Off-Grid Solar System Sizing: Pick the Battery, Inverter, and Panels That Survive Real Outages

Most battery sizing guides assume a stable grid. You charge cheap at night, discharge by day. In Ukraine and parts of Russia the grid is not a partner — it is the failure you design around. This guide uses load math from a real Lviv install, because the textbook formula nearly left that family in the dark.

Start with the load you cannot live without

For the Lviv home we sized, the essentials were a 150 W fridge, 60 W of lights, a 15 W router, a 1.1 kW borehole pump that runs 10 minutes twice a day, and a 40 W gas boiler controller. Daily draw landed near 3.2 kWh. The catch was the pump: at startup it surged to 2.4 kW, three times its running draw. Any inverter sized only to the average would have stalled it.

Size the battery for two cuts, not one

A single 4-hour outage pulls about 1.3 kWh. Outages cluster, so I spec for two. A 5 kWh (48V 100Ah) LiFePO4 pack rides two 4-hour cuts with margin left. Lead-acid at 50% usable depth would need 10 kWh of nameplate to deliver the same — heavier, pricier, and it sulphates within a year under clustered cuts. That is the failure I see most: a 300-cycle lead-acid bank swapped for a 3,000-6,000-cycle LiFePO4 pack after one bad winter.

Match the inverter to the surge, not the average

The 2.4 kW pump spike ruled out the cheap 2 kW inverter. We used a 6 kW hybrid unit with a 10 ms transfer time — fast enough that the fridge never notices the grid drop. A grid-tie-only inverter cuts out completely the moment the line fails, which defeats the point. If your home runs a well pump or a freezer compressor, buy the hybrid rating, not the label.

Oversize panels for winter

Lviv in December gives about 1.2 peak sun hours. To refill a 5 kWh battery before the next cut you need roughly 2.5-3 kW of panels, not the 1 kW a southern-China sizing suggests. We sent 3 kW. Undersize and the second cut of the day hits an empty bank. The same logic applies wherever winter sun is weak — see our Ukraine & Russia off-grid breakdown for a full 5 kW bill of materials.

One number people skip: C-rate

A 100 Ah battery at 1C delivers 5 kW. The 2.4 kW pump surge is safe. A 50 Ah pack at 1C gives only 2.5 kW and the pump stalls. Check C-rate on the datasheet before you trust the watt rating — a label that says "5 kW" on a half-size cell is a callback waiting to happen.

A parts list that ships as one pallet

For the Lviv install we sent a single pallet: a rack-mounted 48V 100Ah LiFePO4, a 6 kW hybrid inverter, and MC4 connectors. The local installer wired it in an afternoon. If you are matching an inverter to your bank, our hybrid inverter sizing guide covers surge, transfer time, and BMS comms in detail.

Frequently asked questions

What size lithium battery do I need for off-grid solar in Ukraine?

For a home that loses power 3-5 times a week, size the battery for two blackouts. A typical essential load draws ~3.2 kWh/day with a 2.4 kW pump surge. A 5 kWh 48V 100Ah LiFePO4 pack rides two 4-hour cuts with margin; lead-acid would need 10 kWh nameplate and sulphates within a year.

Why does a hybrid inverter matter for off-grid solar?

A grid-tie-only inverter stops the moment the grid drops. A hybrid inverter switches to battery in under 20 ms, so fridges and pumps keep running. Match its rating to motor startup surge, not running watts.

How many solar panels for a 5 kWh off-grid battery in winter?

Winter peak sun here drops to ~1.2 hours per day. Refilling a 5 kWh battery needs about 2.5-3 kW of panels. A 1 kW array sized for southern climates leaves the bank empty by the second cut.