Problem
Sizing based on a single brochure number = systems that miss spec.
EmersonEIMS solution
Load profile, PSH, derating and reserve factored in.
Business outcome
Sizing you can sign off on.
Solar Sizing • Engineering Tools
Engineer-grade solar sizing — load, panels, batteries and inverters, done properly.
Use the sizing hub to get realistic numbers, then escalate to EmersonEIMS for a bankable design and supervised install.
Built for
- Commercial buyers
- Procurement & technical teams
- Architects & MEP consultants
- Off-grid sites & lodges
- Estates & developments
- NGO & institutional projects
Problem
Battery autonomy collapses in cloudy weeks.
EmersonEIMS solution
Realistic DOD, ambient and ageing assumptions.
Business outcome
Autonomy that holds in the worst week, not the best.
Problem
Inverter undersized for surge / motor loads.
EmersonEIMS solution
Surge-aware inverter selection with margin.
Business outcome
No nuisance shutdowns at compressor start-up.
- PSH & load-profile based design
- Realistic battery & inverter sizing
- Engineer review on request
- Bridges to bankable proposals
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Solar System Sizing Hub
Professional solar system design tools. Calculate your load, size panels, batteries, and inverters. Get it right the first time with our comprehensive sizing guides.
Load Analysis Reference
Use this reference to calculate your daily energy consumption. Daily kWh = Watts × Hours ÷ 1000
📊 Load Calculation Formula
Daily Energy (kWh) = Σ (Appliance Watts × Hours Used) ÷ 1000
Add 20-30% safety margin for efficiency losses and future expansion
| Appliance | Watts | Typical Hours | Daily kWh |
|---|---|---|---|
| LED Bulb (9W) | 9W | 6h | 0.05 |
| LED Bulb (15W) | 15W | 6h | 0.09 |
| CFL Bulb (20W) | 20W | 6h | 0.12 |
| Fluorescent Tube (36W) | 36W | 8h | 0.29 |
| LED Floodlight (50W) | 50W | 8h | 0.40 |
| Security Light (100W) | 100W | 12h | 1.20 |
Engineering reference
Solar System Sizing: A Step-by-Step Worked Method
Sizing a solar system is a sequence, not a single sum: load first, then array, inverter, battery and cable — each step feeding the next. Here is the full method we follow, worked end-to-end with numbers, so you can see exactly how a system is engineered rather than guessed.
Step 1 — Profile the load (the foundation)
Everything downstream depends on one number done honestly: the daily energy demand in kWh, and how it splits between daytime (served directly by the array) and night (served from battery or grid). List each load, its power and its hours; an air-conditioner at 1.5 kW for 8 hours is 12 kWh, a 200 W fridge running half the day is ~2.4 kWh, and so on. Add a margin for the loads people forget and for growth.
Working from "how many panels fit on the roof" instead of the load is the single most common cause of systems that disappoint — either oversized and wasteful, or undersized and unable to carry the afternoon. The load profile is the brief the whole design answers to.
Step 2 — Array size from the load
P_array (kWp) = E_daily ÷ (PSH × PR)
- E_daily
- = daily energy required (kWh)
- PSH
- = peak sun hours (≈5.5 across much of Kenya)
- PR
- = performance ratio (≈0.78 well-built)
Worked example — A site using 30 kWh/day at 5.5 PSH, PR 0.78 → 30 ÷ (5.5 × 0.78) ≈ 7 kWp ≈ 13 × 550 W modules.
Step 3 — Inverter sizing and the DC/AC ratio
The inverter is sized to the array, but deliberately a little smaller than the array's peak. Panels almost never hit their full STC rating in the field (heat, angle, soiling), so a sensible DC/AC ratioof about 1.1-1.3 means the inverter is well-utilised through more of the day, and the rare moments the array would exceed it are simply "clipped" — a tiny energy loss that is cheaper than buying a bigger inverter. The inverter's MPPT voltage window must also bracket the string voltage at both temperature extremes (see Step 5).
For hybrid and off-grid systems the inverter must additionally be rated for the peak load and the surge of motor starts (fridge compressors, pumps), not just the average — a 5 kW continuous inverter may need to swallow a 10 kW momentary surge. Get this wrong and the inverter trips every time the borehole pump kicks in.
DC/AC (array-to-inverter) ratio
DC/AC = P_array(kWp) ÷ P_inverter(kW) (typical 1.1–1.3)
- P_array
- = installed panel capacity (kWp)
- P_inverter
- = inverter AC rating (kW)
Worked example — A 7 kWp array on a 5.5 kW inverter → DC/AC ≈ 1.27 — a good utilisation with negligible clipping in Kenyan irradiance.
Step 4 — Battery (where night/backup is needed)
C (kWh) = (E_night × Days) ÷ (DoD × η)
- E_night
- = energy served from battery (kWh)
- Days
- = days of autonomy
- DoD
- = usable depth of discharge (0.9 LiFePO₄)
- η
- = round-trip × inverter efficiency (≈0.85)
Worked example — 15 kWh overnight, 1 day autonomy, LiFePO₄: 15 ÷ (0.9 × 0.85) ≈ 20 kWh installed battery.
Step 5 — String voltage and the charge controller
Panels are wired in series into strings whose voltage must stay inside the inverter or charge controller's MPPT window across the temperature range. This is the step that quietly destroys inverters or starves them of harvest: a cold Kenyan highland dawn pushes a string's open-circuit voltage up (potentially past the inverter's maximum), while a hot afternoon pulls operating voltage down (potentially below the MPPT window, so it under-harvests). Both extremes must be checked, not just the nameplate.
On battery systems, the charge controller must be MPPT (not PWM) on anything beyond the smallest installation, and rated for the array current with margin. Matching string design to the controller's window is unglamorous and is exactly where amateur installs go wrong.
Step 6 — Cable sizing by voltage drop
%Vdrop = (2 × L × I × R) ÷ V × 100 (keep DC ≤ ~1–2%)
- L
- = one-way cable length (m)
- I
- = current (A)
- R
- = cable resistance per metre (Ω/m)
- V
- = system voltage
Worked example — Long DC runs on undersized cable bleed away harvest as heat. Size the conductor so DC volt-drop stays ≤ ~2% — a yield leak you never see on a spec sheet otherwise.
| Step | You compute | Key input |
|---|---|---|
| 1. Load | Daily kWh, day/night split | Equipment list + hours |
| 2. Array | kWp of panels | Load, PSH, PR |
| 3. Inverter | AC kW + DC/AC ratio | Array kWp, peak/surge load |
| 4. Battery | Usable kWh | Night energy, autonomy, DoD |
| 5. Strings/MPPT | Panels in series | Temp-extreme voltages |
| 6. Cable | Conductor size | Length, current, %Vdrop |
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