What Size Off-Grid Solar System Do I Need? A Sizing Guide Built on Real Sun Data

Most off-grid calculators plug in "5 peak sun hours" and call it done. Measured data says otherwise: a 1 kW array tilted at 30 degrees produces about 4.0 kWh on an average December day in Phoenix, 1.4 kWh in Chicago and 1.1 kWh in London. An off-grid system has no grid to fall back on, so it has to be sized for the month when the sun is weakest where you live.

This guide walks through the four numbers that decide an off-grid system: your daily energy use, your worst-month solar output, your battery bank and your inverter. Every output figure below comes from the PVGIS solar model for real locations, not from a rule of thumb. If you are sizing a van or motorhome, the RV solar sizing guide applies the same method to a flat roof. For storage, see how many batteries you need for off-grid solar, and for the seasonal picture, how much solar output drops in winter by location.

electric_boltStep 1: Add up your daily energy use in watt-hours

Solar sizing starts with energy, not power. Multiply each device's running watts by the hours it runs per day to get watt-hours (Wh), then add them up. A fridge that draws 100 W while its compressor runs, and runs about a third of the day, uses roughly 800 Wh, not 2,400 Wh. A plug-in power meter or the label on each appliance gives you better numbers than any table.

Two items surprise people. The inverter itself draws power whenever it is on, often 10 to 40 W, which adds up to hundreds of watt-hours a day. And heating with electricity (space heaters, electric water heaters, induction cooking for long periods) usually multiplies the load several times over, which is why most off-grid homes heat with wood, propane or a heat pump sized for the solar budget. The rest of this guide uses 3,000 Wh per day as the example cabin load.

wb_sunnyStep 2: Find your worst-month solar output

Divide your daily load by how many kWh each kilowatt of panels makes per day in your worst month. That gives the array size that breaks even on an average day of that month. The table shows the result for 3,000 Wh per day with panels tilted at 30 degrees.

Two things stand out. First, the spread is huge: the same cabin needs about 750 W of panels in Phoenix but about 2,750 W in Seattle to break even in December. Second, the far-north numbers get impractical. In Stockholm the December figure is about 8,500 W, against 650 W in June. That is the point where most people stop sizing panels for winter and add a generator or accept a smaller winter load instead.

Tilt matters more in winter

A low winter sun favors steeper panels. Comparing flat and 30-degree panels in the same place, December output in Denver rises from 1.71 to 3.32 kWh per kW per day, and in London from 0.45 to 1.10. Off-grid arrays meant for year-round use are often tilted even steeper, close to the latitude plus 10 to 15 degrees, which trades a little summer output for more winter output. The winter output guide has the full month-by-month comparison.

battery_charging_fullStep 3: Size the battery bank for the gaps

Panels cover the average day; the battery covers nights and cloudy stretches. The usual formula is daily load × days of autonomy ÷ usable depth of discharge. For 3,000 Wh per day, two days of autonomy and a lithium iron phosphate (LiFePO4) bank you are willing to use down to 10 percent, that is 3,000 × 2 ÷ 0.9 = about 6,670 Wh. With lead-acid, which should usually stay above 50 percent, the same bank needs about 12,000 Wh of nameplate capacity. The battery sizing guide covers chemistry, voltage and how many physical batteries that works out to.

query_statsStep 4: Check the design against a full-year simulation

Sizing formulas use averages. A stand-alone system simulation runs the panels, battery and load hour by hour through years of weather. We ran one example cabin system through PVGIS at each location: 1,600 W of panels at 45°, a 10,240 Wh LiFePO4 bank (51.2 V, 200 Ah) and a 3,000 Wh daily load. The table shows how often the battery hits its 20% cutoff, meaning some of that day's load goes unserved.

The same hardware that runs short on only 0% of December days in Phoenix and 1% in Denver runs short on 74% in Seattle and 86% in London, while in July the worst location in the table runs short on 0% of days. That pattern is the core trade-off of off-grid solar: you either size for the worst month and waste surplus all summer, or size for the shoulder seasons and plan a backup for winter.

settings_input_componentInverter and charge controller

• check_circleInverter continuous rating: at least the largest set of AC loads you run at the same time, plus about 25 percent headroom.
• check_circleSurge rating: motors in fridges, pumps and power tools can briefly draw two to three times their running watts at startup. Check the inverter surge spec against your largest motor.
• check_circleCharge controller: an MPPT controller converts higher panel voltage into charging current and usually harvests noticeably more energy than PWM, especially in cold weather or with panels wired in series.
• check_circleSystem voltage: 12 V suits small RV and van systems; 24 V or 48 V keeps current and cable size down once the inverter passes roughly 1,500 to 2,000 W.

rv_hookupRV and van systems: same method, smaller roof

Mobile systems follow the same steps with two twists: panels usually lie flat on the roof, which hurts winter output, and roof space caps the array size. A flat 400 W array covers about 78% of a 1,200 Wh daily load in a Phoenix January but only about 21% in Seattle. The RV guide has the numbers for each location and season.