TL;DR â Accurate Calculator Inputs
Off-grid solar cost calculators fail when they receive vague inputs. Feed your calculator your monthly bill and it returns a vague number. Feed it your appliance-level load, your worst-month peak sun hours, your chemistry-specific DoD, and your autonomy requirement, and it returns a specification you can bring to any vendor or permit office. The calculator below accepts all four inputs and outputs a component-level cost estimate based on current 2026 component pricing.
I built my first off-grid system using an online calculator that asked for my ZIP code and monthly bill. The output was a 6kW system. My actual load required 11kW. The problem was the calculator used my average billâwhich included summer air conditioning I planned to replace with passive cooling off-grid, and winter electric heat I was replacing with propane. My real off-grid load was completely different from my on-grid consumption. The correct calculator starts with your appliances, not your bill.
Table of Contents
Why Most Calculators Get It Wrong
Solar calculators are marketing tools before they are engineering tools. Most are designed to produce a fast, attractive number that gets you into a sales conversation. They achieve this by consuming a single inputâyour monthly billâand returning a round number that sounds plausible.
The problems:
- Monthly bills average seasonal variation. Your off-grid load in February is different from August.
- On-grid and off-grid loads differ. Appliances you eliminate (electric heat, clothes dryer) versus appliances you add (well pump, battery-powered tools) change the load profile entirely.
- Annual average sun hours overperform. Sizing against the annual average means your system underperforms every winterâthe season when you need it most.
- Default DoD assumptions are wrong. Calculators that assume 50% DoD for LiFePO4 are using the AGM number. LiFePO4 operates at 80% DoD safely.
"Off-grid residential solar systems sized using bill-average estimating tools were undersized by an average of 34% compared to systems sized using appliance-level load analysis, based on post-installation energy audit data from 280 installations."
â National Renewable Energy Laboratory, Off-Grid System Performance Study, 2023
Run the Accurate Solar Estimator
The free Solar Power Estimator uses all four correct inputsâappliance load, December sun hours, autonomy, and chemistry DoDâto produce a component-level cost estimate you can trust. Run the Free Estimator â
| Input Method | Result Quality | Common Error |
|---|---|---|
| Monthly electric bill | Unreliable | Includes on-grid loads you'll eliminate off-grid |
| Square footage | Unreliable | No correlation with actual consumption |
| Appliance list + hours | Reliable | Requires 30 minutes to compile accurately |
| Annual average sun hours | Optimistic | Ignores winter production deficit |
| December peak sun hours | Reliable | Sizes for worst-case production month |
| Default DoD (50%) | Incorrect for LiFePO4 | Oversizes battery bank unnecessarily |
| Chemistry-specific DoD | Accurate | Requires knowing your battery chemistry upfront |
ð¦ WATTSON'S HARD TRUTH: "Every 'instant estimate' solar calculator is a lead generation form wearing an engineer's lab coat. It produces a number fast enough to feel useful and vague enough to require a sales call. The accurate version takes 30 minutes: you list every plug in your house, you look up PVWatts for your address, and you type those numbers into a tool that knows the difference between LiFePO4 and AGM. Everything else is theater."
Input 1: Appliance Load (Not Your Bill)
Your daily load in watt-hours is the foundation of every sizing calculation. This number must come from your appliancesânot your utility bill.
How to build your appliance load list:
- List every appliance you plan to run off-grid (not your current on-grid listâthese differ)
- Find the nameplate wattage for each (printed on the device, not estimated)
- Estimate daily runtime in hours (observe your actual usage, don't guess)
- Multiply: watts à hours = watt-hours per day for each appliance
- Sum all appliances for your daily total
Common load surprises:
- A 1/2 HP well pump: 750W Ã 1.5 hrs/day = 1,125 Wh (often underestimated by 50%)
- LED lights: 60W total à 5 hrs = 300 Wh (often overestimated by 3Ã)
- Refrigerator: 150W nameplate but 75W effective draw with compressor cycling
- Electric water heater: eliminated in most off-grid conversions (replace with propane or solar thermal)
Input 2: December Peak Sun Hours (Not Annual Average)
Your system must be sized to perform in its worst production month. For most of the continental US, that is December or January.
How to find your December peak sun hours:
- Go to pvwatts.nrel.gov
- Enter your address
- Run the default 4kW array calculation
- Look at the monthly AC Energy output for December
- Divide by the array size (4,000W) and by 31 days to get effective daily peak sun hours
For most locations:
- Southwest desert: 4.5 to 5.5 December peak sun hours
- Southeast / Gulf Coast: 3.5 to 4.5
- Pacific Northwest: 1.5 to 2.5 (winter is brutal; size accordingly)
- Mountain West (Colorado, Montana): 3.0 to 4.5 depending on elevation
Input 3: Days of Autonomy (Site-Specific)
Autonomy is how many consecutive days your battery bank must supply full load without any solar production.
Guidelines by region:
- Southwest desert: 2 days (rare extended overcast)
- Southeast / Gulf Coast: 2 to 3 days (summer storms; winter is moderate)
- Pacific Northwest: 3 to 4 days (extended winter overcast is routine)
- High Plains / Central US: 2 days (clear, cold winters; brief cloudy periods)
- New England / Northeast: 3 days (heavy overcast winters; snowpack on panels)
For most homesteaders, 3 days of autonomy is the conservative, defensible default that avoids rationing in all but the most extreme overcast events.
Get Your Component Specifications
The Solar Buyer's Guide translates your calculator output into specific component recommendationsâpanel models, inverter specs, battery bank configurationsâfor 2026 pricing. Get the Component Guide â
Input 4: Battery Chemistry and Depth of Discharge
The DoD you enter into the calculator determines how large your battery bank must be.
- LiFePO4: 80% usable DoD â bank sized at (daily load à days autonomy) ÷ 0.80
- AGM: 50% usable DoD â bank sized at (daily load à days autonomy) ÷ 0.50
A calculator using the wrong DoD for your planned chemistry oversizes or undersizes your battery bankâadding $3,000 to $5,000 in unnecessary cost, or leaving you short by the same margin. For current 2026 pricing applied to each chemistry and format, the battery bank price-per-kWh comparison shows what your calculated capacity will actually cost.
Reading the Output: What Each Number Means
A properly calibrated solar cost calculator returns four outputs:
Battery bank size (kWh): Total storage capacity required. This drives your chemistry selection and configuration. Don't reduce this number.
Panel array size (W): Total watt-peak capacity to produce your daily load from December sun hours. This will produce significant excess in summerâthat's correct. Size for winter. For the 2026 cost of the panel output your calculator specifies, the current price-per-watt benchmarks for Tier 1 panels give you the market number to verify against any quote.
Charge controller (A): MPPT controller amperage matching your array at your system voltage.
Inverter (W continuous): Your peak simultaneous load plus 25% buffer, sized for the largest motor surge in your load list.
Every output number represents a minimum. Build in 15 to 20% growth margin where your component configuration allows it.
Run the Off-Grid Solar Estimator
The Solar Power Estimator uses all four correct inputsâavoiding every shortcut that produces unreliable results.
Run the Free Solar Power Estimator â
It outputs: battery bank size, panel array, charge controller amperage, and inverter ratingâwith 2026 component pricing ranges applied to each output. The result is a number you can bring to any vendor conversation or permit application.
FAQ
What is the most common mistake in off-grid solar sizing?
Using annual average peak sun hours instead of December peak sun hours. A system sized for the annual average (often 4.5 to 5.0 hours for mid-latitude US) will underperform by 30 to 50% in the worst two months of winterâwhen your heating loads and lighting loads are highest.
How accurate are online solar calculators?
Bill-based calculators are accurate within approximately ±35% and are unsuitable for final system sizing. Appliance-level calculators using December sun hours and chemistry-specific DoD are accurate within ±10% to ±15% for most residential applications. Use the appliance method for any sizing decision that involves a purchase commitment.
What should a solar cost per watt be in 2026?
For a DIY off-grid system using Tier 1 components: $1.00 to $1.40 per watt-peak including panels, inverter, batteries sized for 3 days autonomy, racking, and wiring. For contractor-installed systems: $2.80 to $3.80 per watt including all soft costs and labor. The difference is the labor and overhead captured in the contractor model.
How do I account for future load growth in my sizing calculation?
Add 20% to your calculated battery bank and panel array as a growth margin before purchasing. This accounts for the second refrigerator, the power tools you'll eventually run, and the guest cabin load you haven't built yet. Designing for growth at the initial build is far cheaper than retrofitting a production system.
The 30 minutes that determines your next 25 years
Building an accurate appliance load list takes approximately 30 minutes. Running PVWatts for your address takes five minutes. Entering those numbers into the Solar Power Estimator takes two minutes. The result is a system specification that doesn't leave a $10,000 gap between expectation and reality.
The bill-based estimate takes 30 seconds and frequently produces a system that either undersizes your battery bank by 35% or oversizes your panel array by 40%. Either error costs you thousands of dollars and months of lost production before you figure out what went wrong. Once you have your specification, the Solar Payback Calculator turns your sized system cost into a break-even year and 25-year return.
My first system was wrong because I used the wrong calculator. My second system was right because I spent 30 minutes with my wife listing every appliance in the house and then looked up our December sun hours in PVWatts. Start with those two tasks. The rest of the sizing math is arithmetic. Run the Solar Power Estimator after you have your appliance list and your December sun hour number, and you'll have a specification worth acting on.