SolarVeri Data Team
Irradiance data and off-grid sizing
A 3,000 Wh per day cabin with two days of backup needs about 6,670 Wh of LiFePO4 storage, which is 6 batteries of 12 V 100 Ah, or about 12,000 Wh of lead-acid. But the battery is only half the story: in a northern winter, no reasonable bank can make up for panels that do not collect enough energy.
Battery sizing is step 3 of our off-grid solar sizing guide. This page covers the formula, chemistry, system voltage and what simulations show about winter. If you are building a mobile system, read it alongside the RV solar guide, and check winter solar output for your area before deciding how many days of autonomy you need.
calculateThe battery sizing formula
- Daily load (Wh): the total from your appliance list. Include inverter idle draw.
- Days of autonomy: how many days the bank should carry the load with little or no sun. Common choices are 1 to 2 for RVs and vans, 2 to 4 for cabins.
- Usable depth of discharge (DoD): about 0.9 for LiFePO4 and 0.5 for flooded or AGM lead-acid if you want a reasonable cycle life.
- Bank size (Wh) = daily load × days of autonomy ÷ DoD. Divide by the system voltage to get amp-hours.
| Use case | Daily load (Wh) | Days of autonomy | LiFePO4 bank (Wh) | 12 V 100 Ah LiFePO4 batteries | Lead-acid bank (Wh) | 12 V 100 Ah lead-acid batteries |
|---|---|---|---|---|---|---|
| Van or small RV | 1,200 | 1 | 1,330 | 2 | 2,400 | 2 |
| RV, longer boondocking | 1,200 | 2 | 2,670 | 3 | 4,800 | 4 |
| Weekend cabin | 3,000 | 2 | 6,670 | 6 | 12,000 | 10 |
| Full-time cabin | 3,000 | 3 | 10,000 | 8 | 18,000 | 15 |
| Small off-grid home | 6,000 | 3 | 20,000 | 16 | 36,000 | 30 |
The lead-acid column is roughly double the LiFePO4 column for the same usable energy, and lead-acid is also much heavier per usable watt-hour. That is the main reason new off-grid builds have largely moved to lithium iron phosphate, even though the upfront price per nameplate kWh is higher.
scienceLiFePO4 vs lead-acid for off-grid
- check_circleUsable capacity: LiFePO4 can routinely use 80 to 100 percent of rated capacity; lead-acid lasts much longer if kept above about 50 percent.
- check_circleCycle life: LiFePO4 is typically rated for thousands of cycles; lead-acid for hundreds to around a thousand at moderate depth.
- check_circleCold: most LiFePO4 batteries must not be charged below freezing (0 °C / 32 °F) unless they have built-in heating. In an unheated cabin or RV this matters as much as capacity.
- check_circleMaintenance: flooded lead-acid needs watering and ventilation; sealed AGM and LiFePO4 do not.
- check_circlePartial charge: lead-acid degrades when it sits partly charged for weeks, which is exactly what happens in a cloudy winter. LiFePO4 tolerates it.
bolt12 V, 24 V or 48 V?
Higher system voltage means lower current for the same power, which means thinner cables and less loss. Running a 3,000 W inverter at full load pulls about 250 A from a 12 V bank, 125 A from 24 V and 62.5 A from 48 V, before inverter losses. That is why small van builds use 12 V, while cabins and homes with larger inverters usually go to 48 V. In a 12 V system, the same 10 kWh bank would be about 781 Ah; at 48 V it is about 195 Ah.
ac_unitWhy a bigger battery cannot fix winter
Autonomy covers short gaps. Over a whole month, the battery can only give back what the panels put in. We simulated one cabin system in PVGIS at each location: 1,600 W of panels at 45°, a 10,240 Wh LiFePO4 bank and a 3,000 Wh daily load. Down to its 20% cutoff, the bank alone holds about 2.7 days of that load.
| Location | Average Wh delivered per day, December | Share of load covered | Days battery hits cutoff, December |
|---|---|---|---|
| Phoenix, AZ | 3,005 | 100% | 0% |
| Austin, TX | 2,995 | 100% | 3% |
| Denver, CO | 2,992 | 100% | 1% |
| Nashville, TN | 2,754 | 92% | 16% |
| New York, NY | 2,766 | 92% | 12% |
| Chicago, IL | 2,011 | 67% | 55% |
| Seattle, WA | 1,636 | 55% | 74% |
| Madrid, Spain | 2,956 | 99% | 5% |
| Munich, Germany | 1,977 | 66% | 67% |
| London, UK | 1,703 | 57% | 86% |
In Phoenix and Austin this bank is plenty because the panels refill it most days. In Seattle and London the same bank empties again and again in December, because a month of short gray days simply does not produce 3,000 Wh a day from 1,600 W of panels. Doubling the battery there would mostly add cost; adding panels, tilting them steeper, adding a generator or trimming the winter load are the levers that work. The winter output guide shows how deep the seasonal drop is in each region.
cableWiring several batteries together
- check_circleSeries adds voltage (four 12 V batteries make 48 V); parallel adds amp-hours at the same voltage.
- check_circleUse identical batteries of the same age and charge state in one bank. Mixing old and new, or different chemistries, shortens the life of all of them.
- check_circleCheck the manufacturer limits on how many LiFePO4 batteries can be put in series or parallel and whether their battery management systems support it.
- check_circleFuse each parallel string and size cables for the full inverter current. A licensed electrician should review any system tied to a building.