calc

How big an off-grid solar system to run a house?

Short answer

Running an average US house (about 28.4 kWh a day) off-grid at 3.5 peak sun hours takes roughly 26 × 400 W panels · 1,387 Ah at 51.2 V · 271 A of MPPT (two or more controllers) · 8,000 W inverter (9,000 W surge) — about 71 kWh of LiFePO4 for two days of autonomy, with a 4-ton central air conditioner as the biggest motor.

How to work it out

  1. Battery: 28,400 Wh × 2 days ÷ 80% usable ÷ 51.2 V = 1,387 Ah (56 batteries)
  2. Panels: 28,400 Wh ÷ (3.5 sun h × 86% × 92% battery round trip) = 10,256 W → 26 panels
  3. Controller: 10,400 W ÷ 48 V × 1.25 = 271 A → 271 A
  4. Inverter: 6,000 W × 1.25 = 7,500 W → 8,000 W; surge: 6,000 W + (8,000 − 5,000) W = 9,000 W

The EIA puts the average US residential customer at 863 kWh a month in 2024 — about 10,359 kWh a year, or 28.4 kWh a day. That is a grid-scale appetite: off-grid, every one of those kWh has to be made and stored on site, every day, all winter.

Adjust for your project

From the solar load calculator, inverter losses included.
Days the batteries cover with no sun.
Use your worst month for the season you need power in. See peak sun hours by city.
Sets how much of the rated capacity you plan to use: 80% for LiFePO4 (Victron rates cycle life at 80% depth of discharge), 50% for lead-acid (Trojan recommends 20–50% for long life).
Add up what could run together (microwave + fridge + lights…).
The largest motor or compressor’s starting watts, e.g. 800–1,200 W for a fridge (Champion chart).
Its running watts (already counted in the peak above), e.g. 150–400 W for a fridge. The surge needed is the peak plus that motor’s extra starting watts.
More options: DoD · Headroom · Losses …
%
Blank uses 80% for LiFePO4 and 50% for lead-acid. Check your battery’s data sheet.
%
Your choice of margin so cold, bright days and long loads don’t run equipment at its limit.
%
Wiring, dirt, heat, mismatch and controller losses. 14% is the PVWatts default.
°F
Batteries hold less when cold: Victron gives about 80% for LiFePO4 at 32 °F (0 °C) and 50% at −4 °F (−20 °C); lead-acid about 86% at 32 °F. LiFePO4 must not be charged below 41 °F (5 °C) unless it is self-heating.
Your system26 × 400 W panels · 1,387 Ah at 51.2 V · 271 A of MPPT (two or more controllers) · 8,000 W inverter (9,000 W surge)
Battery bank
1,387 Ah · 48 V
12 V 100 Ah batteries
56 batteries · LiFePO4
Solar array (minimum)
10,256 W
Panels
26 panels · 400 W
MPPT controller output current
271 A
Inverter continuous rating
8,000 W
Inverter surge rating
9,000 W
Battery current at peak AC load
130 A
  • About 271 A of charge current is more than one common controller; split the array across two or more MPPT controllers, or raise the system voltage.
Before you wire itWire and fuse sizes come from NEC/ABYC ampacity tables and the equipment manual — have a qualified installer check them.

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Battery monitor with shuntA shunt monitor on the bank is how you’ll know whether the system matches this plan in practice — especially in the first winter.
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The same system at other sun hours

The same system at other sun hours
Peak sun hours per daySolar array (minimum)PanelsMPPT controller output current
217,947 W45 panels469 A
311,965 W30 panels313 A
48,974 W23 panels240 A
57,179 W18 panels188 A
65,982 W15 panels156 A

What makes a whole house hard off-grid

  • Winter decides everything. At 3.5 peak sun hours this needs about 10.3 kW of panels; at a 2.7-hour Chicago-style winter, about 13.3 kW. Many off-grid homes size for spring and autumn and run a generator through the darkest weeks.
  • The battery is the expensive part. Two days at 28.4 kWh is about 71 kWh of LiFePO4 at 80% usable — dozens of 100 Ah-class modules, or stacks of 48 V rack batteries.
  • Cut the load first. Electric heat, water heating, a dryer and an old fridge can make up much of a bill. Off-grid homes commonly move heating and hot water to propane or wood and run efficient appliances, which can cut the electric load a lot.
  • Big inverter, 48 V. A 6,000 W peak needs an inverter around 8,000 W continuous with headroom, and a surge rating of about 9,000 W to start a 4-ton central AC (Champion: 4,000–5,000 running, 6,500–8,000 starting watts) with everything else running. At 48 V the peak is about 130 A from the battery — at 12 V it would be over 500 A.

Grid-tied instead, the same house needs only panels — how many solar panels to run an average house. For outage backup without going off-grid, see how many kWh to power a house for a day and how many 100Ah batteries to power a house.

Different size? Use the full Off-Grid Solar System Calculator.

What you’ll need

  • 12 V 100 Ah LiFePO4 batteryPremium price

    About 1.28 kWh rated in a box roughly the size of a group 24–31 lead-acid battery at around half the weight. Check the BMS discharge limit and whether it can charge below freezing.

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  • 400 W residential-size solar panelPremium price

    Fewer, larger panels for a cabin, shed or ground mount. Their higher voltage usually means an MPPT controller.

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  • MPPT solar charge controllerMid-range price

    Converts a higher panel voltage into charging current, so you can wire panels in series and lose less in the cables. Size it on output amps and maximum PV voltage.

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Frequently asked questions

How big an off-grid system for a house using 15 kWh a day?

About 14 × 400 W panels · 732 Ah at 51.2 V · 146 A of MPPT (two or more controllers) · 8,000 W inverter (9,000 W surge) at 3.5 sun hours and two days of autonomy.

How many solar panels for an off-grid house in a sunny climate (5.5 sun hours)?

About 17 panels (6,526 W minimum) for 28.4 kWh a day.

Other common sizes

Sources