Skill guide · Energy & Heat

Sizing a Solar Array & Battery System

The Energy & Heat chapter's Solar Generator and Off-Grid Solar Array items both say to size against real load, not marketed capacity. This guide is the actual method: four load tiers, the formulas behind each, and the one Utah-specific number that undersizes most first attempts at this. See Sizing a Fuel-Powered Generator for the same load tiers applied to a generator instead, and an honest comparison between the two.

Download this guide as PDF

What this guide covers, and what it doesn't

This is the sizing math: how big to buy each of the four components below, panels, charge controller, battery, and inverter, and what system voltage to build around. It doesn't cover picking specific panel or battery models, wiring gauge, mounting, or permitting; those are real, separate steps once you know the numbers here.

Four load tiers, not one number

"How much solar do I need" doesn't have one answer, because a household typically wants several different levels of backup, and they don't scale the same way.

TierWhat it coversWhat actually scales it
Whole house Everything: full HVAC, all kitchen appliances, laundry, water heater, every outlet The house itself: square footage, insulation, climate zone. Headcount matters far less than the building does.
House essentials (critical loads) Fridge/freezer, a well or sump pump, a furnace blower, key lighting, internet Also the house: a fridge or a well pump draws the same wattage whether one person lives there or five. A bigger household adds only a smaller secondary load from more hot water, laundry, and cooking.
Non-house essentials A portable power station running medical equipment, radios, laptops, and charging Headcount, closely. This is literally each person's own devices and needs.
Bare essentials (recharging only) Phones, headlamps, handheld radios kept topped off, nothing else Headcount, almost linearly. A family of five's bare-minimum tier is genuinely close to five times a single person's.

Why this distinction matters for sizing, not just planning

A single formula that scales every tier by household size will oversize your portable power station if you live alone, and undersize your critical-loads panel if you have a big family in a small, efficient house. Size the top two tiers against your actual appliances and square footage; size the bottom two against your actual headcount.

Sizing the panels

Panel wattage = daily Wh needed ÷ (peak sun hours × 0.75) × 1.1–1.2

The 0.75 factor covers real-world losses: wiring resistance, charge controller conversion, panel angle, dust, and heat all take a bite out of a panel's rated output. The 1.1–1.2 is a margin for degradation and bad-weather days.

A peak sun hour isn't the same as an hour of daylight. It's the number of hours per day the sun would need to shine at its strongest, exactly 1,000 watts per square meter, to deliver the same total energy your location actually receives spread across a full day of morning, midday, evening, and cloud cover. A place can have 10-plus hours of daylight and still add up to only 5 peak sun hours, since most of that daylight isn't at full intensity.

Warning

Size against your worst month, not the annual average

Salt Lake City averages 5.1 peak sun hours per day annually, but that swings from roughly 8.3 hours in June down to about 1.8 hours in December, more than a 4x difference. A system sized off the annual average works great in summer and does almost nothing in the exact winter storm outage this manual is built around. Always size the panel array against your local worst-month figure, not the yearly average.

Sizing the charge controller

The charge controller sits between the panels and the battery, regulating how the panels' power actually charges the bank, and it comes in two types worth knowing apart. A PWM controller is essentially a fast switch: its output amps equal the array's current regardless of what the panels are rated at in watts, and it only makes sense for small systems under about 300W where the panel voltage closely matches the battery voltage. An MPPT controller is a DC-DC converter that trades surplus panel voltage for extra current, harvesting roughly 10-30% more usable energy from the same array; use MPPT for any system over 500W or where the panel voltage runs meaningfully higher than the battery's.

Pick your battery bank's voltage before this formula makes sense. System voltage isn't something you calculate, it's a build choice, driven mainly by your recommended panel wattage from above: roughly under 1,500W, 12V keeps wiring simple; 1,500–3,000W, 24V keeps the current (and wire gauge) manageable; above 3,000W, or anything approaching whole-house scale, 48V is the standard choice. Higher voltage means lower current for the same power, which means thinner, cheaper wire and less resistive loss.

Controller amps (minimum) = (total array watts ÷ battery charging voltage) × 1.25

Use the battery's actual charging voltage, not its nominal voltage, a 12V-nominal battery often charges closer to 14.4V; using 12V in the formula undersizes the controller by roughly 20%. A 1,600W array on a 24V bank, for example, needs 1,600 ÷ 24 = 66.7A, × 1.25 = 83.3A, so you'd buy a controller rated at least 100A. Also check the controller's maximum input voltage rating against your panels' actual open-circuit voltage in series: cold weather raises a panel's voltage output, sometimes 20% or more above its rated figure on a freezing morning, and exceeding the controller's input rating can damage it.

Sizing the battery

Battery Wh = daily energy needed × days of autonomy ÷ usable depth of discharge

Depth of discharge changes this a lot by battery chemistry: LiFePO4 (lithium iron phosphate) can safely use about 85% of its rated capacity, while lead-acid or AGM (Absorbent Glass Mat, a sealed, spill-proof lead-acid variant) batteries should only be drawn down to about 50%, meaning a lead-acid bank needs nearly double the rated capacity for the same usable power. Most home systems target 2-5 days of autonomy, 3 is a reasonable default; lean toward the higher end in a cloudy winter climate. Cold temperatures also reduce a lithium battery's usable capacity, so an unheated garage or shed installation needs extra margin beyond the autonomy math alone.

Sizing the inverter

Size the inverter's continuous rating to the sum of everything you'd realistically run at once, and its surge rating to your single largest motor-driven load. A fridge compressor or a well pump can spike 3-7x its running wattage for a second or two at startup; an inverter that looks fine on paper for continuous load can still fail to start a well pump if its surge rating is too low.

Warning

Wattage isn't the only spec that matters: pure sine wave vs. modified sine wave

A cheaper modified sine wave inverter outputs a rough, stepped approximation of AC power instead of a smooth wave, and it's fine for simple resistive loads (a basic light bulb, most chargers) but a real problem for anything with a microprocessor or a precision power supply. A CPAP or oxygen concentrator specifically can misread pressure, throw error codes, run hotter, or fail outright on modified sine wave power, the same underlying issue the generator sizing guide's THD discussion covers for generators. If the Access & Functional Needs chapter's medical equipment applies to your household, buy a pure sine wave inverter without exception, not just one rated for enough watts.

Calculate your own numbers

First, find your daily energy needed. List every device you'd run, its watts, and how many hours a day you'd actually run it; each row multiplies straight across, so the only addition left is the Total row. Find watts on the appliance's nameplate/label; if it only lists amps, multiply amps × 120V (or 240V for a large appliance) to get watts.

ApplianceWatts× Hours used/day= Wh/day
Total daily energy needed (add this column)

Typical wattage for common appliances, use the real nameplate value when you can find one; these are reasonable stand-ins when you can't. See Sizing a Fuel-Powered Generator for the same appliances' starting watts too, if you're sizing a generator alongside this.

ApplianceWatts
Refrigerator ~700W
Well pump ~1,000W
Sump pump ~800W
Furnace blower ~800W
Window AC, 5,000 BTU ~500W
Window AC, 10,000 BTU ~900W

You'll also need your worst-month peak sun hours. Global Solar Atlas (a free tool from the World Bank Group) has this, but the site takes a few non-obvious clicks to get there:

  1. Search your address, or click your location directly on the map.
  2. Under "Choose PV system to calculate energy yield," click Choose on any system type, "Small residential" is fine; the exact choice barely changes the result.
  3. Click Open detail.
  4. Scroll to "Average hourly profiles" and read the Sum row at the bottom of that table: one number per month, in Wh.
  5. Divide the lowest number in that row by 1,000. That's your worst-month peak sun hours.

That number already has typical system losses built in; combined with the worksheet's own 0.75 factor below, the result comes out a bit conservative rather than optimistic, the safer side to be wrong on for backup power.

Don't want to tally devices individually? Typical daily figures by tier, use a real device-by-device total when you can; these are reasonable stand-ins when you can't.

TierTypical figure
Bare essentials Roughly 150-300Wh/day per person for phone charging, a headlamp, and a handheld radio; a household's absolute floor is often cited around 500-1,000Wh/day total
Non-house essentials Roughly 2-5kWh/day for a household's first 72 hours running a power station for medical equipment, communication, and charging; higher if a CPAP or oxygen concentrator is in that mix
House essentials (critical loads) Roughly 1-2kW continuous for a typical critical-loads panel, spiking to 3-4kW when a well or sump pump cycles
Whole house Roughly 10-20kWh/day for an energy-conscious off-grid household, versus about 29kWh/day for a typical grid-connected U.S. household; a 5kWh/day site needs roughly 1.5kW of panels, a 20kWh/day site roughly 6kW

Want an exact reading instead of an estimate?

A plug-in power meter (a "Kill A Watt" or equivalent, roughly $20-30) gives you a real number for anything that plugs into a standard outlet: plug the meter into the wall, the appliance into the meter, and leave it connected a full 24 hours to read the accumulated kilowatt-hours, this captures a cycling load like a fridge far better than a nameplate figure or a guess at hours-used. Most of these meters cap out around 1875W, check that against the appliance first. For a hardwired 240V load that can't plug into anything, a well pump, electric dryer, or central AC, a clamp meter at the breaker (a one-time reading, or worth having an electrician do if you're not comfortable near an open panel) or a whole-home monitor like Emporia Vue or Sense, installed at the panel, gives the same kind of real, continuous number per circuit.

Fill in your own load. Each line combines with the one above it; a rule marks a line that's an answer, and a plain number with no blank is a fixed part of the formula, not something to look up.

Panel sizing

hrs worst-month peak sun hours, from above
× 0.75 real-world loss factor (fixed)
= hrs effective sun hours
Wh daily energy needed, from the tally above
÷ hrs effective sun hours, from above
= W base panel wattage
× 1.1-1.2 margin for degradation and bad-weather days (fixed)
= W recommended panel wattage

Charge controller sizing

W total array watts, your recommended panel wattage from above
÷ V battery charging voltage (12, 24, or 48, from your choice above)
= A array amps
× 1.25 safety margin (fixed)
= A minimum controller amps

Battery sizing

Wh daily energy needed, from the tally above
× days days of autonomy (3 is a reasonable default)
= Wh daily energy needed × autonomy
÷ % usable depth of discharge (85% lithium, 50% lead-acid/AGM)
= Wh battery capacity needed

Inverter sizing

This is a different pair of numbers from the daily-energy tally above. That one adds watts × hours across a whole day; this one is only what's running at the exact same moment, plus the single biggest startup surge. List only what you'd realistically run simultaneously.

ApplianceRunning wattsStarting watts (motor-driven only)
Total running watts (add this column) Not a sum, see below

Buy against two numbers from that tally, not one. Continuous rating, at least your running-watts total: ______ W. Surge rating, at least your largest single starting watts, the biggest number in that column above, not a sum: ______ W. That second number is the one a fine-looking continuous rating can still fail on.

Worked example, using Salt Lake City's own December low of 1.8 peak sun hours from the warning above: a household needing 3,600Wh/day needs 3,600 ÷ (1.8 × 0.75) × 1.1-1.2 ≈ 2,933-3,200W of panels. On a 24V bank, that's 3,200 ÷ 24 = 133.3A, × 1.25 ≈ 167A, so a 200A controller. For 3 days of autonomy on a lithium (85% usable) bank, that same household needs (3,600 × 3) ÷ 0.85 ≈ 12,706Wh of battery capacity, nearly double that on a 50%-usable lead-acid bank for the same real backup. If a 700W fridge and an 800W sump pump are the two things most likely to run at once, with the fridge's 2,200W starting watts the largest single surge, that household needs an inverter rated at least 1,500W continuous and 2,200W surge.

Plug in your own figures below to run these formulas directly against your own numbers. This doesn't replace understanding the method, it just does the arithmetic; the exact formula used is shown with your numbers filled in.

Sources