01What this calculator sizes
An off-grid tiny house has no utility to fall back on, so it needs three things sized correctly together: the solar array that generates enough power on a poor-sun day, the battery bank that stores enough energy to ride out a stretch of cloudy weather, and an inverter rated for the highest wattage you will ever draw at once. This calculator sizes all three from one set of inputs — daily energy usage, peak sun hours, autonomy days, battery chemistry and a system derate — instead of treating them as separate calculations.
The daily usage figure is the same number a full appliance-by-appliance load list produces: add up each appliance’s watts × hours used per day and divide by 1,000 to get kWh/day, or take a monthly off-grid generator or utility bill and divide by 30. The U.S. Department of Energy’s solar PV technology basics and the U.S. Energy Information Administration’s solar explainer both use this same load-then-resource approach. For the electrical side of the build, our electrical load calculator and conduit fill calculator help with panel sizing and wiring.
02Typical inputs and ranges
Off-grid sizing is only as good as the inputs you give it. The table below shows sensible ranges seen across manufacturer and DIY off-grid sizing guides.
03What changes your result
A few real-world factors move these numbers up or down from the calculator’s estimate:
- Motor start-up (surge) loads. Compressors and pumps briefly draw far more than their running wattage; an inverter sized only to steady peak load can still stall on start-up.
- Seasonal sun-hour swings. A location averaging 5 sun hours annually might see 3 or fewer in mid-winter — size to the worst month for reliability.
- Battery temperature. Cold weather reduces usable battery capacity, especially for lead-acid; very hot climates shorten battery life.
- Generator or shore-power backup. A backup generator lets you safely size the battery bank smaller since you are not relying on solar alone for every cloudy stretch.
- Wire runs and charge controller type. Long DC wiring runs and PWM (vs MPPT) charge controllers add losses beyond the general derate assumed here.
- Add up your appliance watts × hours/day (or take a monthly bill ÷ 30) for your daily energy usage in kWh/day.
- Estimate the highest wattage you could draw at one time — fridge, pump and a couple of small appliances running together — for peak simultaneous load.
- Enter your local peak sun hours for the worst-sun month (default 4.5), and how many days of autonomy you want (default 2).
- Pick lithium or lead-acid battery chemistry, then adjust the system derate if you have a specific efficiency figure (default 0.78).
- Press Calculate to see the solar array, battery bank and inverter sizes, plus the usage-vs-generation bar.
Once you have a target array size, our electrical load calculator can help cross-check the panel and breaker sizing for the rest of the build.
This is a planning-stage estimate for comparison and awareness, not a certified electrical or system design. It does not account for:
- Motor start-up surge current, which can require a larger inverter than the steady peak load alone suggests
- Month-by-month sun-hour and temperature variation across a full year
- Charge controller type (MPPT vs PWM), wire-run losses and specific battery round-trip efficiency
- Local electrical code, permitting, and manufacturer-specific installation requirements
04Related calculators
Working through a related project? Try our Container Home Cost Calculator, Container Home Construction Calculator, and Tiny House Weight Calculator.
01The off-grid sizing formula
Off-grid sizing has three linked steps: convert daily load into a required array size using your local solar resource, size a battery bank from that same load, your desired autonomy and the battery’s safe depth of discharge, then size an inverter from the highest instantaneous load rather than the daily average.
Where:
- daily usage= kWh you consume per day, from an appliance load list or a monthly bill ÷ 30.
- peak sun hours= equivalent hours of full 1 kW/m² sun per day for your worst-sun month.
- derate= system performance ratio after losses, typically 0.75-0.85.
- autonomy days= days of battery backup you want with no solar input, typically 1-3.
- depth of discharge= safe fraction of battery capacity to use — about 0.50 lead-acid, 0.80 lithium.
- peak simultaneous load= the highest combined wattage you could draw at one instant.
02Worked example
A tiny house uses 6 kWh/day, with a peak simultaneous load of 1,800 W (fridge, water pump and a couple of small appliances together). The site gets 4.5 peak sun hours in its worst month, the owner wants 2 days of autonomy on a lithium battery bank (80% DoD), and assumes a 0.78 derate. Here is the calculation end to end:
So this tiny house needs roughly a 1.71 kW solar array, a 15 kWh lithium battery bank, and an inverter rated at least 2,250 W continuous. Switching to lead-acid at 50% DoD would push the same 2-day battery bank to 24 kWh instead of 15 — nearly double — which is why the battery-chemistry toggle above matters as much as the array size itself. As with grid-tied systems, this off-grid design should still be checked against your actual worst-sun month and a licensed installer’s appliance surge-load review before you buy equipment.