❄ Solar AC Sizing Tool

Solar Air Conditioner Calculator:
BTU to Watts, Surge Check, Panels, Battery, Soft Starter

Convert your AC’s BTU rating to running watts using the actual EER rating, calculate compressor startup surge with and without a soft starter, size your solar panels by US region peak sun hours, and size your battery bank for hours of off-grid operation — all in one tool.

❄ Size Your Solar AC System

Step 1 — AC Type
Compressor surges 4-6x. EER 8-12 typical. Runs ~50% of the time in cooling mode.
Step 2 — AC Specs
BTU
EER
Find the EER on the yellow EnergyGuide label on your AC unit. For mini-splits, divide your SEER rating by 1.1 to approximate EER. A 5,000 BTU window AC at EER 10 draws 500W running.
hours
Step 3 — Soft Starter
I have / plan to install a soft starter
SoftStartRV, MicroAir EasyStart
No soft starter: full compressor surge applies. Size your inverter and battery for the full startup current.
Step 4 — Location & Solar
Step 5 — Battery System
kWh
Enter your existing battery capacity to see how many hours of AC it supports. Leave at 0 to just see the required battery size.

📈 Your Solar AC Specification

❄

Select your AC type, enter BTU and EER rating, set daily runtime and location, then click Calculate. You will get running watts, compressor surge, required inverter size, number of solar panels by region, and battery kWh for your target runtime.

Why Every Solar AC Sizing Guide Gets the Watt Number Wrong

The number one mistake in solar AC sizing is using the BTU rating to estimate watt draw without accounting for EER (Energy Efficiency Ratio). A 5,000 BTU window air conditioner at EER 8 draws 625 watts. The same 5,000 BTU unit at EER 12 draws only 417 watts — a 33% difference that completely changes your required solar panel count and battery size. The EER rating is printed on the yellow EnergyGuide label on every AC unit sold in the United States. If your unit is a mini-split, use the SEER rating divided by 1.1 to approximate the EER for sizing purposes.

The second mistake — and the more dangerous one — is ignoring compressor startup surge. A 5,000 BTU window AC at 500 watts running can surge to 2,500-3,000 watts for the one to three seconds it takes the compressor to spin up. If your inverter’s surge rating is below this number, it will trip its overload protection the moment the AC cycles on. This can happen even after hours of normal operation because AC compressors cycle on and off continuously. An inverter that cannot handle the compressor surge is not a usable inverter for AC operation, regardless of its continuous watt rating.

EER vs SEER vs COP: Which Number to Use for Solar Sizing

EER (Energy Efficiency Ratio) is measured at a fixed condition: 95 degrees Fahrenheit outdoor, 80 degrees indoor, 50% relative humidity. It is the ratio of BTU output to watt input: EER = BTU/hr / Watts. A unit with an EER of 12 produces 12 BTUs of cooling per watt of electricity. This is the number to use for solar sizing because solar production is highest on the same hot sunny days when your AC is working hardest.

SEER (Seasonal Energy Efficiency Ratio) is an average across a full cooling season with varying conditions. It is always higher than EER for the same unit — a mini-split with a SEER of 20 might have an EER of only 14-16 at peak summer conditions. For solar sizing, always use EER, not SEER. If only SEER is listed (common for mini-splits), divide by 1.1 as a conservative approximation of peak-condition EER.

How the Solar Air Conditioner Calculator Works

Select your AC type, enter the BTU rating and EER from your unit’s label, set daily runtime hours, choose whether you have a soft starter, select your US region for location-adjusted peak sun hours, pick your panel wattage, and enter battery system details. The calculator converts BTU to running watts, calculates compressor startup surge with and without soft starter credit, determines the minimum inverter size, calculates daily kWh consumption accounting for duty cycle, divides by peak sun hours to find the number of panels needed, and sizes the battery bank for your target runtime. A bar chart shows whether your panel array covers the AC’s daily demand or leaves a deficit.

Three Real Solar AC Examples Across US Climates

❄

Window AC, Off-Grid Cabin — Southwest Desert

Sedona, AZ | 8,000 BTU window AC | 6.5 peak sun hours | No grid connection

Mike’s off-grid cabin in the Arizona high desert has a 8,000 BTU window AC (EER 11) that runs 10 hours per day in summer. He wants to know how many 400W panels and how much LFP battery to size.

ParameterValueNotes
Running watts727W8,000 / 11 = 727W
Surge (no soft starter)3,636W727W x 5 surge multiplier
Required inverter4,545W surge3,636 x 1.25 NEC margin
Daily kWh (50% duty)3.64 kWh727W x 10h x 0.50 / 1000
Panels (6.5 PSH, 400W)2 panels3.64 / (6.5 x 0.4) = 1.4 — round up to 2
Battery for 10h AC3.94 kWh727W x 0.50 x 10h / 0.92 eff / 1000
Result: 2x 400W panels (800W total), 4 kWh LFP battery, Victron MultiPlus 24/3000 (6,000W surge handles the 4,545W requirement). At 6.5 peak sun hours in Arizona, the 2-panel array produces 5.2 kWh/day — enough to power the AC (3.64 kWh) with 1.56 kWh surplus for lights and charging. With a SoftStartRV, surge drops from 3,636W to about 1,800W, allowing a 2,000W surge-rated inverter.
🏠

Mini-Split, Off-Grid Homestead — Southeast US

Rural Georgia | 9,000 BTU mini-split | SEER 22 | 4.8 peak sun hours

The Johnsons have a 9,000 BTU Mitsubishi mini-split (SEER 22, approx EER 20) that cools their master bedroom 12 hours per day. They have a 48V LFP system and want to calculate their full system requirements.

ParameterValueNotes
Running watts450W9,000 / 20 = 450W
Surge (mini-split, no soft starter)1,125W450W x 2.5 multiplier
Required inverter1,406W surge1,125 x 1.25
Daily kWh (70% duty)3.78 kWh450W x 12h x 0.70 / 1000
Panels (4.8 PSH, 400W)2 panels3.78 / (4.8 x 0.4) = 1.97
Battery for 12h AC4.10 kWh450W x 0.70 x 12h / 0.92 / 1000
The mini-split’s inverter-driven compressor is a game-changer for solar sizing. At 450W running and only 1,125W surge (vs 3,636W for a comparable window unit), the Johnsons can use a 2,000W inverter instead of a 5,000W unit — saving $800. The variable-speed compressor also means the mini-split modulates output rather than cycling on and off, further reducing surge events. Their 2x 400W array produces 3.84 kWh/day in Georgia — essentially matching the 3.78 kWh AC demand with minimal surplus. Adding a third panel provides a comfortable buffer for cloudy days.
⛽

RV Window AC — The Soft Starter Case Study

Full-time RV travel | 13,500 BTU roof AC | EER 9.7 | Victron MultiPlus 12/3000

Carlos has a Dometic 13,500 BTU roof AC on his Class A motorhome (EER 9.7). His existing Victron MultiPlus 12/3000 (3,000W continuous / 6,000W surge) sometimes trips when the AC kicks on. Should he upgrade the inverter or add a soft starter?

ScenarioSurge WattsRequired Inverter SurgeHis MultiPlus 6,000W Surge
No soft starter6,959W8,699W neededFAILS — 6,000W < 8,699W
With MicroAir EasyStart2,784W3,480W neededPASSES — 6,000W > 3,480W
Without a soft starter, the 13,500 BTU AC surges to 6,959W — requiring an 8,699W surge inverter. No standard 12V inverter at that size exists at reasonable cost. With a MicroAir EasyStart 364 ($249), the startup surge drops to approximately 2,784W, well within the Victron 12/3000’s 6,000W surge capacity. Carlos saves $1,500+ by adding a $249 soft starter instead of replacing a perfectly good inverter. The EasyStart also reduces compressor wear and extends the AC’s lifespan. This is the single highest-value upgrade for any RV with a window or roof AC running on battery and inverter power.

Expert Tips for Running Air Conditioning on Solar Power

1

Install a Soft Starter Before Upgrading Your Inverter

If your inverter trips when the AC compressor starts, your first instinct is to buy a bigger inverter. But a $150-300 soft starter (SoftStartRV or MicroAir EasyStart) typically reduces compressor startup surge by 60-70%, often allowing your existing inverter to handle loads it previously could not. The EasyStart 364 is the most popular choice for 13,500-15,000 BTU RV roof ACs. The SoftStartRV works with most residential window ACs. Both install in 30-60 minutes and immediately resolve most inverter tripping problems. The payback is instant compared to buying and installing a larger inverter. Even if you plan to upgrade your inverter eventually, add the soft starter now — it protects the compressor from hard starts that shorten its life.

2

Mini-Splits Are the Right AC for Solar — Windows Are the Wrong One

Watt for watt of cooling capacity, a mini-split is dramatically more compatible with solar than a window unit. A 9,000 BTU mini-split at SEER 22 draws 450W and surges to only 1,125W. A 9,000 BTU window AC at EER 10 draws 900W and surges to 4,500W. The mini-split runs half the time, surges one-quarter as hard, and uses one-half the daily energy. For any permanent off-grid or solar-primary installation, the premium cost of a mini-split pays back within 1-2 years in smaller inverter, smaller battery bank, and fewer panels required. The only scenario where window AC makes sense on solar is for portable or temporary installations where mini-split installation is not practical.

3

Run AC During Peak Sun Hours, Not in the Evening

The most effective strategy for solar-powered AC is timing. Pre-cool your space during peak solar production hours (typically 10am to 3pm) when your array is producing at full capacity and the battery is full from morning charging. Run the AC hard during peak sun to take advantage of direct solar power with minimal battery draw. Then shut off or reduce the AC in the late afternoon as production drops, relying on the thermal mass of your well-insulated space to maintain comfort. This simple scheduling can reduce your required battery bank size by 40-50% compared to running AC continuously through the evening. A programmable thermostat or smart outlet timer makes this automatic.

16 Frequently Asked Questions About Running Air Conditioners on Solar

How do I find the EER rating on my air conditioner?+
The EER (Energy Efficiency Ratio) is printed on the yellow EnergyGuide label required on all room air conditioners sold in the United States by the Federal Trade Commission. It is also on the specification sticker inside the unit near the filter. For window ACs, EER is typically between 8 and 12. For mini-splits, manufacturers usually advertise SEER (Seasonal Energy Efficiency Ratio) rather than EER. To convert: EER is approximately SEER divided by 1.1 for most modern variable-speed units. The EER at actual peak summer conditions (95F outdoor, 80F indoor) is what matters for solar sizing, since that is when both your AC demand and solar production are highest. If you cannot find your EER, use 10 as a conservative estimate for window units and 15 for mini-splits.
Why does my inverter shut off when the AC turns on?+
This is a surge overload. Your AC compressor draws 4-6 times its running current for 1-3 seconds on startup. If this startup surge exceeds your inverter’s surge rating, the overload protection trips and the inverter shuts off. The solution is either a soft starter (reduces surge by 60-70%), a larger inverter (more expensive but effective), or both. A soft starter costing $150-300 often resolves the problem without any inverter replacement. If you have already tried a soft starter and the inverter still trips, check the inverter’s actual surge rating versus the post-soft-starter surge estimate — some budget inverters advertise surge ratings they cannot actually sustain for the full 3-5 seconds required. Victron inverters are known for reliable surge performance; their MultiPlus series regularly starts ACs that trip cheaper inverters of equal nominal rating.
How many solar panels do I need to run a 5,000 BTU window AC?+
It depends on your location and how many hours you run the AC. A 5,000 BTU window AC at EER 10 draws 500W running and uses approximately 2-3 kWh per day at 5-6 hours of operation with 50% duty cycle. In Phoenix (6.5 peak sun hours), a single 400W panel produces 2.6 kWh/day — enough for the AC with minimal surplus. In Seattle (3.5 peak sun hours), that same panel produces only 1.4 kWh/day, requiring 2 panels for the same AC load. The formula: panels needed = (daily kWh) / (peak sun hours x panel kW). Use this calculator with your specific region selected to get the accurate number for your location.
Can I run a central AC on solar?+
Yes, but it requires substantial investment. A standard 3-ton central AC (36,000 BTU) at SEER 16 draws 2,250W running and surges to 13,500W or more on startup. A 5-ton unit draws 3,750W running and surges to 22,500W. To run a 3-ton central AC on solar requires a minimum 5,000W continuous inverter with 10,000W+ surge capacity (such as an EG4 6000XP or Victron Quattro), a 10-15 kWh battery bank for several hours of operation, and 6-10 x 400W solar panels for daily energy replenishment. Total system cost is typically $15,000-30,000 installed. Most homeowners pursuing solar-powered AC choose to replace their central system with mini-splits, which require 3-4x smaller solar and battery systems for equivalent cooling coverage. A 9,000 BTU mini-split at SEER 22 handles a typical bedroom with 450W — roughly 1/5 the solar system size of a whole-home central unit.
What is duty cycle and why does it affect solar sizing?+
AC duty cycle is the percentage of time the compressor is actually running versus sitting idle. A window AC in a hot room might run 60-70% of the time. In mild conditions, it might only run 30-40%. The standard estimate used for sizing is 50% duty cycle for window and portable ACs, 70% for mini-splits (which run more continuously at reduced speed rather than cycling on and off). Duty cycle dramatically affects your daily energy consumption and battery requirements. A 500W AC running at 50% duty cycle for 8 hours uses 500W x 8h x 0.50 = 2,000 Wh = 2 kWh. The same unit at 70% duty cycle uses 2.8 kWh — 40% more. Use the actual setting closest to your expected conditions for accurate sizing.
What size battery do I need to run AC overnight?+
For overnight AC operation (8 hours without solar), calculate: battery kWh needed = running watts x duty cycle x hours / inverter efficiency (0.92). For a 5,000 BTU window AC at 500W, 50% duty cycle, 8 hours: 500 x 0.50 x 8 / 0.92 = 2,174 Wh = 2.2 kWh. For a 9,000 BTU mini-split at 450W, 70% duty cycle, 8 hours: 450 x 0.70 x 8 / 0.92 = 2,739 Wh = 2.7 kWh. For overnight AC in a hot climate like Texas or Florida, a 5-10 kWh LFP battery bank is realistic minimum sizing — the AC is your single largest overnight load and the battery must be large enough to handle it without excessive depth of discharge that shortens battery life.
What is a soft starter and how much does it reduce surge?+
A soft starter is an electronic device that gradually ramps up the voltage applied to an AC compressor motor during startup, instead of applying full voltage instantly. This slow ramp-up reduces the initial current inrush — the startup surge — by approximately 60-70%. The two most popular products for RV and residential use are the MicroAir EasyStart (designed specifically for RV and residential AC units, models for 9,000-15,000 BTU units) and the SoftStartRV (compatible with most window and through-wall ACs). The EasyStart 364 is the most widely recommended for RV roof ACs. Both install in 30-60 minutes and require no electrical license for most DIY installations. At $149-299, they are one of the highest-value upgrades for any solar-plus-AC system. Even if you plan to keep your current inverter long-term, the soft starter reduces compressor wear and extends AC life by eliminating the mechanical stress of hard starts.
What is the difference between running watts and surge watts?+
Running watts is the steady-state power the AC consumes once the compressor is spinning normally. This is what you calculate from the BTU rating and EER: running watts = BTU/hr / EER. Surge watts is the peak power the compressor draws for 1-3 seconds when it first starts. Electric motors require much more current to start than to run because of the energy needed to overcome inertia and overcome back-EMF. For compressor motors, this startup surge is typically 4-6 times the running watts. Both numbers matter: your inverter’s continuous rating must exceed the running watts, and your inverter’s surge rating must exceed the surge watts. An inverter rated 2,000W continuous / 4,000W surge will run a 500W AC but might trip on a 3,000W surge from startup.
How many peak sun hours does my location get?+
Peak sun hours (PSH) is the number of hours per day equivalent to full-intensity solar irradiance (1,000 W/m2). It is not the number of daylight hours. A location with 5 PSH receives the energy equivalent of 5 hours of full sun, even if daylight lasts 14 hours. The US ranges from about 3.5 PSH in the Pacific Northwest to 6.5+ PSH in the Southwest desert. The National Renewable Energy Laboratory (NREL) National Solar Radiation Database provides precise PSH data for any US zip code. For AC sizing, use the summer average PSH for your location since AC demand and solar production are both at their peak in summer. The Southwest advantage is significant: a solar system in Phoenix produces nearly twice the energy per panel per day as the same system in Seattle.
What inverter size do I need for a 13,500 BTU RV AC?+
A standard 13,500 BTU RV rooftop AC (EER approximately 9.7) draws about 1,391W running and surges to approximately 6,955W without a soft starter. To handle this without a soft starter, you need an inverter rated for at least 1,750W continuous and 8,694W surge (6,955 x 1.25). Very few inverters meet this surge requirement at reasonable cost — the Victron MultiPlus 12/3000/120-50 (6,000W surge) comes close but may still trip. With a MicroAir EasyStart 364, the surge drops to approximately 2,500-2,800W, and a Victron MultiPlus 12/3000 (6,000W surge) handles it comfortably. For 12V systems, the MicroAir EasyStart plus a Victron MultiPlus 12/3000 is the standard professional solution for RV AC on battery and solar. For 24V or 48V systems, the choices are wider and more economical.
Is it cheaper to run AC off solar or from the grid?+
For an existing grid-connected home, running AC from solar requires offsetting the cost of the solar system (typically $2.50-3.50 per watt installed before incentives). With the federal 30% Investment Tax Credit and typical electricity rates of $0.12-0.18 per kWh, payback periods for grid-tied solar AC systems are typically 6-10 years. For off-grid installations with no grid access, solar is almost always cheaper than running a generator for AC — a generator consuming 0.5 gallons per hour at $3.50/gallon costs $1.75/hour to run, or $5,000+ per year for daily summer AC use. A solar-plus-battery system that replaces that generator typically pays back in 3-5 years. For temporary installations (RVs, remote worksites), the math favors generator power for occasional use but solar for extended or daily use.
What battery chemistry is best for running AC on solar?+
Lithium iron phosphate (LFP) is the correct choice for any solar system that includes AC loads. The primary reason is discharge rate. AC loads draw high current — a 500W AC at 24V draws about 22 amps continuously, with surge currents of 100+ amps. LFP batteries handle these high discharge rates without damage or significant capacity loss. Lead-acid batteries (AGM, flooded) at 100+ amp discharge rates suffer from Peukert’s law — their effective capacity drops dramatically at high discharge rates. Additionally, lead-acid batteries should not be discharged below 50% state of charge without accelerating degradation, while LFP batteries can be discharged to 20% usable capacity with minimal cycle life impact. For the same usable capacity, an LFP battery costs more upfront but lasts 3,000-5,000 cycles vs. 300-500 cycles for AGM, making it significantly cheaper over 10 years of daily AC operation.
Can I run AC directly from solar panels without a battery?+
You can, but it is impractical for most systems. Without a battery, AC power is limited to the instantaneous panel output, which varies constantly with cloud cover, panel angle, and temperature. The compressor startup surge cannot be met by panels alone — panels do not have the stored energy to deliver 5x running watts for 3 seconds. Even if surge is handled by a soft starter, any cloud temporarily reduces panel output below the running watt threshold, and the inverter will cut power to the AC mid-cycle. The compressor is then required to restart from cold — the highest-surge, highest-wear condition. For reliable AC operation, a battery with at least 30 minutes of AC runtime at full load provides the buffer needed to handle cloud transients, startup surges, and brief periods of reduced solar production. A battery with 2-4 hours of AC capacity covers a full afternoon of partially cloudy operation without interruption.
Does running AC reduce my solar panel lifespan?+
No — solar panels are rated to produce power and running AC simply uses that power. Panel degradation (typically 0.5-0.7% per year) is driven by UV exposure, thermal cycling, and humidity — not by how hard your loads work. The components that do experience increased wear from frequent high-discharge AC cycling are your batteries and your inverter. Battery cycle life is specified by the manufacturer at a given discharge depth (e.g., 3,000 cycles at 80% DoD for LFP). Running AC hard every day means completing one full cycle per day, so a 3,000-cycle battery bank lasts about 8 years of daily AC operation. Your inverter experiences thermal stress during AC operation because it is running at or near capacity — ensure it has adequate ventilation and is not enclosed in a hot space. Quality inverters (Victron, SMA) are rated for 10-15 years of continuous operation when properly ventilated.
What are the most energy-efficient AC options for solar?+
Ranked from most to least solar-friendly: (1) DC mini-splits (e.g., Midea, Senville) — some models run directly from 48V DC without an inverter, eliminating conversion loss and eliminating startup surge entirely. These are the ideal choice for new off-grid AC installations. (2) Inverter-driven mini-splits (SEER 20-30) — low surge, high efficiency, variable speed. (3) Window ACs with EER 12+ — acceptable efficiency with a soft starter installed. (4) Standard window ACs (EER 8-10) — inefficient and high-surge, but widely available and inexpensive. (5) Portable ACs — least efficient category, with additional ductwork heat loss. (6) Central AC — highest energy draw, largest system requirement, least compatible with solar of all options. The efficiency difference between a SEER 10 window unit and a SEER 25 mini-split represents a 2.5x difference in panel and battery requirements for identical cooling output.
Where can I find official US resources for solar AC sizing?+
The US Department of Energy Energy Saver site covers AC efficiency standards and EER/SEER ratings. The NREL National Solar Radiation Database provides precise peak sun hour data for any US location. The ENERGY STAR database lists EER ratings for all certified room air conditioners. For inverter and battery sizing standards, the National Electrical Code (NFPA 70) Article 690 governs photovoltaic system design including battery and inverter specifications.

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Legal Disclaimer and Editorial Transparency

The Solar Air Conditioner Calculator on USCalculators.com provides estimates for educational and planning purposes only. BTU-to-watt conversions use EER as entered by the user — actual energy consumption depends on installation conditions, ambient temperature, insulation quality, and thermostat settings. Compressor surge multipliers are typical industry estimates and vary by specific unit model, age, and condition. Soft starter surge reduction of approximately 60% is an industry average — actual results depend on the specific soft starter model and AC unit.

Peak sun hours are regional averages from NREL data and do not account for local shading, panel orientation, soiling, or seasonal variation. Battery sizing estimates use 92% inverter efficiency and assume rated usable capacity. All permanent electrical installations must comply with NFPA 70 (National Electrical Code), Article 690. See energy.gov for official AC efficiency information.

Editorial policy: USCalculators.com is an independent educational resource. No affiliate or commercial relationship exists with MicroAir, SoftStartRV, Victron, Mitsubishi, Dometic, or any AC or inverter manufacturer.