SUNBLOC
Solar Kit Sizing
Which Off-Grid Kit Should I Buy?
Most calculators assume the system will be grid-tied with the utility as backup. Off-grid assumes the system will have to generate and store 100% of its own power, which means days of autonomy matters as much as panel count.
The formula
Start from daily watt-hour demand, then size storage for your autonomy target at a safe depth of discharge, then size the array to replace that storage during your local peak sun hours.
Storage target (Wh) = (daily Wh × days of autonomy) ÷ depth of discharge
Panel count = (daily Wh ÷ peak sun hours) ÷ panel wattage
Example: A cabin drawing 1,800 Wh/day, targeting 3 days of autonomy at 80% DOD, needs roughly 6,750 Wh of storage. At 4.5 peak sun hours and 400W panels, that's about 1 panel per ~1,800Wh of daily replacement — call it 2-3 panels depending on your exact load mix.
Frequently asked questions
How many solar panels do I need for an off-grid system?
Divide your daily watt-hour use by your local peak sun hours, then divide by the wattage of one panel. Off-grid systems also need a storage buffer for cloudy days that rooftop calculators usually skip.
What's the difference between a DIY off-grid kit and a grid-tied rooftop system?
A grid-tied system leans on the utility as backup and often skips batteries. An off-grid kit has to cover 100% of your load itself, so battery capacity and autonomy days drive the sizing more than panel count alone.
How many days of autonomy should I plan for?
Most DIY builds target 2-3 days. More autonomy means fewer blackout risks but a bigger, pricier battery bank — it's a budget call as much as a technical one.
What size inverter do I need for a DIY solar kit?
Match it to your highest simultaneous continuous load, then confirm it can handle the surge wattage of anything with a motor or compressor.